| 0/0 |
2026/05/20 01:06 |
flow |
assessment-security |
52m
Results: map[DenialOfService:true Explanation:Based on the provided kernel bug report and source code analysis, here is the
assessment of the bug:
**Analysis:**
The bug report indicates a classic lock contention or deadlock scenario
resulting in hung tasks (`INFO: task blocked for more than 143 seconds`).
1. **Lock Contention/Deadlock:**
- Task `syz.9.5484` is in the process of opening a TTY device. It has
acquired the global `tty_mutex` (in `tty_open_by_driver` -> `tty_init_dev`)
and is now blocked trying to acquire the `console_sem` semaphore inside
`con_install()`.
- Because `syz.9.5484` is holding `tty_mutex`, numerous other tasks (e.g.,
`syz.5.5487`, `syz.4.5495`, etc.) that are also trying to open TTYs are
blocked waiting for `tty_mutex`.
- The `console_sem` is held by another task for an extended period. This is a
known architectural issue in the kernel's printk/console subsystem where a
task calling `console_unlock()` can be forced to flush a massive backlog of
kernel messages (a "printk flood") to slow consoles (like serial ports)
without yielding, effectively holding `console_sem` hostage for a very long
time.
2. **Impact:**
- **Denial of Service (DoS):** Yes. The system becomes unresponsive as tasks
hang indefinitely waiting for core locks like `tty_mutex` and `console_sem`.
The softlockup watchdog might also fire if the task holding the lock doesn't
yield.
- **Exploitable:** No. This is a synchronization and resource starvation
issue, not a memory safety violation (like use-after-free or out-of-bounds
access). It cannot be used for memory corruption or privilege escalation.
3. **Accessibility:**
- **Unprivileged / User Namespace:** Yes. Unprivileged users can open TTY
devices (`/dev/tty`, `/dev/ptmx`, etc.) and can often trigger kernel warnings
or printk floods (e.g., by intentionally passing malformed arguments to
certain syscalls) that exacerbate the `console_unlock` stall. No special
capabilities are required.
4. **Other Triggers:**
- The bug is a local DoS triggered via standard syscalls (like `openat` on
device nodes). It does not require malicious filesystems, network packets, or
peripheral devices to trigger.
**Conclusion:**
This is a local Denial of Service (DoS) vulnerability caused by lock starvation
involving `tty_mutex` and `console_sem`, typically exacerbated by a printk
flood. It is accessible to unprivileged users but is not exploitable for
privilege escalation. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/05/20 01:06 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/05/20 01:06 |
action |
kernel-checkouter |
1m
Results: map[KernelSrc:/app/workdir/cache/src/f83d98ac7db585a53b3b7d5c818ce044ae77dc75]
|
| 3/1 |
2026/05/20 01:08 |
action |
kernel-builder |
20m
Results: map[KernelObj:/app/workdir/cache/build/e28dfbec6aabe122a6c9ca7ad3762852df9aa3c3]
|
| 4/1 |
2026/05/20 01:29 |
action |
codesearch-prepare |
22m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/05/20 01:51 |
agent |
expert |
7m
Model:
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
Instruction:
You are an experienced Linux kernel security engineer. Your task is to analyze given kernel bug report
and determine its security impact based on the following dimensions.
Use the provided tools to examine the source code, check for capability checks (e.g., capable(), ns_capable()),
and understand the nature of the bug. Analyze the given kernel build and configuration.
You can check the kernel config by grepping ".config" file; you can check kernel cmdline by greeping
".config" file for "CONFIG_CMDLINE=". Assume sysctl parameters have default values.
But analyze for the corresponding production build w/o debugging tools enabled (like KASAN, KMSAN, UBSAN).
Don't make assumptions; verify them with source code access. Try different strategies when analyzing the bug:
- think of ways in which the vulnerable code is unreachable
- or the other way around: try to come up with different ideas of how an unprivileged user can reach the bug
If still unsure err on the side of the bug being non-exploitable/not-accessible.
In the final reply, provide a reasoning for your assessment.
Analysis dimensions:
* Exploitable:
Determine if the bug can result in memory corruption or elevated privileges.
Memory safety issues are almost always exploitable (KASAN or UBSAN reports for use-after-free, out-of-bounds;
refcounting issues, corrupted lists, etc). When kernel is crashing on a completly wild pointer access
(e.g. user-space address, or non-canonical address, but not on NULL or address corresponding to KASAN shadow
for NULL address), including both data accesses and control tranfers, that's also usually implies possibility
of exploitation. Such reports usually say "unable to handle kernel paging request".
Uses of uninitialized values detected by KMSAN may be exploitable b/c attacker frequently can affect uninit
values with spraying techniques. However, for these exploitabability depends on how exactly the uninit value
is used in the code, and what it affects.
Think of what happens after the bug is triggered. Some bugs cause kernel panic and halt execution,
they are harder to exploit. For example, BUG reports halts the kernel. However, WARNING reports don't halt
execution in production builds. Debug bug detection tools (like KASAN, KMSAN, KCSAN, UBSAN) are also not enabled
in production builds, so attacker can freely exploit these bugs w/o being detected by these tools.
If you see an integer overflow, think how the overflowed value used later (if it's used as allocation size,
or an array index). If you see an out-of-bounds read, think if it's followed by an out-of-bounds write as well.
Some KCSAN data-races may be exploitable by skilled attackers as well. Think what data structures got corrupted
as the result of data races and how. However, note that kernel has lots of "benign" data races that don't lead
to any runtime misbehavior at all.
* Denial Of Service:
Determine if the bug can result in denial-of-service. Most bugs can, since they cause system crash,
hangs, deadlocks, or resource leaks. This is mostly applicable to WARNING bugs that won't cause system crash
in production. For these think what will be consequences of the violation of the kernel assumptions flagged
by the WARNING. In some cases the unexpected condition is also properly handled by the normal control flow
(e.g. with "if (WARN_ON(...))"), these won't cause denial-of-service. If the condition is not handled,
then it may or may not cause denial-of-service.
* Accessible From Unprivileged Processes:
Determine if the bug can be reached from a typical (non-root) user process that does NOT have any special capabilities
(like CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON) or access to device nodes restricted to root.
Assume that unprivileged_bpf_disabled=1, that is eBPF loading is not accessible. However, cBPF (classical BPF)
is still accessible to non-root processes.
Assume that user namespaces are not accessible, that is, the process cannot get the mentioned capabilities even
within a new user namespace (checked by ns_capable() function in the kernel sources).
* Accessible From User Namespaces:
Determine if the bug can be reached within a user-namespace where the process has all capabilities
(including CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON). Such capabilities are checked with ns_capable()
function in the kernel sources.
* VM Guest Trigger:
Determine if the bug can be triggered from the context of a typical KVM guest (e.g., set up by a QEMU VMM).
Consider accesses to standard Linux host paravirtualized features (virtio-blk, virtio-net, etc.),
and handling of VM exits in the KVM code.
* VM Host Trigger in The Confidetial Computing Context:
Determine if the bug can be triggered in a confidential computing guest kernel from the context of a KVM host.
Consider access to standard Linux guest paravirtualized features (virtio-blk, virtio-net, etc.).
* Ethernet Network Trigger:
Determine if the bug can be triggered by processing ingress network Ethernet traffic, either directly (network stack)
or via drivers exposed to network data.
* Other Remote Trigger:
Determine if the bug can be triggered by processing remote traffic other than Ethernet (Wifi, Bluetooth, NFC, etc).
* Peripheral Trigger:
Determine if the bug can be triggered via an untrusted peripheral device that can be physically plugged
into a system, such as a USB device or a niche hardware driver handling external hardware inputs.
This is particularly important for mobile and desktop environments where users can plug in unknown devices.
* Malicious Filesystem Trigger:
Determine if the bug can be triggered by the kernel mounting and parsing a malicious filesystem image.
This is highly critical for Desktop and Mobile environments where external media or downloaded images
might be auto-mounted.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The kernel bug report is:
INFO: task syz.9.5484:27591 blocked for more than 143 seconds.
Tainted: G L syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.9.5484 state:D stack:27224 pid:27591 tgid:27590 ppid:26506 task_flags:0x400040 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5388 [inline]
__schedule+0x17b4/0x5680 kernel/sched/core.c:7189
__schedule_loop kernel/sched/core.c:7268 [inline]
schedule+0x164/0x360 kernel/sched/core.c:7283
schedule_timeout+0xc3/0x2c0 kernel/time/sleep_timeout.c:75
___down_common kernel/locking/semaphore.c:289 [inline]
__down_common+0x3d0/0x850 kernel/locking/semaphore.c:314
down+0x80/0xd0 kernel/locking/semaphore.c:100
console_lock+0x58/0x90 kernel/printk/printk.c:2896
class_console_lock_constructor include/linux/console.h:736 [inline]
con_install+0x97/0x770 drivers/tty/vt/vt.c:3742
tty_driver_install_tty drivers/tty/tty_io.c:1295 [inline]
tty_init_dev+0xd7/0x4d0 drivers/tty/tty_io.c:1407
tty_open_by_driver drivers/tty/tty_io.c:2073 [inline]
tty_open+0x862/0xd70 drivers/tty/tty_io.c:2120
chrdev_open+0x4cd/0x5e0 fs/char_dev.c:411
do_dentry_open+0x785/0x14e0 fs/open.c:947
vfs_open+0x3b/0x340 fs/open.c:1079
do_open fs/namei.c:4699 [inline]
path_openat+0x2e08/0x3860 fs/namei.c:4858
do_file_open+0x23e/0x4a0 fs/namei.c:4887
do_sys_openat2+0x113/0x200 fs/open.c:1364
do_sys_open fs/open.c:1370 [inline]
__do_sys_openat fs/open.c:1386 [inline]
__se_sys_openat fs/open.c:1381 [inline]
__x64_sys_openat+0x138/0x170 fs/open.c:1381
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f1471d5d60e
RSP: 002b:00007f1472c55b28 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007f1472c566c0 RCX: 00007f1471d5d60e
RDX: 0000000000000002 RSI: 00007f1472c55c00 RDI: ffffffffffffff9c
RBP: 00007f1471e32d69 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f1472016038 R14: 00007f1472015fa0 R15: 00007f147213fa48
</TASK>
INFO: task syz.5.5487:27610 blocked for more than 143 seconds.
Tainted: G L syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.5.5487 state:D stack:28120 pid:27610 tgid:27608 ppid:26507 task_flags:0x400040 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5388 [inline]
__schedule+0x17b4/0x5680 kernel/sched/core.c:7189
__schedule_loop kernel/sched/core.c:7268 [inline]
schedule+0x164/0x360 kernel/sched/core.c:7283
schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7340
__mutex_lock_common kernel/locking/mutex.c:726 [inline]
__mutex_lock+0x7f7/0x1550 kernel/locking/mutex.c:820
tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
chrdev_open+0x4cd/0x5e0 fs/char_dev.c:411
do_dentry_open+0x785/0x14e0 fs/open.c:947
vfs_open+0x3b/0x340 fs/open.c:1079
do_open fs/namei.c:4699 [inline]
path_openat+0x2e08/0x3860 fs/namei.c:4858
do_file_open+0x23e/0x4a0 fs/namei.c:4887
do_sys_openat2+0x113/0x200 fs/open.c:1364
do_sys_open fs/open.c:1370 [inline]
__do_sys_openat fs/open.c:1386 [inline]
__se_sys_openat fs/open.c:1381 [inline]
__x64_sys_openat+0x138/0x170 fs/open.c:1381
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f1e20b9cdd9
RSP: 002b:00007f1e219da028 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007f1e20e16090 RCX: 00007f1e20b9cdd9
RDX: 0000000000021800 RSI: 0000200000001840 RDI: ffffffffffffff9c
RBP: 00007f1e20c32d69 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f1e20e16128 R14: 00007f1e20e16090 R15: 00007f1e20f3fa48
</TASK>
INFO: task syz.4.5495:27640 blocked for more than 143 seconds.
Tainted: G L syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.4.5495 state:D stack:27880 pid:27640 tgid:27636 ppid:26502 task_flags:0x400040 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5388 [inline]
__schedule+0x17b4/0x5680 kernel/sched/core.c:7189
__schedule_loop kernel/sched/core.c:7268 [inline]
schedule+0x164/0x360 kernel/sched/core.c:7283
schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7340
__mutex_lock_common kernel/locking/mutex.c:726 [inline]
__mutex_lock+0x7f7/0x1550 kernel/locking/mutex.c:820
tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
chrdev_open+0x4cd/0x5e0 fs/char_dev.c:411
do_dentry_open+0x785/0x14e0 fs/open.c:947
vfs_open+0x3b/0x340 fs/open.c:1079
do_open fs/namei.c:4699 [inline]
path_openat+0x2e08/0x3860 fs/namei.c:4858
do_file_open+0x23e/0x4a0 fs/namei.c:4887
do_sys_openat2+0x113/0x200 fs/open.c:1364
do_sys_open fs/open.c:1370 [inline]
__do_sys_openat fs/open.c:1386 [inline]
__se_sys_openat fs/open.c:1381 [inline]
__x64_sys_openat+0x138/0x170 fs/open.c:1381
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f7f7379cdd9
RSP: 002b:00007f7f746a7028 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007f7f73a16180 RCX: 00007f7f7379cdd9
RDX: 0000000000048000 RSI: 0000200000000200 RDI: ffffffffffffff9c
RBP: 00007f7f73832d69 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f7f73a16218 R14: 00007f7f73a16180 R15: 00007f7f73b3fa48
</TASK>
Showing all locks held in the system:
1 lock held by khungtaskd/31:
#0: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#0: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#0: ffffffff8e95cd60 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180 kernel/locking/lockdep.c:6775
2 locks held by getty/5381:
#0: ffff8880322200a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x25/0x70 drivers/tty/tty_ldisc.c:243
#1: ffffc900032332e8 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x45c/0x13a0 drivers/tty/n_tty.c:2211
3 locks held by kworker/0:10/14405:
2 locks held by syz.7.5468/27542:
2 locks held by syz.9.5484/27591:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
#1: ffff888036be81b0 (&tty->legacy_mutex){+.+.}-{4:4}, at: tty_init_dev+0x74/0x4d0 drivers/tty/tty_io.c:1406
1 lock held by syz.5.5487/27610:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz.4.5495/27640:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
3 locks held by kworker/0:5/27913:
1 lock held by syz.6.5576/27974:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: class_mutex_constructor include/linux/mutex.h:253 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: ptmx_open+0xdc/0x2e0 drivers/tty/pty.c:800
2 locks held by kworker/u8:15/28032:
1 lock held by syz.8.5662/28311:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz.8.5662/28313:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: class_mutex_constructor include/linux/mutex.h:253 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: ptmx_open+0xdc/0x2e0 drivers/tty/pty.c:800
1 lock held by syz.2.5669/28336:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz.2.5669/28338:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz.0.5695/28419:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz.3.5721/28563:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz.5.5732/28633:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz.4.5778/28828:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz.9.5795/28889:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz.7.5812/28955:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz.7.5812/28956:
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open_by_driver drivers/tty/tty_io.c:2037 [inline]
#0: ffffffff8f202780 (tty_mutex){+.+.}-{4:4}, at: tty_open+0x22d/0xd70 drivers/tty/tty_io.c:2120
1 lock held by syz-executor/29025:
1 lock held by syz.1.5831/29041:
=============================================
NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 31 Comm: khungtaskd Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
nmi_cpu_backtrace+0x274/0x2d0 lib/nmi_backtrace.c:113
nmi_trigger_cpumask_backtrace+0x17a/0x300 lib/nmi_backtrace.c:62
trigger_all_cpu_backtrace include/linux/nmi.h:162 [inline]
__sys_info lib/sys_info.c:157 [inline]
sys_info+0x135/0x170 lib/sys_info.c:165
check_hung_uninterruptible_tasks kernel/hung_task.c:353 [inline]
watchdog+0xfd3/0x1030 kernel/hung_task.c:561
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 4992 Comm: udevd Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
RIP: 0010:hlock_class kernel/locking/lockdep.c:234 [inline]
RIP: 0010:check_wait_context kernel/locking/lockdep.c:4854 [inline]
RIP: 0010:__lock_acquire+0x35e/0x2cf0 kernel/locking/lockdep.c:5187
Code: 09 d4 41 09 cc 41 09 dc 45 89 64 c7 20 41 89 7c c7 24 4c 89 7c 24 20 4d 8d 2c c7 41 81 e4 ff 1f 00 00 4c 0f a3 25 f2 1d 9f 12 <73> 15 49 69 c4 c8 00 00 00 48 8d 80 70 d3 e1 93 4c 8b 74 24 08 eb
RSP: 0018:ffffc90002e2f610 EFLAGS: 00000003
RAX: 000000000000000f RBX: 0000000000020000 RCX: 0000000000000007
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000003 R08: ffffffff8176f256 R09: ffffffff8e95cd60
R10: ffffc90002e2f918 R11: ffffffff81b0e910 R12: 0000000000000007
R13: ffff888037742ac8 R14: ffff888037741ec0 R15: ffff888037742a50
FS: 00007fb967051880(0000) GS:ffff888125390000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000007d05a000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000081 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
rcu_read_lock include/linux/rcupdate.h:838 [inline]
class_rcu_constructor include/linux/rcupdate.h:1181 [inline]
unwind_next_frame+0xc3/0x2550 arch/x86/kernel/unwind_orc.c:495
arch_stack_walk+0x11b/0x150 arch/x86/kernel/stacktrace.c:25
stack_trace_save+0xa9/0x100 kernel/stacktrace.c:122
kasan_save_stack+0x3e/0x60 mm/kasan/common.c:57
kasan_record_aux_stack+0xbd/0xd0 mm/kasan/generic.c:556
__call_rcu_common kernel/rcu/tree.c:3131 [inline]
call_rcu+0xee/0x890 kernel/rcu/tree.c:3251
__dentry_kill+0x4a9/0x690 fs/dcache.c:737
finish_dput+0xc9/0x480 fs/dcache.c:927
lookup_one_qstr_excl+0x1ca/0x360 fs/namei.c:-1
__start_dirop fs/namei.c:2916 [inline]
start_dirop fs/namei.c:2938 [inline]
filename_unlinkat+0x2ba/0x610 fs/namei.c:5549
__do_sys_unlink fs/namei.c:5603 [inline]
__se_sys_unlink+0x2e/0x140 fs/namei.c:5600
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fb966915937
Code: 00 00 e9 a9 fd ff ff 66 2e 0f 1f 84 00 00 00 00 00 66 90 b8 5f 00 00 00 0f 05 c3 0f 1f 84 00 00 00 00 00 b8 57 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 01 c3 48 8b 15 91 b4 0d 00 f7 d8 64 89 02 b8
RSP: 002b:00007ffeb2e9aa78 EFLAGS: 00000202 ORIG_RAX: 0000000000000057
RAX: ffffffffffffffda RBX: 0000000000000bb8 RCX: 00007fb966915937
RDX: 0000000000000000 RSI: 000000008044ea00 RDI: 000055785116402e
RBP: 0000000000000000 R08: 000000008046eff1 R09: 3fffffffffffffff
R10: 4000000000000000 R11: 0000000000000202 R12: 0000000000000000
R13: 000055785117f100 R14: 0000000000000001 R15: 0000000000000000
</TASK>
Reply: Based on the provided kernel bug report and source code analysis, here is the assessment of the bug:
**Analysis:**
The bug report indicates a classic lock contention or deadlock scenario resulting in hung tasks (`INFO: task blocked for more than 143 seconds`).
1. **Lock Contention/Deadlock:**
- Task `syz.9.5484` is in the process of opening a TTY device. It has acquired the global `tty_mutex` (in `tty_open_by_driver` -> `tty_init_dev`) and is now blocked trying to acquire the `console_sem` semaphore inside `con_install()`.
- Because `syz.9.5484` is holding `tty_mutex`, numerous other tasks (e.g., `syz.5.5487`, `syz.4.5495`, etc.) that are also trying to open TTYs are blocked waiting for `tty_mutex`.
- The `console_sem` is held by another task for an extended period. This is a known architectural issue in the kernel's printk/console subsystem where a task calling `console_unlock()` can be forced to flush a massive backlog of kernel messages (a "printk flood") to slow consoles (like serial ports) without yielding, effectively holding `console_sem` hostage for a very long time.
2. **Impact:**
- **Denial of Service (DoS):** Yes. The system becomes unresponsive as tasks hang indefinitely waiting for core locks like `tty_mutex` and `console_sem`. The softlockup watchdog might also fire if the task holding the lock doesn't yield.
- **Exploitable:** No. This is a synchronization and resource starvation issue, not a memory safety violation (like use-after-free or out-of-bounds access). It cannot be used for memory corruption or privilege escalation.
3. **Accessibility:**
- **Unprivileged / User Namespace:** Yes. Unprivileged users can open TTY devices (`/dev/tty`, `/dev/ptmx`, etc.) and can often trigger kernel warnings or printk floods (e.g., by intentionally passing malformed arguments to certain syscalls) that exacerbate the `console_unlock` stall. No special capabilities are required.
4. **Other Triggers:**
- The bug is a local DoS triggered via standard syscalls (like `openat` on device nodes). It does not require malicious filesystems, network packets, or peripheral devices to trigger.
**Conclusion:**
This is a local Denial of Service (DoS) vulnerability caused by lock starvation involving `tty_mutex` and `console_sem`, typically exacerbated by a printk flood. It is accessible to unprivileged users but is not exploitable for privilege escalation.
|
| 6/2 |
2026/05/20 01:51 |
llm |
expert |
0m
Model:
Tokens:
input: 12882
output: 41
thoughts: 465
Thoughts: **Analyzing a Potential TTY Locking Issue**
Okay, so I've got a hung task report here. Looks like task `syz.9.5484:27591` has been blocked for over 143 seconds. That's definitely concerning. The call trace points to a deadlock or significant contention. I see `console_lock` right in the middle, specifically during `con_install`. I also see another task being blocked, seemingly in `tty_open_by_driver` while trying to acquire a `tty_mutex`.
Aha, the pieces are starting to fit together. I know that `con_install` is called from `tty_init_dev`, which in turn, is likely called from `tty_open_by_driver` or `tty_open`. This means `console_lock` is being held somewhere that `tty_open_by_driver` is trying to access and therefore the `tty_mutex`. It suggests there's some kind of locking conflict, potentially between the console driver and the TTY driver. Now, the key is to pinpoint the exact source of this contention. Time to dive into the code. Let's start by looking at `drivers/tty/vt/vt.c` for `con_install`. I need to figure out how `console_lock` is acquired and released in relation to how `tty_open_by_driver` operates.
|
| 7/2 |
2026/05/20 01:51 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/tty/vt/vt.c IncludeLines:true Name:con_install]
Results: map[SourceCode:3733: /*
3734: * Allocate the console screen memory.
3735: */
3736: static int con_install(struct tty_driver *driver, struct tty_struct *tty)
3737: {
3738: unsigned int currcons = tty->index;
3739: struct vc_data *vc;
3740: int ret;
3741:
3742: guard(console_lock)();
3743: ret = vc_allocate(currcons);
3744: if (ret)
3745: return ret;
3746:
3747: vc = vc_cons[currcons].d;
3748:
3749: /* Still being freed */
3750: if (vc->port.tty)
3751: return -ERESTARTSYS;
3752:
3753: ret = tty_port_install(&vc->port, driver, tty);
3754: if (ret)
3755: return ret;
3756:
3757: tty->driver_data = vc;
3758: vc->port.tty = tty;
3759: tty_port_get(&vc->port);
3760:
3761: if (!tty->winsize.ws_row && !tty->winsize.ws_col) {
3762: tty->winsize.ws_row = vc_cons[currcons].d->vc_rows;
3763: tty->winsize.ws_col = vc_cons[currcons].d->vc_cols;
3764: }
3765: if (vc->vc_utf)
3766: tty->termios.c_iflag |= IUTF8;
3767: else
3768: tty->termios.c_iflag &= ~IUTF8;
3769:
3770: return 0;
3771: }
SourceFile:drivers/tty/vt/vt.c]
|
| 8/2 |
2026/05/20 01:51 |
llm |
expert |
0m
Model:
Tokens:
input: 14264
output: 39
thoughts: 374
Thoughts: **Debugging a Console Lock Issue**
Okay, so `con_install` is trying to grab `console_lock`. My immediate question is, why is this lock held in the first place? Time to investigate. Let's see who currently possesses `console_lock`. I'll run `lockdep` and see what it tells me.
Alright, the `lockdep` output shows several other processes holding various locks. I see `rcu_read_lock` held by `khungtaskd`, some `ldisc_sem` and `atomic_read_lock` in `getty`, and various `tty_mutex` and `legacy_mutex` instances by `syz` processes. But wait a minute... `console_lock` is a semaphore, right? The output isn't explicitly showing `console_lock`.
That's right, `lockdep` can be tricky with semaphores. Does `lockdep` even track it if it's not explicitly initialized to be tracked? Hmmm, if the semaphore is *not* initialized for lockdep tracking, it won't show up. Okay, so `console_lock` *is* a semaphore.
So, `lockdep` will track semaphores if configured. But, it's possible that `console_lock` wasn't initialized with lockdep. The only way to know is to check the definition of `console_lock` to be sure.
|
| 9/2 |
2026/05/20 01:51 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/printk/printk.c IncludeLines:true Name:console_lock]
Results: map[SourceCode:2880: /**
2881: * console_lock - block the console subsystem from printing
2882: *
2883: * Acquires a lock which guarantees that no consoles will
2884: * be in or enter their write() callback.
2885: *
2886: * Can sleep, returns nothing.
2887: */
2888: void console_lock(void)
2889: {
2890: might_sleep();
2891:
2892: /* On panic, the console_lock must be left to the panic cpu. */
2893: while (panic_on_other_cpu())
2894: msleep(1000);
2895:
2896: down_console_sem();
2897: console_locked = 1;
2898: console_may_schedule = 1;
2899: }
SourceFile:kernel/printk/printk.c]
|
| 10/2 |
2026/05/20 01:51 |
llm |
expert |
0m
Model:
Tokens:
input: 15265
output: 41
thoughts: 11
Thoughts: Okay, here's my attempt at summarizing that thought process, tailored for an expert audience:
**Checking the Down Console Semaphore**
Alright, let's just make sure we're on the right track here. My first thought is to immediately verify the state of `down_console_sem`. It's a fundamental synchronization primitive in this context, and any unexpected behavior there could cascade into some pretty gnarly problems. I want to quickly confirm the semaphore's locked/unlocked state, and, ideally, peek at any associated queue. The implications of this semaphore being held in unexpected scenarios are huge; console output, device access, even system stability could depend on it behaving as expected. So, let's dive into the details surrounding it.
|
| 11/2 |
2026/05/20 01:51 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:kernel/printk/printk.c IncludeLines:true Name:down_console_sem]
Results: map[SourceCode: SourceFile:]
|
| 12/2 |
2026/05/20 01:51 |
llm |
expert |
0m
Model:
Tokens:
input: 15493
output: 19
thoughts: 12
Thoughts: Here's my take on those thoughts:
**Analyzing a Potential Concurrency Issue**
Okay, so I need to understand what's happening with this system, and the phrase "down_console_sem" just pinged my radar. My immediate instinct is to dive in with a `grep`. That semaphore name strongly suggests some kind of synchronization or locking mechanism related to the console or terminal. It's a common naming convention, and I've seen it used in similar contexts before. My thought process is, let's find where this thing is used. Knowing the context of its usage is key to understanding the concurrency model at play, and if there's any risk of a deadlock or livelock. Grepping for the semaphore is the quick first step to trace its uses and figure out if there might be a problem. This should at least give me a starting point.
|
| 13/2 |
2026/05/20 01:51 |
tool |
grepper |
0m
Args: map[Expression:down_console_sem]
Results: map[Output:kernel/printk/printk.c=307=EXPORT_SYMBOL(console_srcu_read_unlock);
--
kernel/printk/printk.c-312- */
kernel/printk/printk.c:313:#define down_console_sem() do { \
kernel/printk/printk.c-314- down(&console_sem);\
--
kernel/printk/printk.c=2888=void console_lock(void)
--
kernel/printk/printk.c-2895-
kernel/printk/printk.c:2896: down_console_sem();
kernel/printk/printk.c-2897- console_locked = 1;
]
|
| 14/2 |
2026/05/20 01:51 |
llm |
expert |
0m
Model:
Tokens:
input: 15885
output: 49
thoughts: 340
Thoughts: **Analyzing a Potential Soft Lockup**
Okay, so `console_sem` is a semaphore, right? Let's figure out who has it. The Non-Maskable Interrupt (NMI) backtrace is showing some interesting things. CPU 1 looks like it's the culprit.
Let's see, CPU 1 is in `lock_acquire`, which leads to `rcu_lock_acquire`, and then `rcu_read_lock`. Okay, nothing immediately alarming there. Following the trace, we see `class_rcu_constructor`, then `unwind_next_frame`, `arch_stack_walk`, `stack_trace_save`, and KASAN-related functions like `kasan_record_aux_stack`. That's where things get interesting. It goes through `__call_rcu_common` and `call_rcu`, then `__dentry_kill`. Alright, after that it looks like it is going through the system call stack with `filename_unlinkat`, and down to the actual system calls `do_syscall_x64`, `do_syscall_64`, `entry_SYSCALL_64_after_hwframe`.
Wait a minute, CPU 1 is effectively traversing the stack: `__dentry_kill` is calling `call_rcu`, which calls `kasan_record_aux_stack`, which is saving the stack trace, and eventually is trying to unwind the stack. And then, it goes back into `rcu_read_lock` and `lock_acquire`. That doesn't seem right. The trace indicates that CPU 1 has been marked as SOFTLOCKUP! So that means CPU 1 has been looping or has been stuck for an extended period without scheduling.
Why is it stuck in `unwind_next_frame`? Let me see what `arch/x86/kernel/unwind_orc.c` says about this.
|
| 15/2 |
2026/05/20 01:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:arch/x86/kernel/unwind_orc.c IncludeLines:true Name:unwind_next_frame]
Results: map[SourceCode: 484: bool unwind_next_frame(struct unwind_state *state)
485: {
486: unsigned long ip_p, sp, tmp, orig_ip = state->ip, prev_sp = state->sp;
487: enum stack_type prev_type = state->stack_info.type;
488: struct orc_entry *orc;
489: bool indirect = false;
490:
491: if (unwind_done(state))
492: return false;
493:
494: /* Don't let modules unload while we're reading their ORC data. */
495: guard(rcu)();
496:
497: /* End-of-stack check for user tasks: */
498: if (state->regs && user_mode(state->regs))
499: goto the_end;
500:
501: /*
502: * Find the orc_entry associated with the text address.
503: *
504: * For a call frame (as opposed to a signal frame), state->ip points to
505: * the instruction after the call. That instruction's stack layout
506: * could be different from the call instruction's layout, for example
507: * if the call was to a noreturn function. So get the ORC data for the
508: * call instruction itself.
509: */
510: orc = orc_find(state->signal ? state->ip : state->ip - 1);
511: if (!orc) {
512: /*
513: * As a fallback, try to assume this code uses a frame pointer.
514: * This is just a guess, so the rest of the unwind is no longer
515: * considered reliable.
516: */
517: orc = &orc_fp_entry;
518: state->error = true;
519: } else {
520: if (orc->type == ORC_TYPE_UNDEFINED)
521: goto err;
522:
523: if (orc->type == ORC_TYPE_END_OF_STACK)
524: goto the_end;
525: }
526:
527: state->signal = orc->signal;
528:
529: /* Find the previous frame's stack: */
530: switch (orc->sp_reg) {
531: case ORC_REG_SP:
532: sp = state->sp + orc->sp_offset;
533: break;
534:
535: case ORC_REG_BP:
536: sp = state->bp + orc->sp_offset;
537: break;
538:
539: case ORC_REG_SP_INDIRECT:
540: sp = state->sp;
541: indirect = true;
542: break;
543:
544: case ORC_REG_BP_INDIRECT:
545: sp = state->bp + orc->sp_offset;
546: indirect = true;
547: break;
548:
549: /*
550: * Any of the below registers may temporarily hold the stack pointer,
551: * typically during a DRAP stack realignment sequence or some other
552: * stack swizzle.
553: */
554:
555: case ORC_REG_AX:
556: if (!get_reg(state, offsetof(struct pt_regs, ax), &sp)) {
557: orc_warn_current("missing AX value at %pB\n",
558: (void *)state->ip);
559: goto err;
560: }
561: break;
562:
563: case ORC_REG_DX:
564: if (!get_reg(state, offsetof(struct pt_regs, dx), &sp)) {
565: orc_warn_current("missing DX value at %pB\n",
566: (void *)state->ip);
567: goto err;
568: }
569: break;
570:
571: case ORC_REG_DI:
572: if (!get_reg(state, offsetof(struct pt_regs, di), &sp)) {
573: orc_warn_current("missing RDI value at %pB\n",
574: (void *)state->ip);
575: goto err;
576: }
577: break;
578:
579: case ORC_REG_R10:
580: if (!get_reg(state, offsetof(struct pt_regs, r10), &sp)) {
581: orc_warn_current("missing R10 value at %pB\n",
582: (void *)state->ip);
583: goto err;
584: }
585: break;
586:
587: case ORC_REG_R13:
588: if (!get_reg(state, offsetof(struct pt_regs, r13), &sp)) {
589: orc_warn_current("missing R13 value at %pB\n",
590: (void *)state->ip);
591: goto err;
592: }
593: break;
594:
595: default:
596: orc_warn("unknown SP base reg %d at %pB\n",
597: orc->sp_reg, (void *)state->ip);
598: goto err;
599: }
600:
601: if (indirect) {
602: if (!deref_stack_reg(state, sp, &sp))
603: goto err;
604:
605: if (orc->sp_reg == ORC_REG_SP_INDIRECT)
606: sp += orc->sp_offset;
607: }
608:
609: /* Find IP, SP and possibly regs: */
610: switch (orc->type) {
611: case ORC_TYPE_CALL:
612: ip_p = sp - sizeof(long);
613:
614: if (!deref_stack_reg(state, ip_p, &state->ip))
615: goto err;
616:
617: state->ip = unwind_recover_ret_addr(state, state->ip,
618: (unsigned long *)ip_p);
619: state->sp = sp;
620: state->regs = NULL;
621: state->prev_regs = NULL;
622: break;
623:
624: case ORC_TYPE_REGS:
625: if (!deref_stack_regs(state, sp, &state->ip, &state->sp)) {
626: orc_warn_current("can't access registers at %pB\n",
627: (void *)orig_ip);
628: goto err;
629: }
630: /*
631: * There is a small chance to interrupt at the entry of
632: * arch_rethook_trampoline() where the ORC info doesn't exist.
633: * That point is right after the RET to arch_rethook_trampoline()
634: * which was modified return address.
635: * At that point, the @addr_p of the unwind_recover_rethook()
636: * (this has to point the address of the stack entry storing
637: * the modified return address) must be "SP - (a stack entry)"
638: * because SP is incremented by the RET.
639: */
640: state->ip = unwind_recover_rethook(state, state->ip,
641: (unsigned long *)(state->sp - sizeof(long)));
642: state->regs = (struct pt_regs *)sp;
643: state->prev_regs = NULL;
644: state->full_regs = true;
645: break;
646:
647: case ORC_TYPE_REGS_PARTIAL:
648: if (!deref_stack_iret_regs(state, sp, &state->ip, &state->sp)) {
649: orc_warn_current("can't access iret registers at %pB\n",
650: (void *)orig_ip);
651: goto err;
652: }
653: /* See ORC_TYPE_REGS case comment. */
654: state->ip = unwind_recover_rethook(state, state->ip,
655: (unsigned long *)(state->sp - sizeof(long)));
656:
657: if (state->full_regs)
658: state->prev_regs = state->regs;
659: state->regs = (void *)sp - IRET_FRAME_OFFSET;
660: state->full_regs = false;
661: break;
662:
663: default:
664: orc_warn("unknown .orc_unwind entry type %d at %pB\n",
665: orc->type, (void *)orig_ip);
666: goto err;
667: }
668:
669: /* Find BP: */
670: switch (orc->bp_reg) {
671: case ORC_REG_UNDEFINED:
672: if (get_reg(state, offsetof(struct pt_regs, bp), &tmp))
673: state->bp = tmp;
674: break;
675:
676: case ORC_REG_PREV_SP:
677: if (!deref_stack_reg(state, sp + orc->bp_offset, &state->bp))
678: goto err;
679: break;
680:
681: case ORC_REG_BP:
682: if (!deref_stack_reg(state, state->bp + orc->bp_offset, &state->bp))
683: goto err;
684: break;
685:
686: default:
687: orc_warn("unknown BP base reg %d for ip %pB\n",
688: orc->bp_reg, (void *)orig_ip);
689: goto err;
690: }
691:
692: /* Prevent a recursive loop due to bad ORC data: */
693: if (state->stack_info.type == prev_type &&
694: on_stack(&state->stack_info, (void *)state->sp, sizeof(long)) &&
695: state->sp <= prev_sp) {
696: orc_warn_current("stack going in the wrong direction? at %pB\n",
697: (void *)orig_ip);
698: goto err;
699: }
700:
701: return true;
702:
703: err:
704: state->error = true;
705:
706: the_end:
707: state->stack_info.type = STACK_TYPE_UNKNOWN;
708: return false;
709: }
SourceFile:arch/x86/kernel/unwind_orc.c]
|
| 16/2 |
2026/05/20 01:52 |
llm |
expert |
0m
Model:
Tokens:
input: 20045
output: 45
thoughts: 1938
Thoughts: **Analyzing a Persistent Softlockup in the Kernel**
Okay, so here's what I've got. The system is showing signs of a prolonged softlockup, and I need to figure out why tasks are being blocked. Let's trace it down.
First off, it looks like `unwind_next_frame` is calling `guard(rcu)();` which acquires `rcu_read_lock`. This is interesting because the backtrace is getting stuck in `lock_acquire`, which should be called *inside* `rcu_read_lock` if lockdep is enabled. But why is `lock_acquire` seemingly looping or taking forever? The NMI backtrace caught CPU 1 in `lock_acquire`, and that CPU is tainted with SOFTLOCKUP, so it's been running without scheduling for far too long.
Looking at the call stack that leads to the lock contention, it appears to be related to `__dentry_kill` which is related to `call_rcu` and eventually `unwind_next_frame`. The question is: is `call_rcu` being called excessively or is `kasan_record_aux_stack` taking a long time? The stack trace says `__dentry_kill` is called from `filename_unlinkat` after a sequence of functions, so it is probably unlinking a single file and not in a loop. I suspect that `__dentry_kill` is freeing a dentry, which triggers `call_rcu`, which records the stack trace, and that recording takes a long time, but it's not clear.
A softlockup usually indicates more than 20 seconds, and that points to a lockdep issue. The NMI backtrace shows CPU 1 in `hlock_class` -> `check_wait_context` -> `__lock_acquire`. Now, `__lock_acquire` can be very slow if the lockdep graph is huge, or if there's a lockdep graph traversal loop. CPU 0 is sitting in `khungtaskd`, because tasks are blocked for longer than 143 seconds. They're blocked on `console_lock` and `tty_mutex`. Hmm... who's holding those locks?
The lockdep output tells a story: `khungtaskd` is holding an `rcu_read_lock`, `getty` holds `ldisc_sem` and `atomic_read_lock`. Then, `syz.9.5484` holds `tty_mutex` *and* `tty->legacy_mutex` while simultaneously being blocked on `console_lock`! That's a strong clue. This task, `syz.9.5484`, acquired those locks during `tty_open_by_driver` or `tty_open` for `tty_mutex` and `tty_init_dev` for `tty->legacy_mutex`, and then it's trying for `console_lock`. Because it's holding `tty_mutex`, all other tasks trying to open a tty are now blocked on that same `tty_mutex`. That means the root cause is `console_lock` being held by someone else, and `syz.9.5484` is waiting for its release.
But who's got `console_lock`? It's a semaphore. The lockdep output doesn't necessarily show every task, especially if they aren't holding other locks or aren't tracked by lockdep. Let's see what the code says. The lock is acquired by `console_lock()`. Is there a task that acquired `console_lock` and never released it? Maybe it's being held on CPU 1.
I checked CPU 1's stack, but it's in `__se_sys_unlink`, and that doesn't hold `console_lock`. Could CPU 1 *be* holding it somehow? If it had called `console_lock()`, it would be visible in the call stack. What if the task holding `console_lock` was preempted, or it's on a *different* CPU? But wait, CPU 1 is still tainted with SOFTLOCKUP! Is the task trying to run on CPU 1, but CPU 1 is stuck? Is it in a softlockup?
I found that CPU 1 is running `udevd` (PID 4992). But `udevd` is in `unlink`, which doesn't acquire `console_lock`! So *is* `console_lock` held by `syz.9.5484` itself? No, that task is blocked on the semaphore. Could it be another task, blocked on the same tty mutex? `sy
|
| 17/2 |
2026/05/20 01:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/tty/tty_io.c IncludeLines:true Name:tty_open_by_driver]
Results: map[SourceCode:2013: /**
2014: * tty_open_by_driver - open a tty device
2015: * @device: dev_t of device to open
2016: * @filp: file pointer to tty
2017: *
2018: * Performs the driver lookup, checks for a reopen, or otherwise performs the
2019: * first-time tty initialization.
2020: *
2021: *
2022: * Claims the global tty_mutex to serialize:
2023: * * concurrent first-time tty initialization
2024: * * concurrent tty driver removal w/ lookup
2025: * * concurrent tty removal from driver table
2026: *
2027: * Return: the locked initialized or re-opened &tty_struct
2028: */
2029: static struct tty_struct *tty_open_by_driver(dev_t device,
2030: struct file *filp)
2031: {
2032: struct tty_struct *tty;
2033: struct tty_driver *driver = NULL;
2034: int index = -1;
2035: int retval;
2036:
2037: mutex_lock(&tty_mutex);
2038: driver = tty_lookup_driver(device, filp, &index);
2039: if (IS_ERR(driver)) {
2040: mutex_unlock(&tty_mutex);
2041: return ERR_CAST(driver);
2042: }
2043:
2044: /* check whether we're reopening an existing tty */
2045: tty = tty_driver_lookup_tty(driver, filp, index);
2046: if (IS_ERR(tty)) {
2047: mutex_unlock(&tty_mutex);
2048: goto out;
2049: }
2050:
2051: if (tty) {
2052: if (tty_port_kopened(tty->port)) {
2053: tty_kref_put(tty);
2054: mutex_unlock(&tty_mutex);
2055: tty = ERR_PTR(-EBUSY);
2056: goto out;
2057: }
2058: mutex_unlock(&tty_mutex);
2059: retval = tty_lock_interruptible(tty);
2060: tty_kref_put(tty); /* drop kref from tty_driver_lookup_tty() */
2061: if (retval) {
2062: if (retval == -EINTR)
2063: retval = -ERESTARTSYS;
2064: tty = ERR_PTR(retval);
2065: goto out;
2066: }
2067: retval = tty_reopen(tty);
2068: if (retval < 0) {
2069: tty_unlock(tty);
2070: tty = ERR_PTR(retval);
2071: }
2072: } else { /* Returns with the tty_lock held for now */
2073: tty = tty_init_dev(driver, index);
2074: mutex_unlock(&tty_mutex);
2075: }
2076: out:
2077: tty_driver_kref_put(driver);
2078: return tty;
2079: }
SourceFile:drivers/tty/tty_io.c]
|
| 18/2 |
2026/05/20 01:52 |
llm |
expert |
0m
Model:
Tokens:
input: 24047
output: 43
thoughts: 78
Thoughts: Okay, here's my breakdown of what's happening, from my expert perspective:
**Investigating the TTY Device Initialization Sequence**
Right, so I'm looking at how a TTY device gets opened, and I see `tty_open_by_driver` is grabbing that `tty_mutex`. Standard. Now, the interesting bit: if the `tty` structure isn't already there β and that's a key condition to watch β the code dives into `tty_init_dev(driver, index)`. Hang on a second...it seems like `tty_init_dev` gets *called* with the `tty_mutex` *already held* and then the mutex is unlocked! That feels a little... unusual. It's not a common pattern I've seen. We really need to check the internals of `tty_init_dev` now, since it might be doing something that assumes exclusive access, and we need to understand how that mutex interaction works for concurrent access. Let's see how that function is implemented and if it contains any race conditions.
|
| 19/2 |
2026/05/20 01:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/tty/tty_io.c IncludeLines:true Name:tty_init_dev]
Results: map[SourceCode:1361: /**
1362: * tty_init_dev - initialise a tty device
1363: * @driver: tty driver we are opening a device on
1364: * @idx: device index
1365: *
1366: * Prepare a tty device. This may not be a "new" clean device but could also be
1367: * an active device. The pty drivers require special handling because of this.
1368: *
1369: * Locking:
1370: * The function is called under the tty_mutex, which protects us from the
1371: * tty struct or driver itself going away.
1372: *
1373: * On exit the tty device has the line discipline attached and a reference
1374: * count of 1. If a pair was created for pty/tty use and the other was a pty
1375: * master then it too has a reference count of 1.
1376: *
1377: * WSH 06/09/97: Rewritten to remove races and properly clean up after a failed
1378: * open. The new code protects the open with a mutex, so it's really quite
1379: * straightforward. The mutex locking can probably be relaxed for the (most
1380: * common) case of reopening a tty.
1381: *
1382: * Return: new tty structure
1383: */
1384: struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx)
1385: {
1386: struct tty_struct *tty;
1387: int retval;
1388:
1389: /*
1390: * First time open is complex, especially for PTY devices.
1391: * This code guarantees that either everything succeeds and the
1392: * TTY is ready for operation, or else the table slots are vacated
1393: * and the allocated memory released. (Except that the termios
1394: * may be retained.)
1395: */
1396:
1397: if (!try_module_get(driver->owner))
1398: return ERR_PTR(-ENODEV);
1399:
1400: tty = alloc_tty_struct(driver, idx);
1401: if (!tty) {
1402: retval = -ENOMEM;
1403: goto err_module_put;
1404: }
1405:
1406: tty_lock(tty);
1407: retval = tty_driver_install_tty(driver, tty);
1408: if (retval < 0)
1409: goto err_free_tty;
1410:
1411: if (!tty->port)
1412: tty->port = driver->ports[idx];
1413:
1414: if (WARN_RATELIMIT(!tty->port,
1415: "%s: %s driver does not set tty->port. This would crash the kernel. Fix the driver!\n",
1416: __func__, tty->driver->name)) {
1417: retval = -EINVAL;
1418: goto err_release_lock;
1419: }
1420:
1421: retval = tty_ldisc_lock(tty, 5 * HZ);
1422: if (retval)
1423: goto err_release_lock;
1424: tty->port->itty = tty;
1425:
1426: /*
1427: * Structures all installed ... call the ldisc open routines.
1428: * If we fail here just call release_tty to clean up. No need
1429: * to decrement the use counts, as release_tty doesn't care.
1430: */
1431: retval = tty_ldisc_setup(tty, tty->link);
1432: if (retval)
1433: goto err_release_tty;
1434: tty_ldisc_unlock(tty);
1435: /* Return the tty locked so that it cannot vanish under the caller */
1436: return tty;
1437:
1438: err_free_tty:
1439: tty_unlock(tty);
1440: free_tty_struct(tty);
1441: err_module_put:
1442: module_put(driver->owner);
1443: return ERR_PTR(retval);
1444:
1445: /* call the tty release_tty routine to clean out this slot */
1446: err_release_tty:
1447: tty_ldisc_unlock(tty);
1448: tty_info_ratelimited(tty, "ldisc open failed (%d), clearing slot %d\n",
1449: retval, idx);
1450: err_release_lock:
1451: tty_unlock(tty);
1452: release_tty(tty, idx);
1453: return ERR_PTR(retval);
1454: }
SourceFile:drivers/tty/tty_io.c]
|
| 20/2 |
2026/05/20 01:52 |
llm |
expert |
0m
Model:
Tokens:
input: 25853
output: 49
thoughts: 1671
Thoughts: **Analyzing a Potential Deadlock in the TTY Subsystem**
Okay, let's break this down. We're looking at a scenario where a softlockup is occurring, and it *seems* related to console operations and potential lock ordering issues. The initial trace pointed us to `tty_init_dev`, which calls `tty_driver_install_tty`. This, in turn, calls the `install` operation of the TTY driver, and for the virtual terminal (`vt`), this resolves to `con_install`. Crucially, we know `con_install` is called while holding `tty_mutex`, and then it calls `console_lock()`. Thus, the lock order is `tty_mutex` -> `console_lock`.
The question immediately arose: could this lock order lead to a deadlock? Specifically, if someone acquired `console_lock` *before* `tty_mutex` and then tried to get the latter, we'd have a classic deadlock. I initially tried to verify this with lockdep, however, the results don't show any direct paths that would result in such a reversed locking order. The lockdep output instead shows `syz.9.5484` holding `tty_mutex` and waiting for `console_lock`.
Then we see CPU 1 is in a softlockup in `unlink`. The thing about a softlockup is that *nothing* is scheduled. So, I need to think about what CPU 1 could be doing to block everything. Could CPU 1 be holding `console_lock`? If `console_lock` is held, then `syz.9.5484` could never acquire the lock. However, `console_lock` is a semaphore, and those are released across syscalls, meaning CPU 1 would never be able to hold the lock from the context of an `unlink` call. I started to explore edge cases like if CPU 1 was in `printk`, which *could* call `console_trylock`, but not `console_lock`. Still, if CPU 1 did call `printk`, it would only *try* to acquire the lock and not necessarily block. So that is not it.
So, where is CPU 1 *really* blocked? The stack trace shows it's deep inside the stack unwinding process: `__dentry_kill` -> `call_rcu` -> `kasan_record_aux_stack` -> `stack_trace_save` -> `arch_stack_walk` -> `unwind_next_frame`. This is where things got interesting. It looks like CPU 1 is looping in `unwind_next_frame` in an infinite loop!
Why would `unwind_next_frame` loop forever? Let's refresh our memory about the specific code. There's a check that should prevent infinite loops: If `state->sp == prev_sp`, the loop breaks. However, if `state->sp > prev_sp`, that's the normal direction for stack unwinding (stack growing down), and this check doesn't catch that. So if ORC data is corrupted, and it points to the same frame or a previous frame but `sp` increases slightly, it could loop forever. If the ORC data is bad, that's definitely a problem.
Now, a softlockup in `unwind_next_frame` means CPU 1 is *stuck*. And there's a vital detail here: `unwind_next_frame` calls `guard(rcu)()`, which effectively acquires `rcu_read_lock`. If it loops forever, *it holds `rcu_read_lock` forever*. That's a huge problem because holding `rcu_read_lock` forever will block `synchronize_rcu()`. So a process calling `synchronize_rcu()` will block forever! So, could a call to `synchronize_rcu()` be the problem? The lockdep output already tells us `syz.9.5484` is blocked in `down(&console_sem)`, which means that lock is held by some other process.
To confirm this, I need to look for any tasks that are holding `console_sem` and simultaneously blocked in `synchronize_rcu()`. The lockdep output also shows `khungtaskd` holds `rcu_read_lock`. I need to review all of the current tasks to verify that no task holding `console_sem` is blocked in `synchronize_rcu()`. However, the bug report snippet doesn't show all tasks in the system! It only shows those that are blocked. However, since we are dealing with a lock that takes longer than 143 seconds, then `khungtaskd` should have reported all of the stuck
|
| 21/2 |
2026/05/20 01:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:arch/x86/kernel/unwind_orc.c IncludeLines:true Name:unwind_next_frame]
Results: map[SourceCode: 484: bool unwind_next_frame(struct unwind_state *state)
485: {
486: unsigned long ip_p, sp, tmp, orig_ip = state->ip, prev_sp = state->sp;
487: enum stack_type prev_type = state->stack_info.type;
488: struct orc_entry *orc;
489: bool indirect = false;
490:
491: if (unwind_done(state))
492: return false;
493:
494: /* Don't let modules unload while we're reading their ORC data. */
495: guard(rcu)();
496:
497: /* End-of-stack check for user tasks: */
498: if (state->regs && user_mode(state->regs))
499: goto the_end;
500:
501: /*
502: * Find the orc_entry associated with the text address.
503: *
504: * For a call frame (as opposed to a signal frame), state->ip points to
505: * the instruction after the call. That instruction's stack layout
506: * could be different from the call instruction's layout, for example
507: * if the call was to a noreturn function. So get the ORC data for the
508: * call instruction itself.
509: */
510: orc = orc_find(state->signal ? state->ip : state->ip - 1);
511: if (!orc) {
512: /*
513: * As a fallback, try to assume this code uses a frame pointer.
514: * This is just a guess, so the rest of the unwind is no longer
515: * considered reliable.
516: */
517: orc = &orc_fp_entry;
518: state->error = true;
519: } else {
520: if (orc->type == ORC_TYPE_UNDEFINED)
521: goto err;
522:
523: if (orc->type == ORC_TYPE_END_OF_STACK)
524: goto the_end;
525: }
526:
527: state->signal = orc->signal;
528:
529: /* Find the previous frame's stack: */
530: switch (orc->sp_reg) {
531: case ORC_REG_SP:
532: sp = state->sp + orc->sp_offset;
533: break;
534:
535: case ORC_REG_BP:
536: sp = state->bp + orc->sp_offset;
537: break;
538:
539: case ORC_REG_SP_INDIRECT:
540: sp = state->sp;
541: indirect = true;
542: break;
543:
544: case ORC_REG_BP_INDIRECT:
545: sp = state->bp + orc->sp_offset;
546: indirect = true;
547: break;
548:
549: /*
550: * Any of the below registers may temporarily hold the stack pointer,
551: * typically during a DRAP stack realignment sequence or some other
552: * stack swizzle.
553: */
554:
555: case ORC_REG_AX:
556: if (!get_reg(state, offsetof(struct pt_regs, ax), &sp)) {
557: orc_warn_current("missing AX value at %pB\n",
558: (void *)state->ip);
559: goto err;
560: }
561: break;
562:
563: case ORC_REG_DX:
564: if (!get_reg(state, offsetof(struct pt_regs, dx), &sp)) {
565: orc_warn_current("missing DX value at %pB\n",
566: (void *)state->ip);
567: goto err;
568: }
569: break;
570:
571: case ORC_REG_DI:
572: if (!get_reg(state, offsetof(struct pt_regs, di), &sp)) {
573: orc_warn_current("missing RDI value at %pB\n",
574: (void *)state->ip);
575: goto err;
576: }
577: break;
578:
579: case ORC_REG_R10:
580: if (!get_reg(state, offsetof(struct pt_regs, r10), &sp)) {
581: orc_warn_current("missing R10 value at %pB\n",
582: (void *)state->ip);
583: goto err;
584: }
585: break;
586:
587: case ORC_REG_R13:
588: if (!get_reg(state, offsetof(struct pt_regs, r13), &sp)) {
589: orc_warn_current("missing R13 value at %pB\n",
590: (void *)state->ip);
591: goto err;
592: }
593: break;
594:
595: default:
596: orc_warn("unknown SP base reg %d at %pB\n",
597: orc->sp_reg, (void *)state->ip);
598: goto err;
599: }
600:
601: if (indirect) {
602: if (!deref_stack_reg(state, sp, &sp))
603: goto err;
604:
605: if (orc->sp_reg == ORC_REG_SP_INDIRECT)
606: sp += orc->sp_offset;
607: }
608:
609: /* Find IP, SP and possibly regs: */
610: switch (orc->type) {
611: case ORC_TYPE_CALL:
612: ip_p = sp - sizeof(long);
613:
614: if (!deref_stack_reg(state, ip_p, &state->ip))
615: goto err;
616:
617: state->ip = unwind_recover_ret_addr(state, state->ip,
618: (unsigned long *)ip_p);
619: state->sp = sp;
620: state->regs = NULL;
621: state->prev_regs = NULL;
622: break;
623:
624: case ORC_TYPE_REGS:
625: if (!deref_stack_regs(state, sp, &state->ip, &state->sp)) {
626: orc_warn_current("can't access registers at %pB\n",
627: (void *)orig_ip);
628: goto err;
629: }
630: /*
631: * There is a small chance to interrupt at the entry of
632: * arch_rethook_trampoline() where the ORC info doesn't exist.
633: * That point is right after the RET to arch_rethook_trampoline()
634: * which was modified return address.
635: * At that point, the @addr_p of the unwind_recover_rethook()
636: * (this has to point the address of the stack entry storing
637: * the modified return address) must be "SP - (a stack entry)"
638: * because SP is incremented by the RET.
639: */
640: state->ip = unwind_recover_rethook(state, state->ip,
641: (unsigned long *)(state->sp - sizeof(long)));
642: state->regs = (struct pt_regs *)sp;
643: state->prev_regs = NULL;
644: state->full_regs = true;
645: break;
646:
647: case ORC_TYPE_REGS_PARTIAL:
648: if (!deref_stack_iret_regs(state, sp, &state->ip, &state->sp)) {
649: orc_warn_current("can't access iret registers at %pB\n",
650: (void *)orig_ip);
651: goto err;
652: }
653: /* See ORC_TYPE_REGS case comment. */
654: state->ip = unwind_recover_rethook(state, state->ip,
655: (unsigned long *)(state->sp - sizeof(long)));
656:
657: if (state->full_regs)
658: state->prev_regs = state->regs;
659: state->regs = (void *)sp - IRET_FRAME_OFFSET;
660: state->full_regs = false;
661: break;
662:
663: default:
664: orc_warn("unknown .orc_unwind entry type %d at %pB\n",
665: orc->type, (void *)orig_ip);
666: goto err;
667: }
668:
669: /* Find BP: */
670: switch (orc->bp_reg) {
671: case ORC_REG_UNDEFINED:
672: if (get_reg(state, offsetof(struct pt_regs, bp), &tmp))
673: state->bp = tmp;
674: break;
675:
676: case ORC_REG_PREV_SP:
677: if (!deref_stack_reg(state, sp + orc->bp_offset, &state->bp))
678: goto err;
679: break;
680:
681: case ORC_REG_BP:
682: if (!deref_stack_reg(state, state->bp + orc->bp_offset, &state->bp))
683: goto err;
684: break;
685:
686: default:
687: orc_warn("unknown BP base reg %d for ip %pB\n",
688: orc->bp_reg, (void *)orig_ip);
689: goto err;
690: }
691:
692: /* Prevent a recursive loop due to bad ORC data: */
693: if (state->stack_info.type == prev_type &&
694: on_stack(&state->stack_info, (void *)state->sp, sizeof(long)) &&
695: state->sp <= prev_sp) {
696: orc_warn_current("stack going in the wrong direction? at %pB\n",
697: (void *)orig_ip);
698: goto err;
699: }
700:
701: return true;
702:
703: err:
704: state->error = true;
705:
706: the_end:
707: state->stack_info.type = STACK_TYPE_UNKNOWN;
708: return false;
709: }
SourceFile:arch/x86/kernel/unwind_orc.c]
|
| 22/2 |
2026/05/20 01:52 |
llm |
expert |
0m
Model:
Tokens:
input: 31952
output: 37
thoughts: 2015
Thoughts: **Analysis of Potential RCU Stalling in a Deep Directory Unlink Scenario**
Okay, let's break this down. My initial thought was about how `unwind_next_frame` uses `rcu_read_lock()` within a loop. The implication is, it acquires and releases the lock *per frame* in the stack walk. This is crucial because it *doesn't* hold the lock across the entire process, which means `synchronize_rcu()` *could* still make progress between frames. The NMI backtrace confirms this: we see `rcu_read_lock` being repeatedly acquired and released within `unwind_next_frame`.
However, the fact that CPU 1 is soft-locked is critical. A softlockup usually means the CPU is stuck in a tight loop and hasn't yielded the processor for an extended period. In `CONFIG_PREEMPT_RCU`, a quiescent state is when the CPU schedules, or goes idle. If CPU 1 is looping in `arch_stack_walk`, it's *never* scheduling, thus *never* reaching a quiescent state. This means CPU 1 is blocking RCU grace periods! If CPU 1 is blocking RCU grace periods, any task calling `synchronize_rcu()` will block forever.
Let's consider who calls `synchronize_rcu()`. I checked if things associated with `console_lock` calls synchronize_rcu(), but it doesn't appear so. But, the lockdep output shows `khungtaskd` holding `rcu_read_lock`. I realized `khungtaskd` uses `rcu_read_lock` to iterate over tasks; thus, it is not blocked in `synchronize_rcu()`. Then there is an INFO message in the bug report which states that `syz.9.5484` is blocked for more than 143 seconds. That sounds like a problem.
Now, if CPU 1 is looping in `arch_stack_walk`, is there a possible infinite loop? `unwind_next_frame` returns `true` to continue. Could there be a stack corruption scenario where `state.sp` increases and it doesn't trigger the `state.sp <= prev_sp` check, or that it doesn't trigger the end of stack condition? But that can't continue forever because eventually it will hit the end of the stack, or it will trigger a page fault, or `orc_type == ORC_TYPE_END_OF_STACK`. Also, `deref_stack_reg` checks if it's on the stack and returns false, and the `unwind_next_frame` returns `false` if it is not. Thus, it can't loop forever unless it jumps around within the stack. But if it jumps around within the stack, `state.sp` must decrease at some point, which will trigger the `state.sp <= prev_sp` check!
So, the issue might be if `state->stack_info.type != prev_type`. Thus the check can be bypassed! Can it alternate between two stack types? Like `STACK_TYPE_TASK` and `STACK_TYPE_IRQ`? I remember that `deref_stack_reg` only reads from the current stack.
Could it be that `lock_acquire` itself is slow? `lock_acquire` *can* take a long time, especially with a massive lockdep graph. But 143 seconds is incredibly long. The NMI backtrace for CPU 1 shows it's stuck in `__lock_acquire`. It uses a graph search algorithm that takes time, but 143 seconds is excessive unless the graph is corrupted. But CPU 1 is actually in `__se_sys_unlink`. Is CPU 1 slow?
Wait! I remember `kasan_record_aux_stack` is called by `call_rcu`, which is called by `__dentry_kill`. If `__dentry_kill` is called a *huge* number of times? `finish_dput` only kills one dentry so it doesn't loop. However, `dentry_kill` can kill the *parent* dentry if its refcount is zero. If a deep directory tree is unlinked, `dput` can *recursively* kill dentries all the way to the root! If the tree is extremely deep (say, 100,000 levels), `dput` will loop 100,000 times, calling `__dentry_kill`, `call_rcu`, and `kasan_record_aux_stack` at each level! With lockdep and KASAN enabled, it could take a very long time, and the
|
| 23/2 |
2026/05/20 01:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:dput]
Results: map[SourceCode: 957: /*
958: * dput - release a dentry
959: * @dentry: dentry to release
960: *
961: * Release a dentry. This will drop the usage count and if appropriate
962: * call the dentry unlink method as well as removing it from the queues and
963: * releasing its resources. If the parent dentries were scheduled for release
964: * they too may now get deleted.
965: */
966: void dput(struct dentry *dentry)
967: {
968: if (!dentry)
969: return;
970: might_sleep();
971: rcu_read_lock();
972: if (likely(fast_dput(dentry))) {
973: rcu_read_unlock();
974: return;
975: }
976: finish_dput(dentry);
977: }
SourceFile:fs/dcache.c]
|
| 24/2 |
2026/05/20 01:53 |
llm |
expert |
0m
Model:
Tokens:
input: 35341
output: 39
thoughts: 10
Thoughts: Okay, here's the summary, assuming I'm the expert thinking through the process:
**Reviewing the `finish_dput` Function**
Alright, let's zero in on `finish_dput`. I need to ensure this is functioning as expected within the pipeline. My expertise tells me this is likely the final step in a data uploading or transformation process. The name itself suggests it's the point where data is 'put' somewhere, perhaps a data warehouse, a cloud bucket, or a production database. I need to quickly assess if it:
* **Handles Errors Robustly:** Does `finish_dput` have proper exception handling? Are failures logged? Are retries implemented if necessary? A single point of failure here could bring down the whole process.
* **Ensures Data Integrity:** I need to confirm data validation and consistency checks are in place before the final 'put'. Are data types correctly handled? Is there a checksum verification to prevent corruption during transfer? Data quality is paramount.
* **Is Efficient and Scalable:** Is `finish_dput` optimized for performance? What happens if the dataset size increases? Are there any bottlenecks or potential performance limitations that might cause delays?
* **Is Atomic:** Does it ensure an all-or-nothing completion? If it's a multi-step operation, does it have a rollback mechanism in case of errors, to maintain data consistency?
In short, I'm checking if this function truly "finishes" the job correctly, efficiently, and reliably. I'm going to start by reviewing the code directly, checking the surrounding context for any potential issues.
|
| 25/2 |
2026/05/20 01:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:finish_dput]
Results: map[SourceCode: 921: static void finish_dput(struct dentry *dentry)
922: __releases(dentry->d_lock)
923: __releases(RCU)
924: {
925: while (lock_for_kill(dentry)) {
926: rcu_read_unlock();
927: dentry = __dentry_kill(dentry);
928: if (!dentry)
929: return;
930: if (retain_dentry(dentry, true)) {
931: spin_unlock(&dentry->d_lock);
932: return;
933: }
934: rcu_read_lock();
935: }
936: rcu_read_unlock();
937: spin_unlock(&dentry->d_lock);
938: }
SourceFile:fs/dcache.c]
|
| 26/2 |
2026/05/20 01:53 |
llm |
expert |
0m
Model:
Tokens:
input: 36012
output: 40
thoughts: 498
Thoughts: **Dentry Kill Loop Analysis**
Okay, so I'm looking at `finish_dput` and I see this `while` loop, right? It's calling `lock_for_kill` and then `__dentry_kill`. The interesting thing is that if `__dentry_kill` returns the parent dentry, this loop just keeps going and kills the parent, then *its* parent, and so on.
Aha! If a directory tree is ridiculously deep, like someone creating `a/b/c/...` with a hundred thousand levels, and then unlinking the bottom directory, this loop will execute a massive number of times. The problem is I don't see any `cond_resched()` in this loop! That's a glaring omission.
This means `finish_dput` will sit there and iteratively kill all those dentries without yielding the CPU. We're talking about a potential softlockup scenario here. And since it's not yielding, doesn't that imply it's blocking RCU grace periods too?
Hold on... I'm missing something. Okay, `rcu_read_unlock()` is called within the loop, and then immediately followed by `rcu_read_lock()`. But the task still isn't scheduling. Hmm... In a `CONFIG_PREEMPT_RCU` setting, does a task *need* to schedule to actually pass a quiescent state? Yes, or it needs to be preempted.
But wait a minute, if the kernel is preemptible -- and I'm looking at the NMI backtrace, it *is* preemptible (`PREEMPT(full)` is visible) -- then the task *can* be preempted! Okay, if it *can* be preempted, then RCU grace periods *can* finish if this task gets preempted.
But if it *isn't* preempted, say, because it's the highest priority task or running in a context where preemption is disabled, then we *might* be blocking RCU. This is a subtle but potentially serious issue.
One more thing... I just noticed that `finish_dput` calls `spin_unlock(&dentry->d_lock)` inside `__dentry_kill`. I need to double-check what's happening within `__dentry_kill`. This might be where the real vulnerability lies... Let's go see what's in that function!
|
| 27/2 |
2026/05/20 01:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:__dentry_kill]
Results: map[SourceCode: 694: static struct dentry *__dentry_kill(struct dentry *dentry)
695: {
696: struct dentry *parent = NULL;
697: bool can_free = true;
698:
699: /*
700: * The dentry is now unrecoverably dead to the world.
701: */
702: lockref_mark_dead(&dentry->d_lockref);
703:
704: /*
705: * inform the fs via d_prune that this dentry is about to be
706: * unhashed and destroyed.
707: */
708: if (dentry->d_flags & DCACHE_OP_PRUNE)
709: dentry->d_op->d_prune(dentry);
710:
711: if (dentry->d_flags & DCACHE_LRU_LIST) {
712: if (!(dentry->d_flags & DCACHE_SHRINK_LIST))
713: d_lru_del(dentry);
714: }
715: /* if it was on the hash then remove it */
716: __d_drop(dentry);
717: if (dentry->d_inode)
718: dentry_unlink_inode(dentry);
719: else
720: spin_unlock(&dentry->d_lock);
721: this_cpu_dec(nr_dentry);
722: if (dentry->d_op && dentry->d_op->d_release)
723: dentry->d_op->d_release(dentry);
724:
725: cond_resched();
726: /* now that it's negative, ->d_parent is stable */
727: if (!IS_ROOT(dentry)) {
728: parent = dentry->d_parent;
729: spin_lock(&parent->d_lock);
730: }
731: spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
732: dentry_unlist(dentry);
733: if (dentry->d_flags & DCACHE_SHRINK_LIST)
734: can_free = false;
735: spin_unlock(&dentry->d_lock);
736: if (likely(can_free))
737: dentry_free(dentry);
738: if (parent && --parent->d_lockref.count) {
739: spin_unlock(&parent->d_lock);
740: return NULL;
741: }
742: return parent;
743: }
SourceFile:fs/dcache.c]
|
| 28/2 |
2026/05/20 01:53 |
llm |
expert |
0m
Model:
Tokens:
input: 37878
output: 41
thoughts: 942
Thoughts: **Debugging a Softlockup in `__dentry_kill`**
Okay, so I'm looking at this softlockup, and it seems to be happening within `__dentry_kill`. The initial thought was, "Wait, `__dentry_kill` calls `cond_resched()`! That *should* yield, so no softlockup, right?" But then, I realized `cond_resched()` only reschedules if `TIF_NEED_RESCHED` is set. Even so, if it *does* reschedule, the watchdog should be reset because the CPU is not locked up. That's not quite right either: `cond_resched()` doesn't directly reset the watchdog, it's the timer or `touch_softlockup_watchdog()` that does. But, the same principle still holds: the scheduler will mark the CPU as not locked up.
Then it hit me: `cond_resched()` is a no-op if preemption is enabled! In a `CONFIG_PREEMPTION=y` kernel, it's defined to do *nothing*. Is this the problem? I started wondering about `PREEMPT_RT` or `PREEMPT_DYNAMIC`, maybe it's a no-op there too?
So, if `cond_resched()` is a no-op, then a task *can* run for too long. If there is a softlockup, that might be why. But, the timer interrupt should preempt the task, so the softlockup watchdog won't fire. But why *isn't* it being preempted? Is preemption disabled in `__dentry_kill`? I had to check.
Looking at the code, `__dentry_kill` is called without any locks held (except for a possible `rcu_read_lock`, no, it gets unlocked), but then it *does* call `spin_unlock(&dentry->d_lock)` and then `cond_resched()`, then `spin_lock(&parent->d_lock)`. So, preemption *should* be enabled when `cond_resched()` is called. That means the timer *should* preempt the task.
Why isn't it preempting?
I considered interrupts being disabled, but `spin_unlock` *enables* preemption, it's not `spin_unlock_irq`, so that's not it. Then I looked at the NMI backtrace, it showed:
```
lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
rcu_read_lock include/linux/rcupdate.h:838 [inline]
class_rcu_constructor include/linux/rcupdate.h:1181 [inline]
unwind_next_frame+0xc3/0x2550 arch/x86/kernel/unwind_orc.c:495
```
This is happening in `kasan_record_aux_stack`. Could something within `kasan_record_aux_stack` be taking too long? A softlockup is 20 seconds, so it seemed unlikely...Unless `unwind_next_frame` is in an infinite loop! That would run forever. But `kasan_record_aux_stack` is called with preemption enabled...or disabled? Wait, `call_rcu` disables interrupts? Time to investigate `call_rcu`!
|
| 29/2 |
2026/05/20 01:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/rcu/tree.c IncludeLines:true Name:call_rcu]
Results: map[SourceCode:3192: /**
3193: * call_rcu() - Queue an RCU callback for invocation after a grace period.
3194: * By default the callbacks are 'lazy' and are kept hidden from the main
3195: * ->cblist to prevent starting of grace periods too soon.
3196: * If you desire grace periods to start very soon, use call_rcu_hurry().
3197: *
3198: * @head: structure to be used for queueing the RCU updates.
3199: * @func: actual callback function to be invoked after the grace period
3200: *
3201: * The callback function will be invoked some time after a full grace
3202: * period elapses, in other words after all pre-existing RCU read-side
3203: * critical sections have completed. However, the callback function
3204: * might well execute concurrently with RCU read-side critical sections
3205: * that started after call_rcu() was invoked.
3206: *
3207: * It is perfectly legal to repost an RCU callback, potentially with
3208: * a different callback function, from within its callback function.
3209: * The specified function will be invoked after another full grace period
3210: * has elapsed. This use case is similar in form to the common practice
3211: * of reposting a timer from within its own handler.
3212: *
3213: * RCU read-side critical sections are delimited by rcu_read_lock()
3214: * and rcu_read_unlock(), and may be nested. In addition, but only in
3215: * v5.0 and later, regions of code across which interrupts, preemption,
3216: * or softirqs have been disabled also serve as RCU read-side critical
3217: * sections. This includes hardware interrupt handlers, softirq handlers,
3218: * and NMI handlers.
3219: *
3220: * Note that all CPUs must agree that the grace period extended beyond
3221: * all pre-existing RCU read-side critical section. On systems with more
3222: * than one CPU, this means that when "func()" is invoked, each CPU is
3223: * guaranteed to have executed a full memory barrier since the end of its
3224: * last RCU read-side critical section whose beginning preceded the call
3225: * to call_rcu(). It also means that each CPU executing an RCU read-side
3226: * critical section that continues beyond the start of "func()" must have
3227: * executed a memory barrier after the call_rcu() but before the beginning
3228: * of that RCU read-side critical section. Note that these guarantees
3229: * include CPUs that are offline, idle, or executing in user mode, as
3230: * well as CPUs that are executing in the kernel.
3231: *
3232: * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
3233: * resulting RCU callback function "func()", then both CPU A and CPU B are
3234: * guaranteed to execute a full memory barrier during the time interval
3235: * between the call to call_rcu() and the invocation of "func()" -- even
3236: * if CPU A and CPU B are the same CPU (but again only if the system has
3237: * more than one CPU).
3238: *
3239: * Implementation of these memory-ordering guarantees is described here:
3240: * Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst.
3241: *
3242: * Specific to call_rcu() (as opposed to the other call_rcu*() functions),
3243: * in kernels built with CONFIG_RCU_LAZY=y, call_rcu() might delay for many
3244: * seconds before starting the grace period needed by the corresponding
3245: * callback. This delay can significantly improve energy-efficiency
3246: * on low-utilization battery-powered devices. To avoid this delay,
3247: * in latency-sensitive kernel code, use call_rcu_hurry().
3248: */
3249: void call_rcu(struct rcu_head *head, rcu_callback_t func)
3250: {
3251: __call_rcu_common(head, func, enable_rcu_lazy);
3252: }
SourceFile:kernel/rcu/tree.c]
|
| 30/2 |
2026/05/20 01:53 |
llm |
expert |
0m
Model:
Tokens:
input: 40876
output: 44
thoughts: 13
Thoughts: Okay, here's my summary, presented as a stream of thought:
**Diving into `__call_rcu_common`**
Alright, time to dig into this. I'm going to start by taking a look at `__call_rcu_common`. I know it's a core function in the RCU (Read-Copy-Update) mechanism, so understanding it thoroughly is key. Let's see how it's implemented and where it sits in the larger picture of concurrent data structure management. I'm expecting to find details about how RCU callbacks are scheduled and executed, and perhaps insights into the nuances of grace periods and quiescent states. I'll be looking for any hints on how it interacts with different scheduler states or memory barriers. Hopefully, by understanding `__call_rcu_common`, I'll be able to better assess its impact on overall system performance and identify any potential bottlenecks or areas for optimization. Let's get started.
|
| 31/2 |
2026/05/20 01:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/rcu/tree.c IncludeLines:true Name:__call_rcu_common]
Results: map[SourceCode:3101: static void
3102: __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
3103: {
3104: static atomic_t doublefrees;
3105: unsigned long flags;
3106: bool lazy;
3107: struct rcu_data *rdp;
3108:
3109: /* Misaligned rcu_head! */
3110: WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
3111:
3112: /* Avoid NULL dereference if callback is NULL. */
3113: if (WARN_ON_ONCE(!func))
3114: return;
3115:
3116: if (debug_rcu_head_queue(head)) {
3117: /*
3118: * Probable double call_rcu(), so leak the callback.
3119: * Use rcu:rcu_callback trace event to find the previous
3120: * time callback was passed to call_rcu().
3121: */
3122: if (atomic_inc_return(&doublefrees) < 4) {
3123: pr_err("%s(): Double-freed CB %p->%pS()!!! ", __func__, head, head->func);
3124: mem_dump_obj(head);
3125: }
3126: WRITE_ONCE(head->func, rcu_leak_callback);
3127: return;
3128: }
3129: head->func = func;
3130: head->next = NULL;
3131: kasan_record_aux_stack(head);
3132:
3133: local_irq_save(flags);
3134: rdp = this_cpu_ptr(&rcu_data);
3135: RCU_LOCKDEP_WARN(!rcu_rdp_cpu_online(rdp), "Callback enqueued on offline CPU!");
3136:
3137: lazy = lazy_in && !rcu_async_should_hurry();
3138:
3139: /* Add the callback to our list. */
3140: if (unlikely(!rcu_segcblist_is_enabled(&rdp->cblist))) {
3141: // This can trigger due to call_rcu() from offline CPU:
3142: WARN_ON_ONCE(rcu_scheduler_active != RCU_SCHEDULER_INACTIVE);
3143: WARN_ON_ONCE(!rcu_is_watching());
3144: // Very early boot, before rcu_init(). Initialize if needed
3145: // and then drop through to queue the callback.
3146: if (rcu_segcblist_empty(&rdp->cblist))
3147: rcu_segcblist_init(&rdp->cblist);
3148: }
3149:
3150: check_cb_ovld(rdp);
3151:
3152: if (unlikely(rcu_rdp_is_offloaded(rdp)))
3153: call_rcu_nocb(rdp, head, func, flags, lazy);
3154: else
3155: call_rcu_core(rdp, head, func, flags);
3156: local_irq_restore(flags);
3157: }
SourceFile:kernel/rcu/tree.c]
|
| 32/2 |
2026/05/20 01:53 |
llm |
expert |
0m
Model:
Tokens:
input: 42123
output: 40
thoughts: 495
Thoughts: **Analyzing the Softlockup in `kasan_record_aux_stack`**
Okay, so I see that `kasan_record_aux_stack(head)` gets called before `local_irq_save(flags)`. That's important! It means interrupts are *not* disabled during the call to `kasan_record_aux_stack`, so preemption is definitely enabled. But if preemption is enabled, how is this triggering a softlockup? It doesn't make sense.
My initial thought was maybe `kasan_record_aux_stack` is taking an excessively long time. But with preemption active, the timer interrupt should fire, the scheduler should preempt the task after its timeslice is up. That would prevent the softlockup watchdog from firing, because the CPU *is* scheduling, even if it's other tasks. Ah, right, the watchdog fires if *the CPU*, not just the task, isn't scheduling for 20 seconds. If the task is preempted, the scheduler runs, and the watchdog is touched.
In fact, the watchdog thread itself (watchdog/N) should be running, touching the watchdog, even if the primary task is delayed! So a softlockup means the task *wasn't* preempted. But why wouldn't it be preempted? What's going on that's preventing the scheduler from kicking in? Maybe it's some code using `preempt_disable()`? I need to track this down.
Let's look at `finish_dput`. Wait a minute...
```c
while (lock_for_kill(dentry)) {
rcu_read_unlock();
dentry = __dentry_kill(dentry);
if (!dentry)
return;
if (retain_dentry(dentry, true)) {
spin_unlock(&dentry->d_lock);
return;
}
rcu_read_lock();
}
```
Does `lock_for_kill` disable preemption? No, I see it takes `dentry->d_lock`, a spinlock! Then `rcu_read_unlock()` is called, and then `__dentry_kill`. But hold on... `__dentry_kill` is called *with* `dentry->d_lock` held! Right, so I need to check `lock_for_kill`. That looks like a promising avenue.
|
| 33/2 |
2026/05/20 01:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:lock_for_kill]
Results: map[SourceCode: 756: static bool lock_for_kill(struct dentry *dentry)
757: {
758: struct inode *inode = dentry->d_inode;
759:
760: if (unlikely(dentry->d_lockref.count))
761: return false;
762:
763: if (!inode || likely(spin_trylock(&inode->i_lock)))
764: return true;
765:
766: do {
767: spin_unlock(&dentry->d_lock);
768: spin_lock(&inode->i_lock);
769: spin_lock(&dentry->d_lock);
770: if (likely(inode == dentry->d_inode))
771: break;
772: spin_unlock(&inode->i_lock);
773: inode = dentry->d_inode;
774: } while (inode);
775: if (likely(!dentry->d_lockref.count))
776: return true;
777: if (inode)
778: spin_unlock(&inode->i_lock);
779: return false;
780: }
SourceFile:fs/dcache.c]
|
| 34/2 |
2026/05/20 01:53 |
llm |
expert |
0m
Model:
Tokens:
input: 43572
output: 39
thoughts: 33
Thoughts: Okay, here's my interpretation of those thoughts, presented as a summary:
**Digging into a Potential Race Condition**
Alright, hang on a second. That `lock_for_kill` function... I just had a hunch that it might assume the `dentry->d_lock` is already held. That's a critical assumption and could lead to some nasty race conditions if it's not handled correctly. We need to be absolutely certain of this. Let me double-check that the calling context actually *does* hold that lock as a pre-requisite.
To do this, I think it's time to take a look at `fast_dput`. That's likely the key spot where this is being used and where any potential error would manifest. I'll need to understand that call graph and the surrounding locking scheme very carefully. I am assuming that `fast_dput` is, itself, called with the `dentry->d_lock` held. If not, we've got a serious issue on our hands!
|
| 35/2 |
2026/05/20 01:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:fast_dput]
Results: map[SourceCode: 851: /*
852: * Try to do a lockless dput(), and return whether that was successful.
853: *
854: * If unsuccessful, we return false, having already taken the dentry lock.
855: * In that case refcount is guaranteed to be zero and we have already
856: * decided that it's not worth keeping around.
857: *
858: * The caller needs to hold the RCU read lock, so that the dentry is
859: * guaranteed to stay around even if the refcount goes down to zero!
860: */
861: static inline bool fast_dput(struct dentry *dentry)
862: {
863: int ret;
864:
865: /*
866: * try to decrement the lockref optimistically.
867: */
868: ret = lockref_put_return(&dentry->d_lockref);
869:
870: /*
871: * If the lockref_put_return() failed due to the lock being held
872: * by somebody else, the fast path has failed. We will need to
873: * get the lock, and then check the count again.
874: */
875: if (unlikely(ret < 0)) {
876: spin_lock(&dentry->d_lock);
877: if (WARN_ON_ONCE(dentry->d_lockref.count <= 0)) {
878: spin_unlock(&dentry->d_lock);
879: return true;
880: }
881: dentry->d_lockref.count--;
882: goto locked;
883: }
884:
885: /*
886: * If we weren't the last ref, we're done.
887: */
888: if (ret)
889: return true;
890:
891: /*
892: * Can we decide that decrement of refcount is all we needed without
893: * taking the lock? There's a very common case when it's all we need -
894: * dentry looks like it ought to be retained and there's nothing else
895: * to do.
896: */
897: if (retain_dentry(dentry, false))
898: return true;
899:
900: /*
901: * Either not worth retaining or we can't tell without the lock.
902: * Get the lock, then. We've already decremented the refcount to 0,
903: * but we'll need to re-check the situation after getting the lock.
904: */
905: spin_lock(&dentry->d_lock);
906:
907: /*
908: * Did somebody else grab a reference to it in the meantime, and
909: * we're no longer the last user after all? Alternatively, somebody
910: * else could have killed it and marked it dead. Either way, we
911: * don't need to do anything else.
912: */
913: locked:
914: if (dentry->d_lockref.count || retain_dentry(dentry, true)) {
915: spin_unlock(&dentry->d_lock);
916: return true;
917: }
918: return false;
919: }
SourceFile:fs/dcache.c]
|
| 36/2 |
2026/05/20 01:53 |
llm |
expert |
0m
Model:
Tokens:
input: 44903
output: 41
thoughts: 264
Thoughts: **Analyzing the Lock Behavior in `__dentry_kill`**
Okay, so I've confirmed that `fast_dput` returns `false`, meaning `finish_dput` is called with `dentry->d_lock` held. That directly implies `__dentry_kill` is *also* called with that lock held. Now, the critical piece here is that `dentry->d_lock` is a spinlock. That immediately flags preemption as disabled within that critical section. I need to be absolutely sure this is handled correctly. Let's revisit the code in `__dentry_kill` again.
Looking at it, I see:
```c
static struct dentry *__dentry_kill(struct dentry *dentry)
{
...
if (dentry->d_inode)
dentry_unlink_inode(dentry);
else
spin_unlock(&dentry->d_lock);
...
cond_resched();
```
Ah, right. The key is in that `if (dentry->d_inode)` condition. If the dentry has an inode, then `dentry_unlink_inode` gets called *while* holding the spinlock. Otherwise, the spinlock is released immediately. This has interesting implications for concurrency and potential deadlocks, depending on what `dentry_unlink_inode` does. Time to investigate what *that* function does.
|
| 37/2 |
2026/05/20 01:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:dentry_unlink_inode]
Results: map[SourceCode: 446: /*
447: * Release the dentry's inode, using the filesystem
448: * d_iput() operation if defined.
449: */
450: static void dentry_unlink_inode(struct dentry * dentry)
451: __releases(dentry->d_lock)
452: __releases(dentry->d_inode->i_lock)
453: {
454: struct inode *inode = dentry->d_inode;
455:
456: raw_write_seqcount_begin(&dentry->d_seq);
457: __d_clear_type_and_inode(dentry);
458: hlist_del_init(&dentry->d_alias);
459: /*
460: * dentry becomes negative, so the space occupied by ->d_alias
461: * belongs to ->waiters now; we could use __hlist_del() instead
462: * of hlist_del_init(), if not for the stunt pulled by nfs
463: * dummy root dentries - positive dentry *not* included into
464: * the alias list of its inode. Open-coding hlist_del_init()
465: * and removing zeroing would be too clumsy...
466: */
467: dentry->waiters = NULL;
468: raw_write_seqcount_end(&dentry->d_seq);
469: spin_unlock(&dentry->d_lock);
470: spin_unlock(&inode->i_lock);
471: if (!inode->i_nlink)
472: fsnotify_inoderemove(inode);
473: if (dentry->d_op && dentry->d_op->d_iput)
474: dentry->d_op->d_iput(dentry, inode);
475: else
476: iput(inode);
477: }
SourceFile:fs/dcache.c]
|
| 38/2 |
2026/05/20 01:53 |
llm |
expert |
0m
Model:
Tokens:
input: 46104
output: 43
thoughts: 1551
Thoughts: **Unraveling the Mystery of the 143-Second Block**
Okay, let's break this down. First, I noted that `dentry_unlink_inode` releases `dentry->d_lock` and `inode->i_lock`, and `__dentry_kill` releases `dentry->d_lock` before `cond_resched()` is called. That initially led me down a rabbit hole about preemption and softlockups, thinking the timer interrupt might be the culprit. I went through the logic around `cond_resched()` and the watchdog, but realized the watchdog checks CPU scheduling, not task scheduling, and the preemption was working as it should. The softlockup question, then, wasn't immediately relevant.
The NMI backtrace showed `khungtaskd` in action. CPU 0 was running `khungtaskd`, and it had detected tasks blocked for an extended period, triggering the backtrace on all CPUs. So, this isn't a simple softlockup triggered by the watchdog, but a diagnostic tool.
The "Tainted: [L]=SOFTLOCKUP" message raised a red flag. It indicated a softlockup *did* happen earlier, likely before `khungtaskd`'s intervention.
The core problem, though, is the 143-second block. The syz test case `syz.9.5484` is blocked on `console_lock()`. My initial thought was that CPU 1, which was running `udevd` (and was also the one that printed the NMI backtrace), might be holding the lock. I checked `udevd`'s actions in relation to `unlink`, but it doesn't directly call `console_lock()` within the `unlink` syscall.
Then, I shifted my focus to RCU. Could RCU grace periods somehow be stalled because of an RCU lock held by CPU 1? I looked at `finish_dput` and the RCU operations there, but it appeared RCU grace periods *could* complete. `__dentry_kill` doesn't hold `rcu_read_lock`. So, that didn't pan out.
I went back to `syz.9.5484` and found it *was* blocked in `down(&console_sem)`, which confirmed it was waiting for the semaphore. This means *someone else* is holding it.
The scenario started to clear: `syz.9.5484` is blocked on the console semaphore, and `console_lock` is used when printing. I had to look at how printing interacts with the console and the tty layer. Maybe the console, usually a serial port, is blocked? The printing could be slow.
Finally, I honed in on the possibility that the console is a TTY and the potential contention with `tty_mutex`. I checked `vt_console_print` to see how it works in `drivers/tty/vt/vt.c`. This seemed the most promising line of investigation. So, the question remains *why* printing to the console is taking so long, especially on a TTY like `/dev/tty0`.
|
| 39/2 |
2026/05/20 01:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/tty/vt/vt.c IncludeLines:true Name:vt_console_print]
Results: map[SourceCode:3447: static void vt_console_print(struct console *co, const char *b, unsigned count)
3448: {
3449: struct vc_data *vc = vc_cons[fg_console].d;
3450: unsigned char c;
3451: static DEFINE_SPINLOCK(printing_lock);
3452: const ushort *start;
3453: ushort start_x, cnt;
3454: int kmsg_console;
3455:
3456: WARN_CONSOLE_UNLOCKED();
3457:
3458: /* this protects against concurrent oops only */
3459: if (!spin_trylock(&printing_lock))
3460: return;
3461:
3462: kmsg_console = vt_get_kmsg_redirect();
3463: if (kmsg_console && vc_cons_allocated(kmsg_console - 1))
3464: vc = vc_cons[kmsg_console - 1].d;
3465:
3466: if (!vc_cons_allocated(fg_console)) {
3467: /* impossible */
3468: /* printk("vt_console_print: tty %d not allocated ??\n", currcons+1); */
3469: goto quit;
3470: }
3471:
3472: if (vc->vc_mode != KD_TEXT)
3473: goto quit;
3474:
3475: /* undraw cursor first */
3476: if (con_is_fg(vc))
3477: hide_cursor(vc);
3478:
3479: start = (ushort *)vc->vc_pos;
3480: start_x = vc->state.x;
3481: cnt = 0;
3482: while (count--) {
3483: c = *b++;
3484: if (c == ASCII_LINEFEED || c == ASCII_CAR_RET ||
3485: c == ASCII_BACKSPACE || vc->vc_need_wrap) {
3486: if (cnt && con_is_visible(vc))
3487: vc->vc_sw->con_putcs(vc, start, cnt, vc->state.y, start_x);
3488: cnt = 0;
3489: if (c == ASCII_BACKSPACE) {
3490: bs(vc);
3491: start = (ushort *)vc->vc_pos;
3492: start_x = vc->state.x;
3493: continue;
3494: }
3495: if (c != ASCII_CAR_RET)
3496: lf(vc);
3497: cr(vc);
3498: start = (ushort *)vc->vc_pos;
3499: start_x = vc->state.x;
3500: if (c == ASCII_LINEFEED || c == ASCII_CAR_RET)
3501: continue;
3502: }
3503: vc_uniscr_putc(vc, c);
3504: scr_writew((vc->vc_attr << 8) + c, (unsigned short *)vc->vc_pos);
3505: notify_write(vc, c);
3506: cnt++;
3507: if (vc->state.x == vc->vc_cols - 1) {
3508: vc->vc_need_wrap = 1;
3509: } else {
3510: vc->vc_pos += 2;
3511: vc->state.x++;
3512: }
3513: }
3514: if (cnt && con_is_visible(vc))
3515: vc->vc_sw->con_putcs(vc, start, cnt, vc->state.y, start_x);
3516: set_cursor(vc);
3517: notify_update(vc);
3518:
3519: quit:
3520: spin_unlock(&printing_lock);
3521: }
SourceFile:drivers/tty/vt/vt.c]
|
| 40/2 |
2026/05/20 01:54 |
llm |
expert |
0m
Model:
Tokens:
input: 49548
output: 41
thoughts: 1465
Thoughts: **Analyzing a Potential Softlockup**
Okay, so `vt_console_print` doesn't grab the `tty_mutex` directly. That's good, but what if something else *holds* `console_lock` and gets blocked? Let's go back and look at the lockdep output... Got it. `getty` is reading from a TTY and it's holding `ldisc_sem` and `atomic_read_lock` which is normal.
But, wait... Could CPU 1 be holding `console_sem`? Nope, it's in `__se_sys_unlink`. Then I see CPU 1 is tainted with SOFTLOCKUP, which means it's been running for a while. Could it be printing a ton of warnings? That's a possibility, and it'd probably hold `console_sem` when printing. Printing would go through `printk` -> `console_unlock` -> `vt_console_print`? But wait, the NMI backtrace shows something else! It's deep in `__lock_acquire` -> `rcu_read_lock` -> `unwind_next_frame` -> `kasan_record_aux_stack` -> `call_rcu` -> `__dentry_kill`.
Hold on, `kasan_record_aux_stack` is called from `call_rcu`. So, if `call_rcu` is in a loop, it might trigger KASAN to record the stack. Does KASAN print anything, though? No, it just saves the stack trace. Hmm, could `kasan_record_aux_stack` be *triggering* a warning somehow? Well, it won't *print* the warning since it's just KASAN, but the backtrace shows it is within `lock_acquire`. Why is it in `lock_acquire`? Because `unwind_next_frame` calls `guard(rcu)();`, which in turn calls `rcu_read_lock()` that calls `lock_acquire()`.
If `unwind_next_frame` is looping, that means it is calling `lock_acquire()` repeatedly. Wait a minute! Lockdep has a limit, or maybe `lock_acquire` is just slow. But why would `unwind_next_frame` loop? Let me dig into `unwind_next_frame`.
```c
/* Prevent a recursive loop due to bad ORC data: */
if (state->stack_info.type == prev_type &&
on_stack(&state->stack_info, (void *)state->sp, sizeof(long)) &&
state->sp <= prev_sp) {
orc_warn_current("stack going in the wrong direction? at %pB\n",
(void *)orig_ip);
goto err;
}
```
Okay, if `state->sp <= prev_sp`, it calls `orc_warn_current`. And that prints a warning!
```c
#define orc_warn_current(args...) \
({ \
if (state->task == current && !state->error) \
orc_warn(args); \
})
```
And `orc_warn` calls `printk`, which calls `printk_deferred_once`!
```c
#define orc_warn(fmt, ...) \
printk_deferred_once(KERN_WARNING "WARNING: " fmt, ##__VA_ARGS__)
```
But, `printk_deferred_once` only prints ONCE, so it won't loop.
What if `state->sp > prev_sp`? Well, then it *doesn't* print, and it continues, meaning the stack pointer is increasing. The stack is finite, so `state->sp` can only increase a limited number of times before it goes out of bounds. If it goes out of bounds, `deref_stack_reg` will fail, and it will return `false`. So it can't loop *forever*.
Then why is CPU 1 in a softlockup? The problem must lie somewhere else... What if `finish_dput` is looping over a *massive* directory tree? If a user creates a directory tree with 1,000,000 levels and then unlinks the leaf, `finish_dput` will loop a million times! And for each level, it calls `__dentry_kill`, which then calls `call_rcu`, which finally calls `kasan_record_aux_stack`, which unwinds the stack. Unwinding the stack might take 10-100 microseconds per level. One million levels times
|
| 41/2 |
2026/05/20 01:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/watchdog.c IncludeLines:true Name:watchdog_timer_fn]
Results: map[SourceCode: 794: /* watchdog kicker functions */
795: static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
796: {
797: unsigned long touch_ts, period_ts, now;
798: struct pt_regs *regs = get_irq_regs();
799: int softlockup_all_cpu_backtrace;
800: int duration, thresh_count;
801: unsigned long flags;
802:
803: if (!watchdog_enabled)
804: return HRTIMER_NORESTART;
805:
806: /*
807: * pass the buddy check if a panic is in process
808: */
809: if (panic_in_progress())
810: return HRTIMER_NORESTART;
811:
812: softlockup_all_cpu_backtrace = (softlockup_si_mask & SYS_INFO_ALL_BT) ?
813: 1 : sysctl_softlockup_all_cpu_backtrace;
814:
815: watchdog_hardlockup_kick();
816:
817: /* kick the softlockup detector */
818: if (completion_done(this_cpu_ptr(&softlockup_completion))) {
819: reinit_completion(this_cpu_ptr(&softlockup_completion));
820: stop_one_cpu_nowait(smp_processor_id(),
821: softlockup_fn, NULL,
822: this_cpu_ptr(&softlockup_stop_work));
823: }
824:
825: /* .. and repeat */
826: hrtimer_forward_now(hrtimer, ns_to_ktime(sample_period));
827:
828: /*
829: * Read the current timestamp first. It might become invalid anytime
830: * when a virtual machine is stopped by the host or when the watchog
831: * is touched from NMI.
832: */
833: now = get_timestamp();
834: /*
835: * If a virtual machine is stopped by the host it can look to
836: * the watchdog like a soft lockup. This function touches the watchdog.
837: */
838: kvm_check_and_clear_guest_paused();
839: /*
840: * The stored timestamp is comparable with @now only when not touched.
841: * It might get touched anytime from NMI. Make sure that is_softlockup()
842: * uses the same (valid) value.
843: */
844: period_ts = READ_ONCE(*this_cpu_ptr(&watchdog_report_ts));
845:
846: update_cpustat();
847:
848: /* Reset the interval when touched by known problematic code. */
849: if (period_ts == SOFTLOCKUP_DELAY_REPORT) {
850: if (unlikely(__this_cpu_read(softlockup_touch_sync))) {
851: /*
852: * If the time stamp was touched atomically
853: * make sure the scheduler tick is up to date.
854: */
855: __this_cpu_write(softlockup_touch_sync, false);
856: sched_clock_tick();
857: }
858:
859: update_report_ts();
860: return HRTIMER_RESTART;
861: }
862:
863: /* Check for a softlockup. */
864: touch_ts = __this_cpu_read(watchdog_touch_ts);
865: duration = is_softlockup(touch_ts, period_ts, now);
866: if (unlikely(duration)) {
867: #ifdef CONFIG_SYSFS
868: ++softlockup_count;
869: #endif
870:
871: /*
872: * Prevent multiple soft-lockup reports if one cpu is already
873: * engaged in dumping all cpu back traces.
874: */
875: if (softlockup_all_cpu_backtrace) {
876: if (test_and_set_bit_lock(0, &soft_lockup_nmi_warn))
877: return HRTIMER_RESTART;
878: }
879:
880: /* Start period for the next softlockup warning. */
881: update_report_ts();
882:
883: printk_cpu_sync_get_irqsave(flags);
884: pr_emerg("BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
885: smp_processor_id(), duration,
886: current->comm, task_pid_nr(current));
887: report_cpu_status();
888: print_modules();
889: print_irqtrace_events(current);
890: if (regs)
891: show_regs(regs);
892: else
893: dump_stack();
894: printk_cpu_sync_put_irqrestore(flags);
895:
896: if (softlockup_all_cpu_backtrace) {
897: trigger_allbutcpu_cpu_backtrace(smp_processor_id());
898: if (!softlockup_panic)
899: clear_bit_unlock(0, &soft_lockup_nmi_warn);
900: }
901:
902: add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK);
903: sys_info(softlockup_si_mask & ~SYS_INFO_ALL_BT);
904: thresh_count = duration / get_softlockup_thresh();
905:
906: if (softlockup_panic && thresh_count >= softlockup_panic)
907: panic("softlockup: hung tasks");
908: }
909:
910: return HRTIMER_RESTART;
911: }
SourceFile:kernel/watchdog.c]
|
| 42/2 |
2026/05/20 01:54 |
llm |
expert |
0m
Model:
Tokens:
input: 53896
output: 39
thoughts: 922
Thoughts: **Analyzing a Softlockup in Kernel File System Operations**
Okay, let's break this down. My initial hunch was that the softlockup watchdog was being triggered via `stop_one_cpu_nowait`, which queues a work item on the CPU's stopper thread. This thread, a high-priority RT thread, is responsible for executing `softlockup_fn`, which in turn updates `watchdog_touch_ts`. If the stopper thread runs and updates this timestamp, the softlockup won't fire. Conversely, a softlockup indicates the stopper thread *didn't* run.
The core of the problem seemed to be tied to preemption. If preemption is enabled, the stopper thread *should* preempt the current task, update the timestamp, and prevent the softlockup. So, the question became: under what circumstances is preemption disabled long enough for this not to happen?
My initial suspect was `__dentry_kill`. The code shows that it releases `dentry->d_lock` and `inode->i_lock` before calling `cond_resched()`, which *should* allow preemption. If `dentry->d_inode` is NULL, then it will call `spin_unlock(&dentry->d_lock)`. After `spin_unlock` and before `spin_lock` `cond_resched()` is called. This indicates that preemption IS enabled between those calls, and the stopper thread SHOULD be able to run.
However, a softlockup did occur. This means the stopper thread was blocked from running. I hypothesized that `cond_resched()` was not giving up the CPU, but this is immediately wrong as the kernel MUST preempt after the timer interrupt, as the timer interrupt sets `TIF_NEED_RESCHED` which should trigger `preempt_schedule()`.
So then I thought perhaps the softlockup *wasn't* in `__dentry_kill` itself, but in a function called by it. I started looking at `unwind_next_frame`, called by `kasan_record_aux_stack`, which is called by `call_rcu`, which is called by `__dentry_kill`.
Now I looked again at `__dentry_kill`. It seems `dentry_free` calls `call_rcu` and `call_rcu` is called with NO locks held, so preemption is definitely enabled there.
Now I will check `dentry_free` and see what it is up to.
|
| 43/2 |
2026/05/20 01:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:dentry_free]
Results: map[SourceCode: 429: static void dentry_free(struct dentry *dentry)
430: {
431: WARN_ON(d_really_is_positive(dentry));
432: if (unlikely(dname_external(dentry))) {
433: struct external_name *p = external_name(dentry);
434: if (likely(atomic_dec_and_test(&p->count))) {
435: call_rcu(&dentry->d_rcu, __d_free_external);
436: return;
437: }
438: }
439: /* if dentry was never visible to RCU, immediate free is OK */
440: if (dentry->d_flags & DCACHE_NORCU)
441: __d_free(&dentry->d_rcu);
442: else
443: call_rcu(&dentry->d_rcu, __d_free);
444: }
SourceFile:fs/dcache.c]
|
| 44/2 |
2026/05/20 01:54 |
llm |
expert |
0m
Model:
Tokens:
input: 55700
output: 43
thoughts: 828
Thoughts: **The Mystery of the Softlockup**
Okay, here's the situation. I'm looking at this `dentry_free` function, and it's calling `call_rcu` without holding *any* locks. That means preemption is definitely *enabled*. So, if preemption is on, why am I seeing a softlockup? It just doesn't make sense!
If `unwind_next_frame` gets stuck in some kind of infinite loop, it should just keep going. But, preemption is enabled, so a timer interrupt should interrupt the process and the softlockup watchdog shouldn't fire. But wait, what is `unwind_next_frame` doing? It calls `guard(rcu)()` which then calls `rcu_read_lock()`.
Now, does `rcu_read_lock()` disable preemption? In `CONFIG_PREEMPT_RCU`, `rcu_read_lock()` *doesn't* disable preemption. But what if `CONFIG_PREEMPT_RCU` isn't set? The NMI backtrace I'm looking at says `PREEMPT(full)`, which means that `CONFIG_PREEMPT_RCU` is definitely set! So again, `rcu_read_lock()` *shouldn't* disable preemption.
Then why is it softlocking?
Aha! Look at the NMI backtrace again: `unwind_next_frame` calls `class_rcu_constructor` which then calls `rcu_read_lock` which then calls `rcu_lock_acquire` which then calls `lock_acquire`. Now, wait a minute, `lock_acquire` disables interrupts, right?
```c
void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
int trylock, int read, int check,
struct lockdep_map *nest_lock, unsigned long ip)
{
unsigned long flags;
if (unlikely(current->lockdep_recursion))
return;
raw_local_irq_save(flags);
check_flags(flags);
current->lockdep_recursion = 1;
trace_lock_acquire(lock, subclass, trylock, read, check, nest_lock, ip);
__lock_acquire(lock, subclass, trylock, read, check,
irqs_disabled_flags(flags), nest_lock, ip, 0, 0);
current->lockdep_recursion = 0;
raw_local_irq_restore(flags);
}
```
Yes! `raw_local_irq_save(flags)` is called. If `__lock_acquire` takes a long time, it runs with interrupts *disabled*. If interrupts are disabled, the timer interrupt can't fire! If the timer interrupt can't fire, the softlockup watchdog won't run, but the hardlockup watchdog will fire after 10 seconds!
But wait... the taint says `[L]=SOFTLOCKUP`. So the softlockup watchdog *did* fire, which means interrupts *were* enabled! That means `__lock_acquire` couldn't have taken 10+ seconds. So now I think the softlockup is happening because the task is looping with preemption disabled, but interrupts are enabled! This is likely due to the call to `raw_local_irq_save(flags)` within `lock_acquire`. Maybe I should check `rcu_read_lock` itself in `include/linux/rcupdate.h`.
|
| 45/2 |
2026/05/20 01:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/rcupdate.h IncludeLines:true Name:rcu_read_lock]
Results: map[SourceCode: 788: /**
789: * rcu_read_lock() - mark the beginning of an RCU read-side critical section
790: *
791: * When synchronize_rcu() is invoked on one CPU while other CPUs
792: * are within RCU read-side critical sections, then the
793: * synchronize_rcu() is guaranteed to block until after all the other
794: * CPUs exit their critical sections. Similarly, if call_rcu() is invoked
795: * on one CPU while other CPUs are within RCU read-side critical
796: * sections, invocation of the corresponding RCU callback is deferred
797: * until after the all the other CPUs exit their critical sections.
798: *
799: * Both synchronize_rcu() and call_rcu() also wait for regions of code
800: * with preemption disabled, including regions of code with interrupts or
801: * softirqs disabled.
802: *
803: * Note, however, that RCU callbacks are permitted to run concurrently
804: * with new RCU read-side critical sections. One way that this can happen
805: * is via the following sequence of events: (1) CPU 0 enters an RCU
806: * read-side critical section, (2) CPU 1 invokes call_rcu() to register
807: * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
808: * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
809: * callback is invoked. This is legal, because the RCU read-side critical
810: * section that was running concurrently with the call_rcu() (and which
811: * therefore might be referencing something that the corresponding RCU
812: * callback would free up) has completed before the corresponding
813: * RCU callback is invoked.
814: *
815: * RCU read-side critical sections may be nested. Any deferred actions
816: * will be deferred until the outermost RCU read-side critical section
817: * completes.
818: *
819: * You can avoid reading and understanding the next paragraph by
820: * following this rule: don't put anything in an rcu_read_lock() RCU
821: * read-side critical section that would block in a !PREEMPTION kernel.
822: * But if you want the full story, read on!
823: *
824: * In non-preemptible RCU implementations (pure TREE_RCU and TINY_RCU),
825: * it is illegal to block while in an RCU read-side critical section.
826: * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPTION
827: * kernel builds, RCU read-side critical sections may be preempted,
828: * but explicit blocking is illegal. Finally, in preemptible RCU
829: * implementations in real-time (with -rt patchset) kernel builds, RCU
830: * read-side critical sections may be preempted and they may also block, but
831: * only when acquiring spinlocks that are subject to priority inheritance.
832: */
833: static __always_inline void rcu_read_lock(void)
834: __acquires_shared(RCU)
835: {
836: __rcu_read_lock();
837: __acquire_shared(RCU);
838: rcu_lock_acquire(&rcu_lock_map);
839: RCU_LOCKDEP_WARN(!rcu_is_watching(),
840: "rcu_read_lock() used illegally while idle");
841: }
SourceFile:include/linux/rcupdate.h]
|
| 46/2 |
2026/05/20 01:54 |
llm |
expert |
0m
Model:
Tokens:
input: 58435
output: 44
thoughts: 13
Thoughts: Okay, here's my attempt at summarizing that thought process, keeping in mind I'm speaking to a fellow expert:
**Diving into RCU Read Locks**
Alright, time to dig into this. My initial thought is to make sure I understand how `__rcu_read_lock` functions. I need to make sure I understand the mechanics. It might be a quick check to refresh my memory, or maybe I'm troubleshooting a problem that might involve contention or incorrect usage. I need to look at the implementation.
|
| 47/2 |
2026/05/20 01:55 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/rcupdate.h IncludeLines:true Name:__rcu_read_lock]
Results: map[SourceCode: 407: /*
408: * Preemptible RCU implementation for rcu_read_lock().
409: * Just increment ->rcu_read_lock_nesting, shared state will be updated
410: * if we block.
411: */
412: void __rcu_read_lock(void)
413: {
414: rcu_preempt_read_enter();
415: if (IS_ENABLED(CONFIG_PROVE_LOCKING))
416: WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
417: if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
418: WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
419: barrier(); /* critical section after entry code. */
420: }
SourceFile:kernel/rcu/tree_plugin.h]
|
| 48/2 |
2026/05/20 01:55 |
llm |
expert |
0m
Model:
Tokens:
input: 58901
output: 37
thoughts: 950
Thoughts: **Analyzing the Softlockup in `finish_dput`**
Okay, so I'm trying to figure out this softlockup, and it's got me thinking. The key observation is that `__rcu_read_lock` doesn't disable preemption, which seems counterintuitive given the problem description. If preemption is enabled, *why* are we seeing this softlockup? The problem arises if `finish_dput` is looping, and within this loop it is calling, and re-entering the following sequence: `rcu_read_unlock()`, `__dentry_kill()`, then `rcu_read_lock()`.
`__dentry_kill` calls `cond_resched()`, which, on its own, seems benign. However, if `cond_resched()` is a no-op, the task won't yield voluntarily. But, the timer interrupt should preempt it! Unless... wait, what about task priority and scheduling policies? If it's a real-time task with `SCHED_FIFO` or `SCHED_RR`, the stopper thread (`migration/N`) is also real-time, and it's at priority 99 β the highest. If the task has a lower priority, the stopper thread *will* preempt it. However, the Syz tasks are `SCHED_NORMAL` which would imply that the stopper thread WILL preempt it. So the softlockup watchdog *shouldn't* fire!
So, what other reason could there be that the stopper thread isn't running? Could it be blocked on something? No, it just calls `softlockup_fn`, which updates `watchdog_touch_ts`. Okay, what if the softlockup watchdog isn't using the stopper thread? But then, in newer kernels, the softlockup watchdog *doesn't* use a thread anymore. Let me check `watchdog_timer_fn` again... Aha! It uses `stop_one_cpu_nowait`! So it queues a work item on the stopper thread. If that thread runs, it calls `softlockup_fn`, and if the stopper thread runs, it *must* update `watchdog_touch_ts`.
This means `duration` is 0. If `duration` is greater than 20 seconds, we get the warning! So if the warning is printed, it means the stopper thread DIDN'T run for 20 seconds! Why? Because preemption was disabled for 20 seconds!
Now, *where* is preemption getting disabled for that long? Is it in `finish_dput`? `finish_dput` calls `__dentry_kill`, which in turn calls `spin_lock(&parent->d_lock)`. Now, could that lock be contended for so long? No, it's just unlinking a single directory tree. The critical connection here is that `finish_dput` could be called *while* `rcu_read_lock()` is held. And checking `dput`, it's clear: `dput` does `rcu_read_lock()`, then `fast_dput()`. If `fast_dput` returns false, it then calls `rcu_read_unlock()` *before* calling `finish_dput`. However, it only calls `rcu_read_unlock()` *before* returning if `fast_dput` is true. But if `finish_dput` is called, it does *not* call `rcu_read_unlock()`! So I have found the problem!
|
| 49/2 |
2026/05/20 01:55 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:dput]
Results: map[SourceCode: 957: /*
958: * dput - release a dentry
959: * @dentry: dentry to release
960: *
961: * Release a dentry. This will drop the usage count and if appropriate
962: * call the dentry unlink method as well as removing it from the queues and
963: * releasing its resources. If the parent dentries were scheduled for release
964: * they too may now get deleted.
965: */
966: void dput(struct dentry *dentry)
967: {
968: if (!dentry)
969: return;
970: might_sleep();
971: rcu_read_lock();
972: if (likely(fast_dput(dentry))) {
973: rcu_read_unlock();
974: return;
975: }
976: finish_dput(dentry);
977: }
SourceFile:fs/dcache.c]
|
| 50/2 |
2026/05/20 01:55 |
llm |
expert |
0m
Model:
Tokens:
input: 60970
output: 45
thoughts: 2451
Thoughts: **Analyzing a Softlockup in dput/finish_dput**
Okay, so `dput` calls `finish_dput`, and I see the crucial detail β `finish_dput` expects the `rcu_read_lock` to be held. Got it. `finish_dput` releases RCU at the end, that's important. Inside the loop, `rcu_read_unlock`, then `__dentry_kill`, then `rcu_read_lock`. So, `__dentry_kill` is called with RCU unlocked. Where is preemption disabled? Wait a minute, `__dentry_kill` gets called with `dentry->d_lock` held. `fast_dput` returns false with that lock held. Thus, `finish_dput` enters the loop *with* `dentry->d_lock` held. `lock_for_kill` also expects the dentry lock to be held. If it returns true, `dentry->d_lock` *is* held. Okay, `rcu_read_unlock`, then `__dentry_kill`, so `__dentry_kill` is definitely called with the dentry lock held, and that is a spinlock. Thus, preemption *is* disabled during that call!
Alright, let's look at `__dentry_kill` again. Wait, `dentry_unlink_inode` *releases* `dentry->d_lock`! That's it! The lock is released *before* `cond_resched()`! So preemption is *enabled* before `cond_resched()`, which allows the stopper thread to run. Wait, could `dentry_unlink_inode` take a long time? `fsnotify_inoderemove`, `iput`... `iput` could be expensive, especially if it evicts the inode, which means writing to disk. But is it really 20 seconds? It should be scheduling if it takes long to finish. It shouldn't lockup.
I must go back to the NMI backtrace. Oh, `__dentry_kill` calls `call_rcu`! Line 737 is `dentry_free(dentry)`. `dentry_free` is called with *no* locks held, so preemption is definitely enabled. So how can this cause a softlockup? The backtrace. It *could* be because of `unwind_next_frame`, but that is unlikely because preemption is enabled, and the timer interrupt *should* interrupt it. But the taint is `[L]=SOFTLOCKUP`, which means the softlockup watchdog *did* fire, which means the stopper thread *couldn't* run. That's a problem. Why wouldn't it run? Maybe the task is running with preemption disabled? `lock_acquire` disables interrupts, right? `__lock_acquire` might take long!
If interrupts are disabled, the timer interrupt cannot fire, so the softlockup watchdog would *not* run! This means the *hard*lockup watchdog, not softlockup, would fire. The taint is `[L]=SOFTLOCKUP`. So, interrupts *were* enabled, the timer interrupt fired, the stopper work got queued! So *why* didn't it run? The stopper thread has a very high priority! Maybe if the stopper is also on priority 99? Nope. It's a syz task. Maybe the stopper thread is blocked? But the stopper thread just runs `softlockup_fn`, and that doesn't block.
Wait... is the softlockup watchdog *actually* using the stopper thread? Let me go back to `watchdog_timer_fn`. Yes, it *does* use the stopper thread. Could the stopper thread be preempted by a higher priority thread? No, priority 99 is the highest. What if the task is running with `preempt_disable()`? Ah, could `kasan_record_aux_stack` disable preemption?
|
| 51/2 |
2026/05/20 01:55 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/kasan/generic.c IncludeLines:true Name:kasan_record_aux_stack]
Results: map[SourceCode: 535: /*
536: * This function avoids dynamic memory allocations and thus can be called from
537: * contexts that do not allow allocating memory.
538: */
539: void kasan_record_aux_stack(void *addr)
540: {
541: struct slab *slab = kasan_addr_to_slab(addr);
542: struct kmem_cache *cache;
543: struct kasan_alloc_meta *alloc_meta;
544: void *object;
545:
546: if (is_kfence_address(addr) || !slab)
547: return;
548:
549: cache = slab->slab_cache;
550: object = nearest_obj(cache, slab, addr);
551: alloc_meta = kasan_get_alloc_meta(cache, object);
552: if (!alloc_meta)
553: return;
554:
555: alloc_meta->aux_stack[1] = alloc_meta->aux_stack[0];
556: alloc_meta->aux_stack[0] = kasan_save_stack(0, 0);
557: }
SourceFile:mm/kasan/generic.c]
|
| 52/2 |
2026/05/20 01:55 |
llm |
expert |
0m
Model:
Tokens:
input: 64723
output: 39
thoughts: 1154
Thoughts: **Analyzing a Softlockup and Task Hang**
Okay, so I'm looking at this softlockup, and initially, I was focusing on `kasan_record_aux_stack`. I thought maybe it was disabling preemption somehow, but that's wrong. `call_rcu` disables interrupts, but `kasan_record_aux_stack` is called *before* the `local_irq_save`! So, preemption is actually *enabled* during the KASAN stack recording. That doesn't seem to be the source of the issue.
Then I realized, hold on... the `[L]` taint for SOFTLOCKUP is *global*! It means the softlockup happened at *some point* in the past. It doesn't mean CPU 1 is currently in a softlockup. It could have happened minutes ago, right? The bug report is about tasks being blocked for 143 seconds, and `khungtaskd` is reporting it. CPU 1 was likely just doing normal work β `unlink` in this case β and the NMI backtrace was triggered on *all* CPUs. CPU 1 happened to be in `unwind_next_frame` at the exact moment the NMI hit.
So, it's not an infinite loop in `unwind_next_frame` as I initially thought. The softlockup taint is just leftover from a prior issue. The *real* problem is these hung tasks.
Why are the tasks hung? One is blocked in `console_lock()`. That's the key. Who holds the `console_sem`? I need to check if a task is holding `console_sem` and didn't release it, or if a task holding it is blocked on something else. If a task *were* holding `console_sem` and *was* blocked, `khungtaskd` would report it. But it only reported three tasks. Wait a minute... `khungtaskd` reports up to ten tasks. If there was another task blocking, holding `console_sem`, and blocked, it would have been reported unless it wasn't in `D` state!
So, could a task be running for 143 seconds holding `console_sem`? If it were, it would trigger a softlockup, and the taint does show `[L]=SOFTLOCKUP`. But if that was the case, the softlockup watchdog would have printed a warning, but that's not in the bug report. Perhaps the softlockup warning was earlier, and syzkaller didn't capture it as the primary crash, or it's truncated?
What if a task *is* holding `console_sem` and looping? What could it be doing? `console_unlock`, right? That's what prints the log buffer to consoles. If the log buffer is constantly being filled by other CPUs, `console_unlock` could loop for a very long time. However, `console_unlock` has a mechanism to avoid looping forever: `cond_resched()`. I should really check `console_unlock` in `kernel/printk/printk.c`.
|
| 53/2 |
2026/05/20 01:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/printk/printk.c IncludeLines:true Name:console_unlock]
Results: map[SourceCode:3395: /**
3396: * console_unlock - unblock the legacy console subsystem from printing
3397: *
3398: * Releases the console_lock which the caller holds to block printing of
3399: * the legacy console subsystem.
3400: *
3401: * While the console_lock was held, console output may have been buffered
3402: * by printk(). If this is the case, console_unlock() emits the output on
3403: * legacy consoles prior to releasing the lock.
3404: *
3405: * console_unlock(); may be called from any context.
3406: */
3407: void console_unlock(void)
3408: {
3409: struct console_flush_type ft;
3410:
3411: printk_get_console_flush_type(&ft);
3412: if (ft.legacy_direct)
3413: __console_flush_and_unlock();
3414: else
3415: __console_unlock();
3416: }
SourceFile:kernel/printk/printk.c]
|
| 54/2 |
2026/05/20 01:56 |
llm |
expert |
0m
Model:
Tokens:
input: 66918
output: 44
thoughts: 14
Thoughts: Here's a summary of my thought process, as requested:
**Checking `__console_flush_and_unlock`**
Okay, so I need to take a look at `__console_flush_and_unlock`. This function immediately raises a few questions in my mind, even without seeing the code. The naming convention, specifically the double underscore prefix, suggests it's likely an internal or low-level function, perhaps one that's not meant for direct external use. The "flush" part immediately signals that it deals with some kind of data buffer, probably related to console output β writing data to the terminal. "Unlock" implies a synchronization primitive β a mutex, semaphore, or something similar β is involved. It's likely ensuring a critical section is released to prevent data corruption or race conditions. I'm guessing this would be something I'd want to inspect to see how it works within the context of a potential bug I may be debugging, to see if there is an issue with how the buffer is written to, or if the lock is not handled correctly. Time to dig in and see the implementation.
|
| 55/2 |
2026/05/20 01:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/printk/printk.c IncludeLines:true Name:__console_flush_and_unlock]
Results: map[SourceCode:3350: static void __console_flush_and_unlock(void)
3351: {
3352: bool do_cond_resched;
3353: bool handover;
3354: bool flushed;
3355: u64 next_seq;
3356:
3357: /*
3358: * Console drivers are called with interrupts disabled, so
3359: * @console_may_schedule should be cleared before; however, we may
3360: * end up dumping a lot of lines, for example, if called from
3361: * console registration path, and should invoke cond_resched()
3362: * between lines if allowable. Not doing so can cause a very long
3363: * scheduling stall on a slow console leading to RCU stall and
3364: * softlockup warnings which exacerbate the issue with more
3365: * messages practically incapacitating the system. Therefore, create
3366: * a local to use for the printing loop.
3367: */
3368: do_cond_resched = console_may_schedule;
3369:
3370: do {
3371: console_may_schedule = 0;
3372:
3373: flushed = console_flush_all(do_cond_resched, &next_seq, &handover);
3374: if (!handover)
3375: __console_unlock();
3376:
3377: /*
3378: * Abort if there was a failure to flush all messages to all
3379: * usable consoles. Either it is not possible to flush (in
3380: * which case it would be an infinite loop of retrying) or
3381: * another context has taken over printing.
3382: */
3383: if (!flushed)
3384: break;
3385:
3386: /*
3387: * Some context may have added new records after
3388: * console_flush_all() but before unlocking the console.
3389: * Re-check if there is a new record to flush. If the trylock
3390: * fails, another context is already handling the printing.
3391: */
3392: } while (prb_read_valid(prb, next_seq, NULL) && console_trylock());
3393: }
SourceFile:kernel/printk/printk.c]
|
| 56/2 |
2026/05/20 01:56 |
llm |
expert |
0m
Model:
Tokens:
input: 67893
output: 21
thoughts: 1638
Thoughts: **Unraveling a Kernel Hang: A Deep Dive**
Okay, so here's what I'm seeing. `__console_flush_and_unlock` calls `console_flush_all`, which, under certain conditions, can call `cond_resched()`. But wait, in the current preemptive kernel configuration, `cond_resched()` is effectively a no-op! That means if there's a lot of console output, this function *won't* yield. We're talking potentially slow console devices like serial ports, which could be up to 10ms per line. If there are 10,000 lines? That's 100 seconds! During this process, `console_sem` is held, and this seems like the root of the problem.
If preemption is enabled, it *should* be preempted by the timer interrupt. However, syzkaller is blocked in `down(&console_sem)`, so other tasks *can't* run. So this task, `syz.9.5484`, is blocked, and will be for an extremely long time. Specifically, it's blocked for more than 143 seconds. Then, `khungtaskd` reports it as a hung task. And because `syz.9.5484` holds `tty_mutex`, *every* task trying to open a TTY is also blocked for > 143 seconds! That perfectly explains the hang!
The core problem is that `console_unlock` is taking forever, holding `console_sem`. Why? It has to be a `printk` flood! If someone is flooding the kernel log, the task calling `console_unlock` gets stuck trying to print everything. And since `cond_resched()` is a no-op, this might even trigger a softlockup if it's not preempted. But if it *is* preempted, it'll still hold `console_sem` for a while.
Is this a bug? Absolutely, if an unprivileged user can trigger a `printk` flood, it's a DoS vulnerability. But, the bug report doesn't reveal what's flooding the logs.
Digging deeper into the syzkaller report, specifically: "INFO: task syz.9.5484:27591 blocked for more than 143 seconds." The fact that it's blocked on both `tty_mutex` and `console_sem` is crucial. Is it possible that the bug *isn't* a `printk` flood, but a deadlock? I have to investigate. `syz.9.5484` is holding `tty_mutex` and waiting on `console_sem` (specifically, in `con_install`). Is there a task holding `console_sem` and *waiting* for `tty_mutex`? That would be a deadlock!
I've checked the known console drivers, `vt_console_print` and the serial consoles, and they don't seem to acquire `tty_mutex`. But, what if `console_sem` is held by a task blocked on *something else*? Not OOM, because syzkaller usually reports that differently. No, console sem is not held during VFS.
Wait! The lockdep output reveals a crucial clue: `syz.6.5576` is trying to acquire `tty_mutex` in `ptmx_open`, and a lot of tasks are blocked on `tty_mutex`. `syz.9.5484` holds `tty_mutex` but is blocked on `console_sem`.
If it's a `printk` flood, then syzkaller should report it differently: "softlockup" or "rcu stall" or "printk flood". This, however, reports "task blocked". This means the task holding `console_sem` is either running for a long time, OR blocked. If it's blocked, what's it blocked on?
Could it be blocked on `tty_mutex`? Aha! If a task holds `console_sem` and *then* tries to get `tty_mutex`, that's a deadlock! I need to search for a code path where `console_lock` is followed by an attempt to acquire `tty_mutex`, or calls a function that acquires `tty_mutex`. Let's check `tty_mutex` users. That should reveal something.
|
| 57/2 |
2026/05/20 01:56 |
tool |
codesearch-find-references |
0m
Args:
Results: map[References:<nil> TruncatedOutput:false]
|
| 58/2 |
2026/05/20 01:56 |
llm |
expert |
0m
Model:
Tokens:
input: 70534
output: 23
thoughts: 22
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Investigating `tty_mutex`**
Okay, so `tty_mutex`... that rings a bell. Ah, right, it's a global, which immediately flags it for closer scrutiny. Global variables, especially mutexes, are notorious for introducing contention and potential deadlocks in kernel code. So, my initial thought is, let's track down *every* place this thing is being used. References are key here. I need a comprehensive view of how it's acquired, released, and crucially, within what execution contexts. Gotta identify the hot spots, the critical sections it protects. The goal is to build a mental map of its role in the TTY subsystem and anticipate potential concurrency issues. Let's see what the code has to show.
|
| 59/2 |
2026/05/20 01:56 |
tool |
grepper |
0m
Args: map[Expression:mutex_lock\(&tty_mutex\)]
Results: map[Output:drivers/tty/tty_io.c=1646=void tty_kclose(struct tty_struct *tty)
--
drivers/tty/tty_io.c-1660- */
drivers/tty/tty_io.c:1661: mutex_lock(&tty_mutex);
drivers/tty/tty_io.c-1662- tty_port_set_kopened(tty->port, 0);
--
drivers/tty/tty_io.c=1676=void tty_release_struct(struct tty_struct *tty, int idx)
--
drivers/tty/tty_io.c-1690- */
drivers/tty/tty_io.c:1691: mutex_lock(&tty_mutex);
drivers/tty/tty_io.c-1692- release_tty(tty, idx);
--
drivers/tty/tty_io.c=1944=static struct tty_struct *tty_kopen(dev_t device, int shared)
--
drivers/tty/tty_io.c-1949-
drivers/tty/tty_io.c:1950: mutex_lock(&tty_mutex);
drivers/tty/tty_io.c-1951- driver = tty_lookup_driver(device, NULL, &index);
--
drivers/tty/tty_io.c=2029=static struct tty_struct *tty_open_by_driver(dev_t device,
--
drivers/tty/tty_io.c-2036-
drivers/tty/tty_io.c:2037: mutex_lock(&tty_mutex);
drivers/tty/tty_io.c-2038- driver = tty_lookup_driver(device, filp, &index);
--
fs/proc/proc_tty.c=104=static void *t_start(struct seq_file *m, loff_t *pos)
fs/proc/proc_tty.c-105-{
fs/proc/proc_tty.c:106: mutex_lock(&tty_mutex);
fs/proc/proc_tty.c-107- return seq_list_start(&tty_drivers, *pos);
]
|
| 60/2 |
2026/05/20 01:56 |
llm |
expert |
0m
Model:
Tokens:
input: 71318
output: 39
thoughts: 198
Thoughts: **Analyzing Potential `tty_mutex` Contention**
Okay, so I'm trying to figure out where `tty_mutex` is acquired and if there's any risk of a potential deadlock with `console_sem`. Based on the code, `tty_mutex` is definitely being acquired in a few key places: `tty_kclose`, `tty_release_struct`, `tty_kopen`, `tty_open_by_driver`, and also within `t_start`, which I know is in `fs/proc/proc_tty.c`.
My immediate concern is whether any of these functions that grab the `tty_mutex` could *also* be called while `console_sem` is held, since that would be a classic recipe for trouble. Let's start with `t_start`. I recall that's called when you're reading `/proc/tty/drivers`. Hmm, does *that* hold `console_sem`? A quick mental scan... nope, reading that proc entry shouldn't be holding it. Good.
What about `tty_kclose` and `tty_release_struct`? They're clearly invoked when a TTY is being closed or released. The question is, could they be called *from* a console driver while it's holding `console_sem`? Think about it⦠for instance, what happens when a console is unregistered? Does that unregistration process involve closing a TTY? I need to check the `unregister_console` function now. That's the key.
|
| 61/2 |
2026/05/20 01:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/printk/printk.c IncludeLines:true Name:unregister_console]
Results: map[SourceCode:4325: int unregister_console(struct console *console)
4326: {
4327: int res;
4328:
4329: console_list_lock();
4330: res = unregister_console_locked(console);
4331: console_list_unlock();
4332: return res;
4333: }
SourceFile:kernel/printk/printk.c]
|
| 62/2 |
2026/05/20 01:56 |
llm |
expert |
0m
Model:
Tokens:
input: 72017
output: 41
thoughts: 49
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm already quite familiar with kernel concurrency and console drivers:
**Analyzing `unregister_console` Concurrency**
Alright, I'm diving into this `unregister_console` function. The first thing that jumps out is that it grabs `console_list_lock`. Wait a second... isn't that a mutex? I need to be sure I have the right mental model. I always have to double check these things, the semantics of these locks and semaphores are crucial for avoiding deadlocks and race conditions. Okay, *that* part makes sense now. Let me see... does `unregister_console_locked` use `console_sem`? I need to know what other locks are acquired, and what the overall locking order is to ensure that I'm not creating any concurrency issues. This needs to be checked carefully. Let's see...
|
| 63/2 |
2026/05/20 01:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/printk/printk.c IncludeLines:true Name:unregister_console_locked]
Results: map[SourceCode:4227: /* Must be called under console_list_lock(). */
4228: static int unregister_console_locked(struct console *console)
4229: {
4230: bool use_device_lock = (console->flags & CON_NBCON) && console->write_atomic;
4231: bool found_legacy_con = false;
4232: bool found_nbcon_con = false;
4233: bool found_boot_con = false;
4234: unsigned long flags;
4235: struct console *c;
4236: int res;
4237:
4238: lockdep_assert_console_list_lock_held();
4239:
4240: con_printk(KERN_INFO, console, "disabled\n");
4241:
4242: res = _braille_unregister_console(console);
4243: if (res < 0)
4244: return res;
4245: if (res > 0)
4246: return 0;
4247:
4248: if (!console_is_registered_locked(console))
4249: res = -ENODEV;
4250: else if (console_is_usable(console, console->flags, true))
4251: __pr_flush(console, 1000, true);
4252:
4253: /* Disable it unconditionally */
4254: console_srcu_write_flags(console, console->flags & ~CON_ENABLED);
4255:
4256: if (res < 0)
4257: return res;
4258:
4259: /*
4260: * Use the driver synchronization to ensure that the hardware is not
4261: * in use while this console transitions to being unregistered.
4262: */
4263: if (use_device_lock)
4264: console->device_lock(console, &flags);
4265:
4266: hlist_del_init_rcu(&console->node);
4267:
4268: if (use_device_lock)
4269: console->device_unlock(console, flags);
4270:
4271: /*
4272: * <HISTORICAL>
4273: * If this isn't the last console and it has CON_CONSDEV set, we
4274: * need to set it on the next preferred console.
4275: * </HISTORICAL>
4276: *
4277: * The above makes no sense as there is no guarantee that the next
4278: * console has any device attached. Oh well....
4279: */
4280: if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV)
4281: console_srcu_write_flags(console_first(), console_first()->flags | CON_CONSDEV);
4282:
4283: /*
4284: * Ensure that all SRCU list walks have completed. All contexts
4285: * must not be able to see this console in the list so that any
4286: * exit/cleanup routines can be performed safely.
4287: */
4288: synchronize_srcu(&console_srcu);
4289:
4290: /*
4291: * With this console gone, the global flags tracking registered
4292: * console types may have changed. Update them.
4293: */
4294: for_each_console(c) {
4295: if (c->flags & CON_BOOT)
4296: found_boot_con = true;
4297:
4298: if (c->flags & CON_NBCON)
4299: found_nbcon_con = true;
4300: else
4301: found_legacy_con = true;
4302: }
4303: if (!found_boot_con)
4304: have_boot_console = found_boot_con;
4305: if (!found_legacy_con)
4306: have_legacy_console = found_legacy_con;
4307: if (!found_nbcon_con)
4308: have_nbcon_console = found_nbcon_con;
4309:
4310: /* @have_nbcon_console must be updated before calling nbcon_free(). */
4311: if (console->flags & CON_NBCON)
4312: nbcon_free(console);
4313:
4314: console_sysfs_notify();
4315:
4316: if (console->exit)
4317: res = console->exit(console);
4318:
4319: /* Changed console list, may require printer threads to start/stop. */
4320: printk_kthreads_check_locked();
4321:
4322: return res;
4323: }
SourceFile:kernel/printk/printk.c]
|
| 64/2 |
2026/05/20 01:56 |
llm |
expert |
0m
Model:
Tokens:
input: 73714
output: 43
thoughts: 56
Thoughts: Here's my summary, as the expert:
**Investigating the Console Unregistration**
Okay, so `unregister_console_locked` is the key function here. The crucial point is that it calls `console->exit(console)`. Right, that's where things get interesting, because `console->exit` is a driver-specific function. I need to understand what that actually *does* for the virtual terminal (`vt`). Time to dive into the source. Let's head over to `drivers/tty/vt/vt.c` and specifically examine the `vt_console_driver` structure. I need to find the `exit` function pointer implementation for `vt`, and figure out what cleaning up it does. Thatβll tell me whatβs happening in this unregistration scenario.
|
| 65/2 |
2026/05/20 01:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/tty/vt/vt.c IncludeLines:true Name:vt_console_driver]
Results: map[SourceCode:3534: static struct console vt_console_driver = {
3535: .name = "tty",
3536: .setup = vt_console_setup,
3537: .write = vt_console_print,
3538: .device = vt_console_device,
3539: .unblank = unblank_screen,
3540: .flags = CON_PRINTBUFFER,
3541: .index = -1,
3542: };
SourceFile:drivers/tty/vt/vt.c]
|
| 66/2 |
2026/05/20 01:56 |
llm |
expert |
0m
Model:
Tokens:
input: 74150
output: 43
thoughts: 227
Thoughts: **Analyzing a Potential Console Lock-Up**
Okay, so I'm digging into this `vt_console_driver` and I see there's no explicit `exit` callback defined. That's a bit unusual, but not necessarily a problem, just something to note. Now, my mind is jumping ahead β I'm thinking about how the console might get stuck. The critical piece here is `console_sem`. Could the semaphore, `console_sem`, *itself* be the culprit, held by something unexpected like `syz.9.5484`? No, wait, that's wrong. If `syz.9.5484` is blocked in `down(&console_sem)`, it's *waiting* for the semaphore, not holding it.
Okay, let's step back and follow the code path. I see `__down_common` gets called from `down`. So if the function's blocked in `down`, it's definitely waiting for the semaphore to be released. Now the real question is *who* holds the semaphore? And what's keeping it held? If a task is running `console_unlock`, and that function is outputting a massive amount of console messages, it *could* hold `console_sem` for a significant amount of time, essentially blocking other processes. But that's not the only possibility.
Is there some other way this `console_sem` can be held indefinitely? What if a task calls `console_lock()` and then goes to sleep, perhaps in the kernel? I need to track down potential sleep paths after calling `console_lock()`. Thinking about it, routines like `do_blank_screen` or `do_unblank_screen` in `vt.c` seem like they might be contenders. Let's start by looking specifically at `do_blank_screen`.
|
| 67/2 |
2026/05/20 01:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/tty/vt/vt.c IncludeLines:true Name:do_blank_screen]
Results: map[SourceCode:4664: void do_blank_screen(int entering_gfx)
4665: {
4666: struct vc_data *vc = vc_cons[fg_console].d;
4667: int i;
4668:
4669: might_sleep();
4670:
4671: WARN_CONSOLE_UNLOCKED();
4672:
4673: if (console_blanked) {
4674: if (blank_state == blank_vesa_wait) {
4675: blank_state = blank_off;
4676: vc->vc_sw->con_blank(vc, vesa_blank_mode + 1, 0);
4677: }
4678: return;
4679: }
4680:
4681: /* entering graphics mode? */
4682: if (entering_gfx) {
4683: hide_cursor(vc);
4684: save_screen(vc);
4685: vc->vc_sw->con_blank(vc, VESA_VSYNC_SUSPEND, 1);
4686: console_blanked = fg_console + 1;
4687: blank_state = blank_off;
4688: set_origin(vc);
4689: return;
4690: }
4691:
4692: blank_state = blank_off;
4693:
4694: /* don't blank graphics */
4695: if (vc->vc_mode != KD_TEXT) {
4696: console_blanked = fg_console + 1;
4697: return;
4698: }
4699:
4700: hide_cursor(vc);
4701: timer_delete_sync(&console_timer);
4702: blank_timer_expired = 0;
4703:
4704: save_screen(vc);
4705: /* In case we need to reset origin, blanking hook returns 1 */
4706: i = vc->vc_sw->con_blank(vc, vesa_off_interval ? VESA_VSYNC_SUSPEND :
4707: (vesa_blank_mode + 1), 0);
4708: console_blanked = fg_console + 1;
4709: if (i)
4710: set_origin(vc);
4711:
4712: if (console_blank_hook && console_blank_hook(1))
4713: return;
4714:
4715: if (vesa_off_interval && vesa_blank_mode) {
4716: blank_state = blank_vesa_wait;
4717: mod_timer(&console_timer, jiffies + vesa_off_interval);
4718: }
4719: vt_event_post(VT_EVENT_BLANK, vc->vc_num, vc->vc_num);
4720: }
SourceFile:drivers/tty/vt/vt.c]
|
| 68/2 |
2026/05/20 01:56 |
llm |
expert |
0m
Model:
Tokens:
input: 75680
output: 43
thoughts: 89
Thoughts: **Deadlock! A Deep Dive into Screen Blanking**
Okay, here's what's hitting me. We've got this function, `do_blank_screen`, which is supposed to handle, well, blanking the screen. And it calls `timer_delete_sync(&console_timer)`. Fine, makes sense, we're cleaning up the screen blanking timer. But wait a minute... I know that `console_timer` is specifically *for* blanking the screen. The alarm bells are starting to go off in my head.
If `timer_delete_sync` is being called *while* we're already holding `console_sem`... and *simultaneously* the timer's callback is running and *also* trying to grab `console_sem`... bingo! Deadlock! That's a classic recipe for a system hang, and we've got a potential race condition right here.
I need to immediately check the actual timer callback associated with `console_timer`. I've got to confirm if it's indeed contending for `console_sem` in a potentially problematic way. This could be a critical bug; I need to understand this timing and resource dependency perfectly.
|
| 69/2 |
2026/05/20 01:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/tty/vt/vt.c IncludeLines:true Name:blank_screen_t]
Results: map[SourceCode:4781: /*
4782: * We defer the timer blanking to work queue so it can take the console mutex
4783: * (console operations can still happen at irq time, but only from printk which
4784: * has the console mutex. Not perfect yet, but better than no locking
4785: */
4786: static void blank_screen_t(struct timer_list *unused)
4787: {
4788: blank_timer_expired = 1;
4789: schedule_work(&console_work);
4790: }
SourceFile:drivers/tty/vt/vt.c]
|
| 70/2 |
2026/05/20 01:56 |
llm |
expert |
0m
Model:
Tokens:
input: 76257
output: 41
thoughts: 56
Thoughts: Okay, let's break this down.
**Analyzing Potential Deadlock Scenarios**
Right, so the `blank_screen_t` timer callback... it's just scheduling `console_work`. That's it. It's not bothering with acquiring `console_sem`, which is good. That means `timer_delete_sync` isn't going to get tangled up in a deadlock situation with `console_sem`. That's a relief; we've averted a potential issue there.
But, hang on, what's the deal with `console_work` itself? We need to look at how *that* function interacts with `console_sem`. We still need to examine the function in detail before we can be fully confident in the absence of deadlock.
|
| 71/2 |
2026/05/20 01:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/tty/vt/vt.c IncludeLines:true Name:console_callback]
Results: map[SourceCode:3340: /*
3341: * This is the console switching callback.
3342: *
3343: * Doing console switching in a process context allows
3344: * us to do the switches asynchronously (needed when we want
3345: * to switch due to a keyboard interrupt). Synchronization
3346: * with other console code and prevention of re-entrancy is
3347: * ensured with console_lock.
3348: */
3349: static void console_callback(struct work_struct *ignored)
3350: {
3351: guard(console_lock)();
3352:
3353: if (want_console >= 0) {
3354: if (want_console != fg_console &&
3355: vc_cons_allocated(want_console)) {
3356: hide_cursor(vc_cons[fg_console].d);
3357: change_console(vc_cons[want_console].d);
3358: /* we only changed when the console had already
3359: been allocated - a new console is not created
3360: in an interrupt routine */
3361: }
3362: want_console = -1;
3363: }
3364: if (do_poke_blanked_console) { /* do not unblank for a LED change */
3365: do_poke_blanked_console = 0;
3366: poke_blanked_console();
3367: }
3368: if (scrollback_delta) {
3369: struct vc_data *vc = vc_cons[fg_console].d;
3370: clear_selection();
3371: if (vc->vc_mode == KD_TEXT && vc->vc_sw->con_scrolldelta)
3372: vc->vc_sw->con_scrolldelta(vc, scrollback_delta);
3373: scrollback_delta = 0;
3374: }
3375: if (blank_timer_expired) {
3376: do_blank_screen(0);
3377: blank_timer_expired = 0;
3378: }
3379: notify_update(vc_cons[fg_console].d);
3380: }
SourceFile:drivers/tty/vt/vt.c]
|
| 72/2 |
2026/05/20 01:56 |
llm |
expert |
0m
Model:
Tokens:
input: 77182
output: 41
thoughts: 5165
Thoughts: **Deep Dive into a Console Deadlock**
Alright, let's break this down. We've got a reported deadlock, `syz.9.5484` blocked on `console_sem`, which is itself a semaphore. My initial thought was the classic: is the `console_callback` routine hogging the semaphore while waiting for something else, like `tty_mutex`? Nope, not directly. We can rule out `fbcon_switch` too, no dice there.
The report nails it: `con_install` is called under the umbrella of `tty_mutex`. So, if `syz.9.5484` is *also* trying to grab `tty_mutex` while waiting for `console_sem`, we've got a textbook deadlock scenario. We search, but no direct `console_sem` then `tty_mutex` acquisition seems to happen. We consider other options.
But what if the lock isn't the problem? Could there be another lock or resource that a thread holding `console_sem` is blocked on? I immediately thought of file system locks - `dentry->d_lock`, `rcu_read_lock` etc. We see CPU 1, doing `__dentry_kill` leading to a call to `kasan_record_aux_stack`, leading to `unwind_next_frame`. Is CPU 1 looped in `finish_dput`? CPU 1 *isn't* holding the `console_sem`...but wait, what if it *was*? Does something called by `unlink` have any connection? Nope. But, if a `WARN` had been hit during this process, that could lead to `printk`, which calls `console_trylock` and potentially `console_unlock`, which calls `vt_console_print`. If it prints a message, it will *release* it. This isn't the issue. The NMI backtrace showed no `printk` or `console_unlock`. Good.
If CPU 1 is not holding it, who is? It isn't `khungtaskd`, because it hasn't printed anything yet. But the syz bot was stuck for 143 seconds before this happened, meaning `khungtaskd`'s printout is after the problem is manifest. It couldn't have been `khungtaskd` who held the lock.
Okay, let's look at the lock details. `syz.9.5484` *is* blocked on `console_sem` and we know the timer has run and it's been blocked for 143 seconds. That's a long time. Flood of `printk`? Again, we considered it. We can't see the specific code, but it is known. But what if a task called `console_lock` and just...vanished? Never scheduled again?
If CPU 1 is in a softlockup, that means it's stuck in `finish_dput`. If it's a softlockup, there is one CPU, CPU 0, that's available. If `console_sem` is held by a task that's bound to CPU 1, but CPU 1 is stuck, it will NEVER be released. Okay, if we check the stack, is there any process bound to CPU 1? A workqueue. `schedule_work` queues work on a local CPU. If `finish_dput` is running in process context (`udevd`), the kernel worker is going to be blocked! And CPU 1 is available. It needs to yield! But in `finish_dput` it isn't. So it's not being released.
So, *if* the `udevd` is not yielding, that has to be the problem. But if it does, the kworker thread on CPU 1, `kworker/1:X`, should run, execute `console_callback`, and release the semaphore. That should be the solution, it should not be held that long. But, maybe `console_work` isn't holding the semaphore.
Okay, what about `udevd` itself? `udevd` doesn't hold `console_sem`.
What if `finish_dput` is *not* yielding because `cond_resched()` is a no-op? Is it `CONFIG_PREEMPT_NONE` or `CONFIG_PREEMPT_VOLUNTARY`? But `PREEMPT(full)` is enabled. The timer interrupt *will* preempt `udevd`! `kworker` will run! It will release it!
What about the case that the held lock could also be from `udevd`? `dentry->d_lock`, `inode->i_lock`, not global, so
|
| 73/2 |
2026/05/20 01:57 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/tty/vt/vt.c IncludeLines:true Name:con_install]
Results: map[SourceCode:3733: /*
3734: * Allocate the console screen memory.
3735: */
3736: static int con_install(struct tty_driver *driver, struct tty_struct *tty)
3737: {
3738: unsigned int currcons = tty->index;
3739: struct vc_data *vc;
3740: int ret;
3741:
3742: guard(console_lock)();
3743: ret = vc_allocate(currcons);
3744: if (ret)
3745: return ret;
3746:
3747: vc = vc_cons[currcons].d;
3748:
3749: /* Still being freed */
3750: if (vc->port.tty)
3751: return -ERESTARTSYS;
3752:
3753: ret = tty_port_install(&vc->port, driver, tty);
3754: if (ret)
3755: return ret;
3756:
3757: tty->driver_data = vc;
3758: vc->port.tty = tty;
3759: tty_port_get(&vc->port);
3760:
3761: if (!tty->winsize.ws_row && !tty->winsize.ws_col) {
3762: tty->winsize.ws_row = vc_cons[currcons].d->vc_rows;
3763: tty->winsize.ws_col = vc_cons[currcons].d->vc_cols;
3764: }
3765: if (vc->vc_utf)
3766: tty->termios.c_iflag |= IUTF8;
3767: else
3768: tty->termios.c_iflag &= ~IUTF8;
3769:
3770: return 0;
3771: }
SourceFile:drivers/tty/vt/vt.c]
|
| 74/2 |
2026/05/20 01:57 |
llm |
expert |
1m
Model:
Tokens:
input: 83986
output: 66
thoughts: 7136
Thoughts: **Analysis of a Kernel Lock Contention Issue**
Alright, let's break this down. It seems we have a lock contention issue, most likely a Denial-of-Service (DoS) vulnerability. The core problem revolves around the `console_sem` semaphore and its interaction with the `tty_mutex`. `con_install` uses `guard(console_lock)()`, ensuring that `console_unlock()` is automatically called, so that's not the leak point. Other functions that handle the console also appear to release the lock correctly, such as `console_callback` and `vt_ioctl`.
The key here is understanding why tasks are blocking. The system is reporting tasks blocked on `console_sem` and, critically, `tty_mutex`. This suggests a chain reaction: one task (let's say `syz.X`) acquires `console_sem`, *then* calls `console_unlock()`. If, *while* holding this `console_sem` and *in* `console_unlock()` there's a flood of `printk` messages, the `console_unlock()` routine can get stuck. While looping, it holds `console_sem`.
If the `console_unlock()` routine is stuck, any other task attempting to acquire `console_sem` (e.g., `syz.9.5484` attempting to open a TTY and calling `con_install`) will block indefinitely. Moreover, if a task *also* holds `tty_mutex` and tries to call `console_lock()`, it blocks, so, any other task trying to open a TTY will block, this blocking the TTY subsystem, leading to a DoS! The report shows that `syz.9.5484` and others are blocked, which means someone else is holding `console_sem`, and based on the lock report, it isn't listed, nor is it in a list of hung tasks, which means it may be running, and the lock isn't getting released in a timely manner.
The fact that lockdep *doesn't* list `console_sem` as held is telling. We know that `console_sem` is tracked by lockdep. The issue must be that *some task* got the lock and exited while holding it (which isn't something that generates a warning). Or it got it, then went to interruptible sleep, and `khungtaskd` didn't report it since it only reports uninterruptible tasks.
The NMI backtrace from CPU 1 is likely a distraction, it is unrelated to the core issue. CPU 1 is likely busy with an unrelated task and its `unlink` process.
Since this issue involves blocking on the console, a flood of `printk` messages or a slow serial console, and given that the lock (`console_sem`) is held, but the holding task isn't listed, this is a likely DoS condition. The system is showing that tasks are blocking because of `console_sem` and `tty_mutex`. Since `tty_mutex` is involved, any unprivileged user can trigger this (by causing a flood of `printk`). The known syzkaller bug is likely the cause. So, unprivileged users can, indeed, cause this DoS. The vulnerability doesn't appear to be exploitable for privilege escalation; it's a denial-of-service condition.
|
| 75/2 |
2026/05/20 01:58 |
tool |
set-results |
0m
Args: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
|
| 76/2 |
2026/05/20 01:58 |
llm |
expert |
0m
Model:
Tokens:
input: 91972
output: 602
thoughts: 211
Thoughts: **Final Response: Hung Task Analysis**
Okay, the analysis is done, time to write up the response. Based on my review, this bug is a clear case of a hung task. It appears to be stemming from a lock contention or deadlock situation, specifically involving `tty_mutex` and `console_sem`. I'm seeing that `syz.9.5484` is the culprit, currently holding `tty_mutex` and waiting on `console_sem` within the `con_install` function.
Because `tty_mutex` is being held, it's blocking any other tasks that need to open a TTY, like `syz.5.5487` and `syz.4.5495`. Those are all blocked waiting to get access to the TTY. My guess is that the task holding the `console_sem` is either stuck in `console_unlock()`, probably spewing a ton of print messages, or it is blocked on another resource.
The end result here is a straightforward Denial of Service. The system becomes completely unresponsive, with tasks hanging indefinitely. Good news is that it's not a memory corruption issue, so there's no exploitable vulnerability for privilege escalation. However, I can see this being triggered by unprivileged users, they just need to open up TTY devices and potentially trigger a `printk` flood.
|
| 77/1 |
2026/05/20 01:58 |
action |
format-explanation |
0m
Results: map[Explanation:Based on the provided kernel bug report and source code analysis, here is the
assessment of the bug:
**Analysis:**
The bug report indicates a classic lock contention or deadlock scenario
resulting in hung tasks (`INFO: task blocked for more than 143 seconds`).
1. **Lock Contention/Deadlock:**
- Task `syz.9.5484` is in the process of opening a TTY device. It has
acquired the global `tty_mutex` (in `tty_open_by_driver` -> `tty_init_dev`)
and is now blocked trying to acquire the `console_sem` semaphore inside
`con_install()`.
- Because `syz.9.5484` is holding `tty_mutex`, numerous other tasks (e.g.,
`syz.5.5487`, `syz.4.5495`, etc.) that are also trying to open TTYs are
blocked waiting for `tty_mutex`.
- The `console_sem` is held by another task for an extended period. This is a
known architectural issue in the kernel's printk/console subsystem where a
task calling `console_unlock()` can be forced to flush a massive backlog of
kernel messages (a "printk flood") to slow consoles (like serial ports)
without yielding, effectively holding `console_sem` hostage for a very long
time.
2. **Impact:**
- **Denial of Service (DoS):** Yes. The system becomes unresponsive as tasks
hang indefinitely waiting for core locks like `tty_mutex` and `console_sem`.
The softlockup watchdog might also fire if the task holding the lock doesn't
yield.
- **Exploitable:** No. This is a synchronization and resource starvation
issue, not a memory safety violation (like use-after-free or out-of-bounds
access). It cannot be used for memory corruption or privilege escalation.
3. **Accessibility:**
- **Unprivileged / User Namespace:** Yes. Unprivileged users can open TTY
devices (`/dev/tty`, `/dev/ptmx`, etc.) and can often trigger kernel warnings
or printk floods (e.g., by intentionally passing malformed arguments to
certain syscalls) that exacerbate the `console_unlock` stall. No special
capabilities are required.
4. **Other Triggers:**
- The bug is a local DoS triggered via standard syscalls (like `openat` on
device nodes). It does not require malicious filesystems, network packets, or
peripheral devices to trigger.
**Conclusion:**
This is a local Denial of Service (DoS) vulnerability caused by lock starvation
involving `tty_mutex` and `console_sem`, typically exacerbated by a printk
flood. It is accessible to unprivileged users but is not exploitable for
privilege escalation.]
|