| 0/0 |
2026/05/22 20:26 |
flow |
assessment-security |
38m
Results: map[DenialOfService:true Explanation:Based on the analysis of the bug report and the kernel source code, here is the
assessment of the vulnerability:
**Bug Mechanism:**
The bug is a slab-use-after-free of a `struct dentry` object in the `ocfs2`
filesystem.
The crash occurs in `rt_mutex_slowunlock`, which is called from
`spin_unlock(&dentry->d_lock)` (on `PREEMPT_RT` kernels, spinlocks are
implemented as `rt_mutex`es). This happens during `dput(dentry)` called from
`end_dirop()` in the cleanup path of the `mknod` system call
(`filename_mknodat`).
The `dentry` is initially allocated by the `mknod` task. However, due to a race
condition with another concurrent filesystem operation (such as `creat` or
`unlink`), the `ocfs2` cluster locking code (`ocfs2_dentry_attach_lock` ->
`ocfs2_find_local_alias` -> `dput`) prematurely drops the reference count of
this `dentry` to 0. This causes the `dentry` to be killed and freed via RCU
(`__dentry_kill` -> `call_rcu`).
When the original `mknod` task resumes and reaches its cleanup path
(`end_dirop`), it calls `dput(dentry)` on the already freed object. The `dput`
function attempts to acquire and release the `dentry->d_lock` (which resides at
offset 208 inside the `d_lockref` field), leading to the KASAN use-after-free
report.
**Security Impact:**
* **Exploitable:** **Yes**. Use-after-free vulnerabilities on heavily used
kernel structures like `struct dentry` are generally exploitable. The structure
contains spinlocks and pointers (including `d_op` function pointers). An
attacker could potentially spray the heap to reallocate the freed `dentry` with
controlled data, leading to arbitrary code execution or memory corruption when
the kernel attempts to unlock the overwritten spinlock.
* **Denial Of Service:** **Yes**. Triggering the bug will cause a kernel panic
and crash the system.
* **Accessible From Unprivileged Processes:** **Yes**. The bug is triggered by
standard filesystem system calls (`mknod` for regular files/FIFOs, `creat`,
`unlink`). As long as an unprivileged user has write access to a directory
within an `ocfs2` mount, they can trigger the race condition.
* **Malicious Filesystem Trigger:** **No**. The bug is a concurrency issue in
the filesystem's runtime operations (specifically the distributed lock
management `dentry_attach_lock`), not a vulnerability in parsing a maliciously
crafted filesystem image.
**Conclusion:**
This is a local privilege escalation and denial-of-service vulnerability
reachable by unprivileged users interacting with an `ocfs2` filesystem. Exploitable:true FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/05/22 20:26 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/05/22 20:26 |
action |
kernel-checkouter |
1m
Results: map[KernelSrc:/app/workdir/cache/src/a6dd3c7c36afbd009cb1a2e2b3c929bc0d13fb93]
|
| 3/1 |
2026/05/22 20:27 |
action |
kernel-builder |
17m
Results: map[KernelObj:/app/workdir/cache/build/050edac12a46eb35ff1748048118496eab891a6c]
|
| 4/1 |
2026/05/22 20:44 |
action |
codesearch-prepare |
12m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/05/22 20:57 |
agent |
expert |
7m
Model:
Results: map[DenialOfService:true Exploitable:true FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:false 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).
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.
Don't make assumptions about the kernel source code (it may be different from what you assume it is).
Extensively use the provided code access tools (codesearch-*, git-*, grepper, etc)
to examine the actual source code, and confirm any assumptions.
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:
==================================================================
BUG: KASAN: slab-use-after-free in __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:132 [inline]
BUG: KASAN: slab-use-after-free in _raw_spin_lock_irqsave+0x40/0x60 kernel/locking/spinlock.c:166
Read of size 1 at addr ffff8880632827b0 by task syz.0.740/12158
CPU: 0 UID: 0 PID: 12158 Comm: syz.0.740 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_address_description+0x55/0x1e0 mm/kasan/report.c:378
print_report+0x58/0x70 mm/kasan/report.c:482
kasan_report+0x117/0x150 mm/kasan/report.c:595
__kasan_check_byte+0x2a/0x40 mm/kasan/common.c:574
kasan_check_byte include/linux/kasan.h:402 [inline]
lock_acquire+0x84/0x350 kernel/locking/lockdep.c:5842
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:132 [inline]
_raw_spin_lock_irqsave+0x40/0x60 kernel/locking/spinlock.c:166
rt_mutex_slowunlock+0xbf/0x8b0 kernel/locking/rtmutex.c:1426
end_dirop fs/namei.c:2952 [inline]
end_creating include/linux/namei.h:123 [inline]
end_creating_path fs/namei.c:4990 [inline]
filename_mknodat+0x464/0x670 fs/namei.c:5184
__do_sys_mknod fs/namei.c:5207 [inline]
__se_sys_mknod+0x3a/0x150 fs/namei.c:5204
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: 0033:0x7fcd4a24c819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fcd3fc41028 EFLAGS: 00000246 ORIG_RAX: 0000000000000085
RAX: ffffffffffffffda RBX: 00007fcd4a4c6270 RCX: 00007fcd4a24c819
RDX: 0000000000000701 RSI: 100000000000600d RDI: 0000200000000080
RBP: 00007fcd4a2e2c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fcd4a4c6308 R14: 00007fcd4a4c6270 R15: 00007ffc4aeb1dc8
</TASK>
Allocated by task 12158:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
unpoison_slab_object mm/kasan/common.c:340 [inline]
__kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:366
kasan_slab_alloc include/linux/kasan.h:253 [inline]
slab_post_alloc_hook mm/slub.c:4569 [inline]
slab_alloc_node mm/slub.c:4898 [inline]
kmem_cache_alloc_lru_noprof+0x33c/0x680 mm/slub.c:4917
__d_alloc+0x37/0x6f0 fs/dcache.c:1740
d_alloc+0x4b/0x190 fs/dcache.c:1819
lookup_one_qstr_excl+0xd8/0x360 fs/namei.c:1801
__start_dirop fs/namei.c:2916 [inline]
start_dirop fs/namei.c:2938 [inline]
filename_create+0x20e/0x370 fs/namei.c:4950
filename_mknodat+0x18c/0x670 fs/namei.c:5158
__do_sys_mknod fs/namei.c:5207 [inline]
__se_sys_mknod+0x3a/0x150 fs/namei.c:5204
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
Freed by task 20:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:253 [inline]
__kasan_slab_free+0x5c/0x80 mm/kasan/common.c:285
kasan_slab_free include/linux/kasan.h:235 [inline]
slab_free_hook mm/slub.c:2689 [inline]
slab_free mm/slub.c:6246 [inline]
kmem_cache_free+0x187/0x6c0 mm/slub.c:6373
rcu_do_batch kernel/rcu/tree.c:2617 [inline]
rcu_core kernel/rcu/tree.c:2869 [inline]
rcu_cpu_kthread+0x99e/0x1470 kernel/rcu/tree.c:2957
smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
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
Last potentially related work creation:
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+0x469/0x5e0 fs/dcache.c:689
finish_dput+0xc9/0x480 fs/dcache.c:879
ocfs2_dentry_attach_lock+0x72c/0xd50 fs/ocfs2/dcache.c:328
ocfs2_lookup+0x3fc/0xa20 fs/ocfs2/namei.c:163
lookup_open fs/namei.c:4484 [inline]
open_last_lookups fs/namei.c:4611 [inline]
path_openat+0x11cb/0x38a0 fs/namei.c:4855
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_creat fs/open.c:1448 [inline]
__se_sys_creat fs/open.c:1442 [inline]
__x64_sys_creat+0x8f/0xc0 fs/open.c:1442
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
The buggy address belongs to the object at ffff8880632826e0
which belongs to the cache dentry of size 376
The buggy address is located 208 bytes inside of
freed 376-byte region [ffff8880632826e0, ffff888063282858)
The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x63282
head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
memcg:ffff888063283ef1
flags: 0x80000000000040(head|node=0|zone=1)
page_type: f5(slab)
raw: 0080000000000040 ffff88801b6c33c0 dead000000000100 dead000000000122
raw: 0000000000000000 0000000800120012 00000000f5000000 ffff888063283ef1
head: 0080000000000040 ffff88801b6c33c0 dead000000000100 dead000000000122
head: 0000000000000000 0000000800120012 00000000f5000000 ffff888063283ef1
head: 0080000000000001 ffffffffffffff81 00000000ffffffff 00000000ffffffff
head: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000002
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 1, migratetype Reclaimable, gfp_mask 0xd20d0(__GFP_RECLAIMABLE|__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5183, tgid 5183 (udevd), ts 108377682153, free_ts 0
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x231/0x280 mm/page_alloc.c:1858
prep_new_page mm/page_alloc.c:1866 [inline]
get_page_from_freelist+0x27c8/0x2840 mm/page_alloc.c:3946
__alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5226
alloc_slab_page mm/slub.c:3278 [inline]
allocate_slab+0x77/0x660 mm/slub.c:3467
new_slab mm/slub.c:3525 [inline]
refill_objects+0x33c/0x3d0 mm/slub.c:7251
refill_sheaf mm/slub.c:2816 [inline]
__pcs_replace_empty_main+0x373/0x720 mm/slub.c:4651
alloc_from_pcs mm/slub.c:4749 [inline]
slab_alloc_node mm/slub.c:4883 [inline]
kmem_cache_alloc_lru_noprof+0x433/0x680 mm/slub.c:4917
__d_alloc+0x37/0x6f0 fs/dcache.c:1740
d_alloc+0x4b/0x190 fs/dcache.c:1819
lookup_one_qstr_excl+0xd8/0x360 fs/namei.c:1801
__start_dirop fs/namei.c:2916 [inline]
start_dirop fs/namei.c:2938 [inline]
filename_unlinkat+0x2c0/0x610 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
page_owner free stack trace missing
Memory state around the buggy address:
ffff888063282680: fb fb fb fb fc fc fc fc fc fc fc fc fa fb fb fb
ffff888063282700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff888063282780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff888063282800: fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc fc
ffff888063282880: fc fc fc 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================
Reply: Based on the analysis of the bug report and the kernel source code, here is the assessment of the vulnerability:
**Bug Mechanism:**
The bug is a slab-use-after-free of a `struct dentry` object in the `ocfs2` filesystem.
The crash occurs in `rt_mutex_slowunlock`, which is called from `spin_unlock(&dentry->d_lock)` (on `PREEMPT_RT` kernels, spinlocks are implemented as `rt_mutex`es). This happens during `dput(dentry)` called from `end_dirop()` in the cleanup path of the `mknod` system call (`filename_mknodat`).
The `dentry` is initially allocated by the `mknod` task. However, due to a race condition with another concurrent filesystem operation (such as `creat` or `unlink`), the `ocfs2` cluster locking code (`ocfs2_dentry_attach_lock` -> `ocfs2_find_local_alias` -> `dput`) prematurely drops the reference count of this `dentry` to 0. This causes the `dentry` to be killed and freed via RCU (`__dentry_kill` -> `call_rcu`).
When the original `mknod` task resumes and reaches its cleanup path (`end_dirop`), it calls `dput(dentry)` on the already freed object. The `dput` function attempts to acquire and release the `dentry->d_lock` (which resides at offset 208 inside the `d_lockref` field), leading to the KASAN use-after-free report.
**Security Impact:**
* **Exploitable:** **Yes**. Use-after-free vulnerabilities on heavily used kernel structures like `struct dentry` are generally exploitable. The structure contains spinlocks and pointers (including `d_op` function pointers). An attacker could potentially spray the heap to reallocate the freed `dentry` with controlled data, leading to arbitrary code execution or memory corruption when the kernel attempts to unlock the overwritten spinlock.
* **Denial Of Service:** **Yes**. Triggering the bug will cause a kernel panic and crash the system.
* **Accessible From Unprivileged Processes:** **Yes**. The bug is triggered by standard filesystem system calls (`mknod` for regular files/FIFOs, `creat`, `unlink`). As long as an unprivileged user has write access to a directory within an `ocfs2` mount, they can trigger the race condition.
* **Malicious Filesystem Trigger:** **No**. The bug is a concurrency issue in the filesystem's runtime operations (specifically the distributed lock management `dentry_attach_lock`), not a vulnerability in parsing a maliciously crafted filesystem image.
**Conclusion:**
This is a local privilege escalation and denial-of-service vulnerability reachable by unprivileged users interacting with an `ocfs2` filesystem.
|
| 6/2 |
2026/05/22 20:57 |
llm |
expert |
0m
Model:
Tokens:
input: 9734
output: 39
thoughts: 200
Thoughts: **Analyzing a KASAN Slab-Use-After-Free Report**
Okay, so I've got a KASAN report, specifically a slab-use-after-free situation. The bug is pinpointed within the `_raw_spin_lock_irqsave` function. That's a low-level spinlock function, so it's likely a concurrency issue. The call stack reveals this function is called from `rt_mutex_slowunlock`, which is in turn called from `end_dirop`, and that last function is in `fs/namei.c`. The problematic access is a read, and it's accessing a single byte at `ffff8880632827b0`. The object in question is a `dentry`, which is 376 bytes, and this buggy address is 208 bytes into that freed memory region. That's a crucial detail. I need to understand what's happening to that `dentry` and when it's being freed, and why this later access is occurring.
So, let's start with `fs/namei.c` and focus on `end_dirop`. The call stack information shows that `end_dirop` is called by `end_creating`, which is called by `end_creating_path`, which is then called by `filename_mknodat`. My immediate priority is to examine the code in `fs/namei.c`, starting with `end_dirop`. I need to trace the control flow and understand the timing of these functions relative to the allocation and deallocation of the `dentry` to identify the potential race condition or incorrect use of memory.
|
| 7/2 |
2026/05/22 20:57 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:end_dirop]
Results: map[SourceCode:2941: /**
2942: * end_dirop - signal completion of a dirop
2943: * @de: the dentry which was returned by start_dirop or similar.
2944: *
2945: * If the de is an error, nothing happens. Otherwise any lock taken to
2946: * protect the dentry is dropped and the dentry itself is release (dput()).
2947: */
2948: void end_dirop(struct dentry *de)
2949: {
2950: if (!IS_ERR(de)) {
2951: inode_unlock(de->d_parent->d_inode);
2952: dput(de);
2953: }
2954: }
SourceFile:fs/namei.c]
|
| 8/2 |
2026/05/22 20:57 |
llm |
expert |
0m
Model:
Tokens:
input: 10569
output: 38
thoughts: 275
Thoughts: **Debugging a Kernel RT-Mutex Crash**
Okay, so I've got a crash, and the stack trace points directly at `rt_mutex_slowunlock`. That's not good. The trace says it’s called from `end_dirop`, which makes me think this is likely a file system issue. `end_dirop` itself calls `inode_unlock` on the parent directory inode and then does a `dput`. Hmm. Let me see... the key lines are from `end_dirop` and `filename_mknodat`.
Let me double-check the code. Line 2952, right, that's where `dput(de);` is. But the trace is showing `rt_mutex_slowunlock` *after* `inode_unlock`. Wait, `inode_unlock` is just a wrapper for `up_write(&inode->i_rwsem)`. If PREEMPT_RT is enabled, then the `rwsem` will be implemented as an `rt_mutex`. So, is `inode_unlock` *actually* calling `rt_mutex_slowunlock`? Right. That makes sense. Need to confirm that. Let me look at `inode_unlock` again...
|
| 9/2 |
2026/05/22 20:57 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:inode_unlock]
Results: map[SourceCode:1037: static inline void inode_unlock(struct inode *inode)
1038: {
1039: up_write(&inode->i_rwsem);
1040: }
SourceFile:include/linux/fs.h]
|
| 10/2 |
2026/05/22 20:57 |
llm |
expert |
0m
Model:
Tokens:
input: 11228
output: 37
thoughts: 8
Thoughts: Okay, here's my attempt at summarizing the thought process, as requested, tailored for an expert audience and in the first person:
**Checking `dput`**
Alright, let's take a look at this. The first thing I always do when I encounter something potentially requiring an output is to consider `dput`. I mean, it's practically ingrained at this point. `dput` provides such a clean, replicable way to share data structures, especially those complex objects that often trip people up. It’s concise and eliminates so many potential issues. It's the go-to for ensuring reproducibility, reducing the back-and-forth, and just generally making collaboration with colleagues much smoother. Honestly, I'm already mentally evaluating if the structure here is even worth the time to work with if there's no way to reproduce the output with a `dput` call. Efficiency is key! Now, let's just make sure it's easily callable...
|
| 11/2 |
2026/05/22 20:57 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:dput]
Results: map[SourceCode: 909: /*
910: * dput - release a dentry
911: * @dentry: dentry to release
912: *
913: * Release a dentry. This will drop the usage count and if appropriate
914: * call the dentry unlink method as well as removing it from the queues and
915: * releasing its resources. If the parent dentries were scheduled for release
916: * they too may now get deleted.
917: */
918: void dput(struct dentry *dentry)
919: {
920: if (!dentry)
921: return;
922: might_sleep();
923: rcu_read_lock();
924: if (likely(fast_dput(dentry))) {
925: rcu_read_unlock();
926: return;
927: }
928: finish_dput(dentry);
929: }
SourceFile:fs/dcache.c]
|
| 12/2 |
2026/05/22 20:57 |
llm |
expert |
0m
Model:
Tokens:
input: 11789
output: 39
thoughts: 468
Thoughts: **Debugging a Kernel Use-After-Free in Filesystem Code**
Okay, the stack trace points me to `rt_mutex_slowunlock` within the kernel's locking code, which then leads to `end_dirop` in `fs/namei.c`. Specifically, the stack is showing a crash on line 2952, which is `dput(de)`. Now, `dput` itself calls `rcu_read_lock` and `fast_dput`. Wait a second. The crucial part here is that `rt_mutex_slowunlock` is invoked from `inode_unlock`, which, if the line numbers are correct, is right *before* that, at line 2951.
I need to confirm what's happening at those lines in `fs/namei.c`. Let's see… line 2952: that's `dput(de)`. And `end_dirop` is defined as:
```c
void end_dirop(struct dentry *de)
{
if (!IS_ERR(de)) {
inode_unlock(de->d_parent->d_inode);
dput(de);
}
}
```
So the crash *could* be within the `dput(de)` function or, since `inode_unlock` is inline here, somewhere in the call to that. It might be assigned to line 2951, or 2952.
The crash report says the freed object is a `dentry`. Okay, it gives the address of the freed object: `ffff8880632826e0`, and specifies that it's part of a `dentry` cache, with a size of 376 bytes. The report states that "The buggy address is located 208 bytes inside of freed 376-byte region".
Given that information, it looks like either `de` *or* `de->d_parent` is being accessed after being freed. And considering that `de->d_parent` is accessed right before `dput(de)` in `inode_unlock(de->d_parent->d_inode);`, it means we may be dealing with a classic use-after-free issue. If `de` is freed, then accessing `de->d_parent` will be a use-after-free.
I'm going to investigate that offset of 208 bytes within the `struct dentry`. That's where I need to focus my attention.
|
| 13/2 |
2026/05/22 20:57 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:fs/namei.c FieldOffset:208 Name:dentry]
Results: map[Fields:[map[Name:d_lockref OffsetBits:1472 SizeBits:1088]]]
|
| 14/2 |
2026/05/22 20:57 |
llm |
expert |
0m
Model:
Tokens:
input: 12896
output: 37
thoughts: 42
Thoughts: Okay, here's my take on those thoughts, framed as my own:
**Analyzing Memory Layout - Quick Math & Verification**
Alright, so I'm looking at this offset, 208, and immediately my brain jumps to bits. The number 1664 comes up, a gut feeling suggests it's related. I quickly do the mental math - 208 multiplied by 8 *does* indeed equal 1664. That likely implies we're dealing with byte offsets somewhere, which is a very reasonable assumption in this context. It's almost certain this is bit manipulation to figure out byte sizes.
Now, I need to confirm this understanding, given my expert understanding of kernel internals. I need to refresh my knowledge of the `struct dentry` layout to be certain. I need a bit more data about this `dentry` structure itself to confirm my understanding of this offset to start to make an intelligent guess about its function.
|
| 15/2 |
2026/05/22 20:57 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:fs/namei.c FieldOffset:<nil> Name:dentry]
Results: map[Fields:[map[Name:d_flags OffsetBits:0 SizeBits:32] map[Name:d_seq OffsetBits:64 SizeBits:448] map[Name:d_hash OffsetBits:512 SizeBits:128] map[Name:d_parent OffsetBits:640 SizeBits:64] map[Name: OffsetBits:704 SizeBits:128] map[Name:d_inode OffsetBits:832 SizeBits:64] map[Name:d_shortname OffsetBits:896 SizeBits:320] map[Name:d_op OffsetBits:1216 SizeBits:64] map[Name:d_sb OffsetBits:1280 SizeBits:64] map[Name:d_time OffsetBits:1344 SizeBits:64] map[Name:d_fsdata OffsetBits:1408 SizeBits:64] map[Name:d_lockref OffsetBits:1472 SizeBits:1088] map[Name: OffsetBits:2560 SizeBits:128] map[Name:d_sib OffsetBits:2688 SizeBits:128] map[Name:d_children OffsetBits:2816 SizeBits:64] map[Name:d_u OffsetBits:2880 SizeBits:128]]]
|
| 16/2 |
2026/05/22 20:57 |
llm |
expert |
0m
Model:
Tokens:
input: 13535
output: 39
thoughts: 244
Thoughts: **Analyzing a Dentry Lock Contention Issue**
Okay, so I'm looking at this crash dump, and immediately something jumps out at me. The crash is related to a spinlock, specifically inside `_raw_spin_lock_irqsave`, which is being called from `rt_mutex_slowunlock`. Now, I know `rt_mutex_slowunlock` *should* be grabbing a lock on an `rt_mutex`... that's usually within the `inode->i_rwsem`. But the offset and size calculations are key here – `d_lockref` is at offset 184 bytes and size of 136 bytes, so 208 is inside `d_lockref`. The crash is happening within this section of memory. That's immediately interesting because `d_lockref` typically *contains* a spinlock.
Wait a second. The stack shows a lock being taken on the `rt_mutex` which, as I mentioned, lives within the inode, and somehow it's accessing the `d_lockref` of a dentry. That just seems off. Why would we be hitting a dentry spinlock in the context of releasing an `rt_mutex`? I have to trace the path backward.
Okay, let's think... the dentry is likely being freed, which triggers a `dput`. And `dput` usually uses `rcu_read_lock` and then calls `fast_dput`. Bingo! Now it's starting to make sense. `fast_dput` is the culprit. That function is known to take `spin_lock(&dentry->d_lock)`. That's where I need to focus my attention. It seems my crash is related to contention on the dentry lock. Let's dig deeper into the code for `fast_dput` now.
|
| 17/2 |
2026/05/22 20:57 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/dcache.c IncludeLines:true Name:fast_dput]
Results: map[SourceCode: 803: /*
804: * Try to do a lockless dput(), and return whether that was successful.
805: *
806: * If unsuccessful, we return false, having already taken the dentry lock.
807: * In that case refcount is guaranteed to be zero and we have already
808: * decided that it's not worth keeping around.
809: *
810: * The caller needs to hold the RCU read lock, so that the dentry is
811: * guaranteed to stay around even if the refcount goes down to zero!
812: */
813: static inline bool fast_dput(struct dentry *dentry)
814: {
815: int ret;
816:
817: /*
818: * try to decrement the lockref optimistically.
819: */
820: ret = lockref_put_return(&dentry->d_lockref);
821:
822: /*
823: * If the lockref_put_return() failed due to the lock being held
824: * by somebody else, the fast path has failed. We will need to
825: * get the lock, and then check the count again.
826: */
827: if (unlikely(ret < 0)) {
828: spin_lock(&dentry->d_lock);
829: if (WARN_ON_ONCE(dentry->d_lockref.count <= 0)) {
830: spin_unlock(&dentry->d_lock);
831: return true;
832: }
833: dentry->d_lockref.count--;
834: goto locked;
835: }
836:
837: /*
838: * If we weren't the last ref, we're done.
839: */
840: if (ret)
841: return true;
842:
843: /*
844: * Can we decide that decrement of refcount is all we needed without
845: * taking the lock? There's a very common case when it's all we need -
846: * dentry looks like it ought to be retained and there's nothing else
847: * to do.
848: */
849: if (retain_dentry(dentry, false))
850: return true;
851:
852: /*
853: * Either not worth retaining or we can't tell without the lock.
854: * Get the lock, then. We've already decremented the refcount to 0,
855: * but we'll need to re-check the situation after getting the lock.
856: */
857: spin_lock(&dentry->d_lock);
858:
859: /*
860: * Did somebody else grab a reference to it in the meantime, and
861: * we're no longer the last user after all? Alternatively, somebody
862: * else could have killed it and marked it dead. Either way, we
863: * don't need to do anything else.
864: */
865: locked:
866: if (dentry->d_lockref.count || retain_dentry(dentry, true)) {
867: spin_unlock(&dentry->d_lock);
868: return true;
869: }
870: return false;
871: }
SourceFile:fs/dcache.c]
|
| 18/2 |
2026/05/22 20:57 |
llm |
expert |
0m
Model:
Tokens:
input: 15253
output: 43
thoughts: 524
Thoughts: **Debugging a Dentry Use-After-Free in the Kernel**
Okay, the stack trace is staring me in the face, and it's pointing to something interesting. It looks like `_raw_spin_lock_irqsave`, then `rt_mutex_slowunlock`, and then it's inlined in `end_dirop` at `fs/namei.c:2952`. Right, that's the starting point. I know `rt_mutex_slowunlock` is called from `end_dirop`, that much is clear. But wait, `rt_mutex_slowunlock` is also called by `inode_unlock`, isn't it? And `inode_unlock` operates on `inode->i_rwsem`, which could be an `rt_mutex` if we're in a PREEMPT_RT environment. If that's the case, then `inode_unlock` calls `up_write`, which ultimately ends up calling `rt_mutex_slowunlock`.
Now, the crucial question is why `rt_mutex_slowunlock` is accessing `dentry->d_lockref`. The stack trace reveals that the freed object is a `dentry`... it's not a mutex itself. And the trace confirms the free operation... and that, in the critical path, the code is accessing `de->d_parent->d_inode`. If `de` has been freed, then accessing `de->d_parent` is a classic Use-After-Free scenario. Let's see... `de->d_parent` is at offset 80, which is 640 bits... and `de->d_parent->d_inode` is at offset 104, or 832 bits. But the buggy address is 208 bytes into the freed `dentry`. 208 bytes corresponds to `d_lockref`.
So, if `de` is freed, the problem is that `dput(de)` is accessing `de->d_lockref` after the object has been freed. The stack trace matches: `dput(de)` is indeed at line 2952. It all comes together. `rt_mutex_slowunlock` is at line 1426, so let's jump over there in `kernel/locking/rtmutex.c` and dig in.
|
| 19/2 |
2026/05/22 20:58 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/locking/rtmutex.c IncludeLines:true Name:rt_mutex_slowunlock]
Results: map[SourceCode:1417: /*
1418: * Slow path to release a rt-mutex.
1419: */
1420: static void __sched rt_mutex_slowunlock(struct rt_mutex_base *lock)
1421: {
1422: DEFINE_RT_WAKE_Q(wqh);
1423: unsigned long flags;
1424:
1425: /* irqsave required to support early boot calls */
1426: raw_spin_lock_irqsave(&lock->wait_lock, flags);
1427:
1428: debug_rt_mutex_unlock(lock);
1429:
1430: /*
1431: * We must be careful here if the fast path is enabled. If we
1432: * have no waiters queued we cannot set owner to NULL here
1433: * because of:
1434: *
1435: * foo->lock->owner = NULL;
1436: * rtmutex_lock(foo->lock); <- fast path
1437: * free = atomic_dec_and_test(foo->refcnt);
1438: * rtmutex_unlock(foo->lock); <- fast path
1439: * if (free)
1440: * kfree(foo);
1441: * raw_spin_unlock(foo->lock->wait_lock);
1442: *
1443: * So for the fastpath enabled kernel:
1444: *
1445: * Nothing can set the waiters bit as long as we hold
1446: * lock->wait_lock. So we do the following sequence:
1447: *
1448: * owner = rt_mutex_owner(lock);
1449: * clear_rt_mutex_waiters(lock);
1450: * raw_spin_unlock(&lock->wait_lock);
1451: * if (cmpxchg(&lock->owner, owner, 0) == owner)
1452: * return;
1453: * goto retry;
1454: *
1455: * The fastpath disabled variant is simple as all access to
1456: * lock->owner is serialized by lock->wait_lock:
1457: *
1458: * lock->owner = NULL;
1459: * raw_spin_unlock(&lock->wait_lock);
1460: */
1461: while (!rt_mutex_has_waiters(lock)) {
1462: /* Drops lock->wait_lock ! */
1463: if (unlock_rt_mutex_safe(lock, flags) == true)
1464: return;
1465: /* Relock the rtmutex and try again */
1466: raw_spin_lock_irqsave(&lock->wait_lock, flags);
1467: }
1468:
1469: /*
1470: * The wakeup next waiter path does not suffer from the above
1471: * race. See the comments there.
1472: *
1473: * Queue the next waiter for wakeup once we release the wait_lock.
1474: */
1475: mark_wakeup_next_waiter(&wqh, lock);
1476: raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
1477:
1478: rt_mutex_wake_up_q(&wqh);
1479: }
SourceFile:kernel/locking/rtmutex.c]
|
| 20/2 |
2026/05/22 20:58 |
llm |
expert |
0m
Model:
Tokens:
input: 17318
output: 40
thoughts: 583
Thoughts: **The Dentry Demise: A PREEMPT_RT Spinlock Saga**
Okay, let me piece this together. The crash in `rt_mutex_slowunlock` is accessing `lock->wait_lock`. If `lock` is the semaphore for an inode I'd expect the offset to be... wait. The report specifies the address is within a `dentry` and it's 208 bytes into a freed one of size 376. Hmm. Is the `lock` actually `dentry->d_lockref`?
No, hold on... `dput` calls `fast_dput` which then calls `lockref_put_return`, and if that fails, it grabs a spinlock. But `dentry->d_lock` is an alias for `dentry->d_lockref.lock`. Aha! On PREEMPT_RT, a `spinlock_t` *is* an `rt_mutex`. So `spin_lock(&dentry->d_lock)` is an `rt_mutex_lock` and `spin_unlock(&dentry->d_lock)` is an `rt_mutex_unlock`.
The stack trace confirms this: `_raw_spin_lock_irqsave`, `rt_mutex_slowunlock`, `end_dirop`. And `end_dirop` is what's calling `dput(de)`. `dput(de)` is calling `fast_dput(de)`. And then `fast_dput(de)` calls `spin_unlock(&dentry->d_lock)`. That's where it's happening! `spin_unlock` on PREEMPT_RT calls `rt_mutex_unlock`, which calls `rt_mutex_slowunlock`. And `rt_mutex_slowunlock` is accessing `raw_spin_lock_irqsave(&lock->wait_lock, flags)`.
Therefore, the `lock` is indeed `&dentry->d_lock`. And the `lock->wait_lock` is the *inner* `raw_spinlock_t` within the `rt_mutex` that is inside the `spinlock_t` that is inside the `d_lockref`. This perfectly explains the crash: It's happening in `dput(de)` because `de` is already freed. That explains the out-of-bounds access.
The critical question is, *why* is the `de` already freed? Time to trace `filename_mknodat` and figure out what caused this.
|
| 21/2 |
2026/05/22 20:58 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:filename_mknodat]
Results: map[SourceCode:5144: int filename_mknodat(int dfd, struct filename *name, umode_t mode,
5145: unsigned int dev)
5146: {
5147: struct delegated_inode di = { };
5148: struct mnt_idmap *idmap;
5149: struct dentry *dentry;
5150: struct path path;
5151: int error;
5152: unsigned int lookup_flags = 0;
5153:
5154: error = may_mknod(mode);
5155: if (error)
5156: return error;
5157: retry:
5158: dentry = filename_create(dfd, name, &path, lookup_flags);
5159: if (IS_ERR(dentry))
5160: return PTR_ERR(dentry);
5161:
5162: error = security_path_mknod(&path, dentry,
5163: mode_strip_umask(path.dentry->d_inode, mode), dev);
5164: if (error)
5165: goto out2;
5166:
5167: idmap = mnt_idmap(path.mnt);
5168: switch (mode & S_IFMT) {
5169: case 0: case S_IFREG:
5170: error = vfs_create(idmap, dentry, mode, &di);
5171: if (!error)
5172: security_path_post_mknod(idmap, dentry);
5173: break;
5174: case S_IFCHR: case S_IFBLK:
5175: error = vfs_mknod(idmap, path.dentry->d_inode,
5176: dentry, mode, new_decode_dev(dev), &di);
5177: break;
5178: case S_IFIFO: case S_IFSOCK:
5179: error = vfs_mknod(idmap, path.dentry->d_inode,
5180: dentry, mode, 0, &di);
5181: break;
5182: }
5183: out2:
5184: end_creating_path(&path, dentry);
5185: if (is_delegated(&di)) {
5186: error = break_deleg_wait(&di);
5187: if (!error)
5188: goto retry;
5189: }
5190: if (retry_estale(error, lookup_flags)) {
5191: lookup_flags |= LOOKUP_REVAL;
5192: goto retry;
5193: }
5194: return error;
5195: }
SourceFile:fs/namei.c]
|
| 22/2 |
2026/05/22 20:58 |
llm |
expert |
0m
Model:
Tokens:
input: 19294
output: 40
thoughts: 78
Thoughts: Okay, let's break this down.
**Diving into the Filesystem Creation Process**
Alright, so `filename_mknodat` is our starting point. It seems that this function is responsible for initiating the process of creating a new file or directory, given a filename and some context. Crucially, it's calling `filename_create`. This suggests `filename_create` is the workhorse here, actually handling the low-level dentry creation. `filename_create` returns a `dentry`, which makes perfect sense – we're building up the filesystem tree, one dentry at a time.
Now, things get interesting. After obtaining that `dentry`, `filename_mknodat` must perform some operations. We're not told what they are specifically yet, but presumably these are the necessary setup steps that are needed before finally calling `end_creating_path`. It makes me want to look into `filename_mknodat` to see what those operations are!
Then, the critical bit: we're calling `end_creating_path(&path, dentry)`. This sounds like the cleanup and finalization of the creation process. And within that, the code is calling `end_creating(&path.dentry, dentry)`. The double referencing, with the ampersand, is standard here in C, and I'd expect it. *But wait*. Let's pause and double-check exactly what's going on within `end_creating_path`. It's worth digging into that function to understand how the path information is being integrated with the new `dentry` and how the whole thing is being completed.
|
| 23/2 |
2026/05/22 20:58 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:end_creating_path]
Results: map[SourceCode:4977: /**
4978: * end_creating_path - finish a code section started by start_creating_path()
4979: * @path: the path instantiated by start_creating_path()
4980: * @dentry: the dentry returned by start_creating_path()
4981: *
4982: * end_creating_path() will unlock and locks taken by start_creating_path()
4983: * and drop an references that were taken. It should only be called
4984: * if start_creating_path() returned a non-error.
4985: * If vfs_mkdir() was called and it returned an error, that error *should*
4986: * be passed to end_creating_path() together with the path.
4987: */
4988: void end_creating_path(const struct path *path, struct dentry *dentry)
4989: {
4990: end_creating(dentry);
4991: mnt_drop_write(path->mnt);
4992: path_put(path);
4993: }
SourceFile:fs/namei.c]
|
| 24/2 |
2026/05/22 20:58 |
llm |
expert |
0m
Model:
Tokens:
input: 20090
output: 38
thoughts: 12
Thoughts: Here's my understanding of the thought process, from my expert perspective:
**Thinking About `end_creating` and `dentry`**
Okay, so I'm processing something, and the key here is the function `end_creating`. I'm immediately recognizing that it takes a `dentry` as an argument. That's a crucial point, and it's making me consider the implications within my area of expertise. I'm thinking about how the `dentry` is being used, what the context is, and what this argument implies about the overall operation. This detail is directing my focus to how the system interacts with directory entries during the creation process, and I'm mentally checking what I know about this function. I'm already starting to build a picture of the complete design.
|
| 25/2 |
2026/05/22 20:58 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:end_creating]
Results: map[SourceCode: 106: /* end_creating - finish action started with start_creating
107: * @child: dentry returned by start_creating() or vfs_mkdir()
108: *
109: * Unlock and release the child. This can be called after
110: * start_creating() whether that function succeeded or not,
111: * but it is not needed on failure.
112: *
113: * If vfs_mkdir() was called then the value returned from that function
114: * should be given for @child rather than the original dentry, as vfs_mkdir()
115: * may have provided a new dentry.
116: *
117: *
118: * If vfs_mkdir() was not called, then @child will be a valid dentry and
119: * @parent will be ignored.
120: */
121: static inline void end_creating(struct dentry *child)
122: {
123: end_dirop(child);
124: }
SourceFile:include/linux/namei.h]
|
| 26/2 |
2026/05/22 20:58 |
llm |
expert |
0m
Model:
Tokens:
input: 20635
output: 39
thoughts: 175
Thoughts: **Investigating Potential Dentry Lifecycle Issues**
Okay, hold on a second. I need to be absolutely sure about this. Could either `vfs_create` or `vfs_mknod` have *inadvertently* freed the `dentry`? This is a crucial point, and I need to verify it. I'm going to carefully review the implementation of both `vfs_create` and `vfs_mknod` to be certain.
Hmm, let's see. The `vfs_create` function *does* take the `dentry` as an argument. Does the function then go ahead and release it? That would be a major problem if it does. I need to understand that side of things.
Right, let me think. Ah, here's a comment: "If `vfs_mkdir()` was called then the value returned from that function should be given for `@child` rather than the original dentry, as `vfs_mkdir()` may have provided a new dentry." This is interesting because the code path is calling `filename_mknodat`, which then calls either `vfs_create` or `vfs_mknod`.
Wait, something seems wrong! `vfs_create` returns an `int` -- not a `struct dentry *`! This is key. The returned integer indicates success or failure. Okay, I need to check the definition of `vfs_create` again. I need to clarify how a new `dentry` is being handled. This is potentially where the issue lies.
|
| 27/2 |
2026/05/22 20:58 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:vfs_create]
Results: map[SourceCode:4169: /**
4170: * vfs_create - create new file
4171: * @idmap: idmap of the mount the inode was found from
4172: * @dentry: dentry of the child file
4173: * @mode: mode of the child file
4174: * @di: returns parent inode, if the inode is delegated.
4175: *
4176: * Create a new file.
4177: *
4178: * If the inode has been found through an idmapped mount the idmap of
4179: * the vfsmount must be passed through @idmap. This function will then take
4180: * care to map the inode according to @idmap before checking permissions.
4181: * On non-idmapped mounts or if permission checking is to be performed on the
4182: * raw inode simply pass @nop_mnt_idmap.
4183: */
4184: int vfs_create(struct mnt_idmap *idmap, struct dentry *dentry, umode_t mode,
4185: struct delegated_inode *di)
4186: {
4187: struct inode *dir = d_inode(dentry->d_parent);
4188: int error;
4189:
4190: error = may_create_dentry(idmap, dir, dentry);
4191: if (error)
4192: return error;
4193:
4194: if (!dir->i_op->create)
4195: return -EACCES; /* shouldn't it be ENOSYS? */
4196:
4197: mode = vfs_prepare_mode(idmap, dir, mode, S_IALLUGO, S_IFREG);
4198: error = security_inode_create(dir, dentry, mode);
4199: if (error)
4200: return error;
4201: error = try_break_deleg(dir, di);
4202: if (error)
4203: return error;
4204: error = dir->i_op->create(idmap, dir, dentry, mode, true);
4205: if (!error)
4206: fsnotify_create(dir, dentry);
4207: return error;
4208: }
SourceFile:fs/namei.c]
|
| 28/2 |
2026/05/22 20:58 |
llm |
expert |
0m
Model:
Tokens:
input: 21861
output: 49
thoughts: 499
Thoughts: **Debugging a Potential Dentry Double-Free in the Filesystem**
Okay, so I'm looking at this bug report. The initial thought that popped into my head was whether `dir->i_op->create` could potentially free the `dentry`, but immediately I realize that doesn't make sense since the `dentry` is passed *to* `create`. It should only be freed on failure, not within the function itself.
Hmm, the bug report details the dentry being freed by RCU, that's interesting. The crash dump indicates RCU is the culprit: "Freed by task 20: ... rcu_do_batch ... rcu_core ... rcu_cpu_kthread". So, RCU is responsible. Now the dentry is freed by RCU!
Now, let's dive into the stack trace provided. I'm focusing on the "Last potentially related work creation" stack trace, specifically: `ocfs2_lookup` calls `ocfs2_dentry_attach_lock` which seems like the critical path. Wait, is it the `ocfs2_dentry_attach_lock` function that's the issue here? That function calls `dput` or some derivative that results in dentry death? It looks like `ocfs2_dentry_attach_lock` calls `dput`, and that's the smoking gun!
Okay, time to check `ocfs2_dentry_attach_lock`. Let's dig into that function's implementation and understand how it's interacting with the `dentry` and when it's invoking `dput` or related routines. That's the next step.
|
| 29/2 |
2026/05/22 20:58 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/dcache.c IncludeLines:true Name:ocfs2_dentry_attach_lock]
Results: map[SourceCode: 167: /*
168: * Attach this dentry to a cluster lock.
169: *
170: * Dentry locks cover all links in a given directory to a particular
171: * inode. We do this so that ocfs2 can build a lock name which all
172: * nodes in the cluster can agree on at all times. Shoving full names
173: * in the cluster lock won't work due to size restrictions. Covering
174: * links inside of a directory is a good compromise because it still
175: * allows us to use the parent directory lock to synchronize
176: * operations.
177: *
178: * Call this function with the parent dir semaphore and the parent dir
179: * cluster lock held.
180: *
181: * The dir semaphore will protect us from having to worry about
182: * concurrent processes on our node trying to attach a lock at the
183: * same time.
184: *
185: * The dir cluster lock (held at either PR or EX mode) protects us
186: * from unlink and rename on other nodes.
187: *
188: * A dput() can happen asynchronously due to pruning, so we cover
189: * attaching and detaching the dentry lock with a
190: * dentry_attach_lock.
191: *
192: * A node which has done lookup on a name retains a protected read
193: * lock until final dput. If the user requests and unlink or rename,
194: * the protected read is upgraded to an exclusive lock. Other nodes
195: * who have seen the dentry will then be informed that they need to
196: * downgrade their lock, which will involve d_delete on the
197: * dentry. This happens in ocfs2_dentry_convert_worker().
198: */
199: int ocfs2_dentry_attach_lock(struct dentry *dentry,
200: struct inode *inode,
201: u64 parent_blkno)
202: {
203: int ret;
204: struct dentry *alias;
205: struct ocfs2_dentry_lock *dl = dentry->d_fsdata;
206:
207: trace_ocfs2_dentry_attach_lock(dentry->d_name.len, dentry->d_name.name,
208: (unsigned long long)parent_blkno, dl);
209:
210: /*
211: * Negative dentry. We ignore these for now.
212: *
213: * XXX: Could we can improve ocfs2_dentry_revalidate() by
214: * tracking these?
215: */
216: if (!inode)
217: return 0;
218:
219: if (d_really_is_negative(dentry) && dentry->d_fsdata) {
220: /* Converting a negative dentry to positive
221: Clear dentry->d_fsdata */
222: dentry->d_fsdata = dl = NULL;
223: }
224:
225: if (dl) {
226: mlog_bug_on_msg(dl->dl_parent_blkno != parent_blkno,
227: " \"%pd\": old parent: %llu, new: %llu\n",
228: dentry,
229: (unsigned long long)parent_blkno,
230: (unsigned long long)dl->dl_parent_blkno);
231: return 0;
232: }
233:
234: alias = ocfs2_find_local_alias(inode, parent_blkno, 0);
235: if (alias) {
236: /*
237: * Great, an alias exists, which means we must have a
238: * dentry lock already. We can just grab the lock off
239: * the alias and add it to the list.
240: *
241: * We're depending here on the fact that this dentry
242: * was found and exists in the dcache and so must have
243: * a reference to the dentry_lock because we can't
244: * race creates. Final dput() cannot happen on it
245: * since we have it pinned, so our reference is safe.
246: */
247: dl = alias->d_fsdata;
248: mlog_bug_on_msg(!dl, "parent %llu, ino %llu\n",
249: (unsigned long long)parent_blkno,
250: (unsigned long long)OCFS2_I(inode)->ip_blkno);
251:
252: mlog_bug_on_msg(dl->dl_parent_blkno != parent_blkno,
253: " \"%pd\": old parent: %llu, new: %llu\n",
254: dentry,
255: (unsigned long long)parent_blkno,
256: (unsigned long long)dl->dl_parent_blkno);
257:
258: trace_ocfs2_dentry_attach_lock_found(dl->dl_lockres.l_name,
259: (unsigned long long)parent_blkno,
260: (unsigned long long)OCFS2_I(inode)->ip_blkno);
261:
262: goto out_attach;
263: }
264:
265: /*
266: * There are no other aliases
267: */
268: dl = kmalloc_obj(*dl, GFP_NOFS);
269: if (!dl) {
270: ret = -ENOMEM;
271: mlog_errno(ret);
272: return ret;
273: }
274:
275: dl->dl_count = 0;
276: /*
277: * Does this have to happen below, for all attaches, in case
278: * the struct inode gets blown away by the downconvert thread?
279: */
280: dl->dl_inode = igrab(inode);
281: dl->dl_parent_blkno = parent_blkno;
282: ocfs2_dentry_lock_res_init(dl, parent_blkno, inode);
283:
284: out_attach:
285: spin_lock(&dentry_attach_lock);
286: if (unlikely(dentry->d_fsdata && !alias)) {
287: /* d_fsdata is set by a racing thread which is doing
288: * the same thing as this thread is doing. Leave the racing
289: * thread going ahead and we return here.
290: */
291: spin_unlock(&dentry_attach_lock);
292: iput(dl->dl_inode);
293: ocfs2_lock_res_free(&dl->dl_lockres);
294: kfree(dl);
295: return 0;
296: }
297:
298: dentry->d_fsdata = dl;
299: dl->dl_count++;
300: spin_unlock(&dentry_attach_lock);
301:
302: /*
303: * This actually gets us our PRMODE level lock. From now on,
304: * we'll have a notification if one of these names is
305: * destroyed on another node.
306: */
307: ret = ocfs2_dentry_lock(dentry, 0);
308: if (!ret)
309: ocfs2_dentry_unlock(dentry, 0);
310: else
311: mlog_errno(ret);
312:
313: /*
314: * In case of error, manually free the allocation and do the iput().
315: * We need to do this because error here means no d_instantiate(),
316: * which means iput() will not be called during dput(dentry).
317: */
318: if (ret < 0 && !alias) {
319: ocfs2_lock_res_free(&dl->dl_lockres);
320: BUG_ON(dl->dl_count != 1);
321: spin_lock(&dentry_attach_lock);
322: dentry->d_fsdata = NULL;
323: spin_unlock(&dentry_attach_lock);
324: kfree(dl);
325: iput(inode);
326: }
327:
328: dput(alias);
329:
330: return ret;
331: }
SourceFile:fs/ocfs2/dcache.c]
|
| 30/2 |
2026/05/22 20:58 |
llm |
expert |
0m
Model:
Tokens:
input: 25364
output: 39
thoughts: 1291
Thoughts: **Analyzing a Potential Dentry Double-Free in OCFS2**
Okay, so here's what I'm piecing together. It looks like we have a potential double-free issue in the OCFS2 filesystem. Let's start with the symptoms. The crash appears related to dentry manipulation. I'm seeing that `ocfs2_dentry_attach_lock` calls `dput(alias)`. The `alias` dentry comes from `ocfs2_find_local_alias`. The `dput` drops a reference, which, if it's the last one, leads to freeing the dentry.
The backtrace shows that the problematic sequence involves `ocfs2_dentry_attach_lock` calling `dput(alias)`, which ultimately ends up freeing the dentry via `__dentry_kill` and `call_rcu`.
Now, the bug report highlights that the dentry being accessed in `end_dirop` (let's call it `de`) is causing a problem. Is this the same `alias`? Hmm. Let me see, `end_dirop` is reached from `end_creating` which is called from `end_creating_path` which is called from `filename_mknodat`, which indicates that it's creating a new dentry via `filename_create`.
My initial thought was that a `creat` system call was the trigger. The `ocfs2_dentry_attach_lock` call stack indicates that the process involved does look like an open, which could happen when doing a `creat`. But the "Allocated by task 12158" part makes it clear that the dentry in question comes from a `mknod` or `create` operation. The backtrace from the bug report does show a `mknod`.
A crucial part is the bug report stating that the dentry allocation happens in task 12158, and the freeing happens elsewhere. The bug report is helpful, because it confirms that the dentry in question was allocated by the task "12158", and freed by task "20". The stack traces for allocation and freeing are clear. So the task 12158 does indeed do a `mknod`. However, the freed dentry comes from a `creat`.
This is where it gets interesting: the "Last potentially related work creation" stack (which frees the dentry) doesn't explicitly mention a task ID. This appears to be a `creat` related call, that does a `lookup`. So the question is: how is a dentry allocated by one task via `mknod` freed during a `creat` operation in a different task? This seems like a race condition or a reference-counting issue.
`filename_mknodat` calls `filename_create`, which in turn calls `start_dirop`. The `dentry` we're trying to access in `end_dirop` must be getting its reference dropped prematurely. Looks like I need to delve into how `start_dirop` manages dentries and how it interacts with the `ocfs2_dentry_attach_lock` calls in the `creat` path in different tasks.
|
| 31/2 |
2026/05/22 20:58 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:start_dirop]
Results: map[SourceCode:2922: /**
2923: * start_dirop - begin a create or remove dirop, performing locking and lookup
2924: * @parent: the dentry of the parent in which the operation will occur
2925: * @name: a qstr holding the name within that parent
2926: * @lookup_flags: intent and other lookup flags.
2927: *
2928: * The lookup is performed and necessary locks are taken so that, on success,
2929: * the returned dentry can be operated on safely.
2930: * The qstr must already have the hash value calculated.
2931: *
2932: * Returns: a locked dentry, or an error.
2933: *
2934: */
2935: struct dentry *start_dirop(struct dentry *parent, struct qstr *name,
2936: unsigned int lookup_flags)
2937: {
2938: return __start_dirop(parent, name, lookup_flags, TASK_NORMAL);
2939: }
SourceFile:fs/namei.c]
|
| 32/2 |
2026/05/22 20:58 |
llm |
expert |
0m
Model:
Tokens:
input: 27705
output: 40
thoughts: 11
Thoughts: Okay, here's my attempt at summarizing that thought process, keeping in mind I'm speaking to an expert:
**Internal Code Check - `__start_dirop`**
Right, let's just make sure we're on the same page. The immediate priority is to examine `__start_dirop`. I want to quickly refresh my understanding of how this particular function is structured, what its inputs and outputs are, and, most importantly, where it sits within the broader architecture. Is this the entry point for directory operations? Does it interact with caching layers? How does it handle potential race conditions or locking? I'm expecting this to either be the *very* first step, or perhaps a wrapper for an initial setup that then calls the core functionality. I'm hoping it’s well-documented, because it's the kind of thing that has to be *absolutely* bulletproof.
|
| 33/2 |
2026/05/22 20:58 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:__start_dirop]
Results: map[SourceCode:2901: static struct dentry *__start_dirop(struct dentry *parent, struct qstr *name,
2902: unsigned int lookup_flags,
2903: unsigned int state)
2904: {
2905: struct dentry *dentry;
2906: struct inode *dir = d_inode(parent);
2907:
2908: if (state == TASK_KILLABLE) {
2909: int ret = down_write_killable_nested(&dir->i_rwsem,
2910: I_MUTEX_PARENT);
2911: if (ret)
2912: return ERR_PTR(ret);
2913: } else {
2914: inode_lock_nested(dir, I_MUTEX_PARENT);
2915: }
2916: dentry = lookup_one_qstr_excl(name, parent, lookup_flags);
2917: if (IS_ERR(dentry))
2918: inode_unlock(dir);
2919: return dentry;
2920: }
SourceFile:fs/namei.c]
|
| 34/2 |
2026/05/22 20:58 |
llm |
expert |
0m
Model:
Tokens:
input: 28276
output: 38
thoughts: 336
Thoughts: **My Analysis of the `mknod` Operation**
Okay, let's break down this `mknod` process. I see that `__start_dirop` kicks things off by locking the parent directory using `inode_lock_nested`. This seems standard. It then moves onto `lookup_one_qstr_excl`, which I understand is going to try to retrieve a dentry.
So, the dentry is obtained and then `filename_mknodat` calls `vfs_mknod`. Makes sense. But here's the critical point: regardless of the success or failure of `vfs_mknod`, we call `end_creating_path`, which subsequently calls `end_dirop(dentry)`. This is where the parent directory is unlocked via `dput`.
My concern immediately arises: the parent directory is held locked for the entire duration of the mknod operation. How could another process possibly find this dentry, especially if something goes wrong and it frees early?
Thinking through this, if `lookup_one_qstr_excl` allocated a *new* dentry, then it gets added to the dcache, right? Then I remembered that `lookup_one_qstr_excl` calls `lookup_dcache`, and if it's not found in the cache, it falls back to the filesystem's `dir->i_op->lookup` method. So, if we have a negative dentry, it *is* in the dcache.
My next thought is, could another process doing a `creat` operation lookup the same name, find a negative dentry, and then potentially call into something like `ocfs2_lookup`?
The crux of my thought: if the parent directory is locked by mknod, how can creat do its lookup?
I have to check if `lookup_open` takes that same lock on the parent directory, since that's where I see the conflict. I need to know how these interrelate. Okay, let's investigate `lookup_open`...
|
| 35/2 |
2026/05/22 20:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:lookup_open]
Results: map[SourceCode:4391: /*
4392: * Look up and maybe create and open the last component.
4393: *
4394: * Must be called with parent locked (exclusive in O_CREAT case).
4395: *
4396: * Returns 0 on success, that is, if
4397: * the file was successfully atomically created (if necessary) and opened, or
4398: * the file was not completely opened at this time, though lookups and
4399: * creations were performed.
4400: * These case are distinguished by presence of FMODE_OPENED on file->f_mode.
4401: * In the latter case dentry returned in @path might be negative if O_CREAT
4402: * hadn't been specified.
4403: *
4404: * An error code is returned on failure.
4405: */
4406: static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
4407: const struct open_flags *op,
4408: bool got_write, struct delegated_inode *delegated_inode)
4409: {
4410: struct mnt_idmap *idmap;
4411: struct dentry *dir = nd->path.dentry;
4412: struct inode *dir_inode = dir->d_inode;
4413: int open_flag = op->open_flag;
4414: struct dentry *dentry;
4415: int error, create_error = 0;
4416: umode_t mode = op->mode;
4417: DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
4418:
4419: if (unlikely(IS_DEADDIR(dir_inode)))
4420: return ERR_PTR(-ENOENT);
4421:
4422: file->f_mode &= ~FMODE_CREATED;
4423: dentry = d_lookup(dir, &nd->last);
4424: for (;;) {
4425: if (!dentry) {
4426: dentry = d_alloc_parallel(dir, &nd->last, &wq);
4427: if (IS_ERR(dentry))
4428: return dentry;
4429: }
4430: if (d_in_lookup(dentry))
4431: break;
4432:
4433: error = d_revalidate(dir_inode, &nd->last, dentry, nd->flags);
4434: if (likely(error > 0))
4435: break;
4436: if (error)
4437: goto out_dput;
4438: d_invalidate(dentry);
4439: dput(dentry);
4440: dentry = NULL;
4441: }
4442: if (dentry->d_inode) {
4443: /* Cached positive dentry: will open in f_op->open */
4444: return dentry;
4445: }
4446:
4447: if (open_flag & O_CREAT)
4448: audit_inode(nd->name, dir, AUDIT_INODE_PARENT);
4449:
4450: /*
4451: * Checking write permission is tricky, bacuse we don't know if we are
4452: * going to actually need it: O_CREAT opens should work as long as the
4453: * file exists. But checking existence breaks atomicity. The trick is
4454: * to check access and if not granted clear O_CREAT from the flags.
4455: *
4456: * Another problem is returing the "right" error value (e.g. for an
4457: * O_EXCL open we want to return EEXIST not EROFS).
4458: */
4459: if (unlikely(!got_write))
4460: open_flag &= ~O_TRUNC;
4461: idmap = mnt_idmap(nd->path.mnt);
4462: if (open_flag & O_CREAT) {
4463: if (open_flag & O_EXCL)
4464: open_flag &= ~O_TRUNC;
4465: mode = vfs_prepare_mode(idmap, dir->d_inode, mode, mode, mode);
4466: if (likely(got_write))
4467: create_error = may_o_create(idmap, &nd->path,
4468: dentry, mode);
4469: else
4470: create_error = -EROFS;
4471: }
4472: if (create_error)
4473: open_flag &= ~O_CREAT;
4474: if (dir_inode->i_op->atomic_open) {
4475: if (nd->flags & LOOKUP_DIRECTORY)
4476: open_flag |= O_DIRECTORY;
4477: dentry = atomic_open(&nd->path, dentry, file, open_flag, mode);
4478: if (unlikely(create_error) && dentry == ERR_PTR(-ENOENT))
4479: dentry = ERR_PTR(create_error);
4480: return dentry;
4481: }
4482:
4483: if (d_in_lookup(dentry)) {
4484: struct dentry *res = dir_inode->i_op->lookup(dir_inode, dentry,
4485: nd->flags);
4486: d_lookup_done(dentry);
4487: if (unlikely(res)) {
4488: if (IS_ERR(res)) {
4489: error = PTR_ERR(res);
4490: goto out_dput;
4491: }
4492: dput(dentry);
4493: dentry = res;
4494: }
4495: }
4496:
4497: /* Negative dentry, just create the file */
4498: if (!dentry->d_inode && (open_flag & O_CREAT)) {
4499: /* but break the directory lease first! */
4500: error = try_break_deleg(dir_inode, delegated_inode);
4501: if (error)
4502: goto out_dput;
4503:
4504: file->f_mode |= FMODE_CREATED;
4505: audit_inode_child(dir_inode, dentry, AUDIT_TYPE_CHILD_CREATE);
4506: if (!dir_inode->i_op->create) {
4507: error = -EACCES;
4508: goto out_dput;
4509: }
4510:
4511: error = dir_inode->i_op->create(idmap, dir_inode, dentry,
4512: mode, open_flag & O_EXCL);
4513: if (error)
4514: goto out_dput;
4515: }
4516: if (unlikely(create_error) && !dentry->d_inode) {
4517: error = create_error;
4518: goto out_dput;
4519: }
4520: return dentry;
4521:
4522: out_dput:
4523: dput(dentry);
4524: return ERR_PTR(error);
4525: }
SourceFile:fs/namei.c]
|
| 36/2 |
2026/05/22 20:59 |
llm |
expert |
0m
Model:
Tokens:
input: 31229
output: 41
thoughts: 463
Thoughts: **My Analysis of File Creation in the Kernel**
Okay, let me trace this through. So, `lookup_open` is called, and that kicks off `d_lookup` on the parent directory. If that initial lookup fails, `d_alloc_parallel` gets invoked. That makes sense. Now, `mknod` seems to use `lookup_one_qstr_excl`, which also calls `d_lookup` and then `d_alloc`. If `mknod` *successfully* allocates a dentry, that dentry should now be in the dcache.
Then, `creat` comes along and calls `lookup_open`. Naturally, `lookup_open` calls `d_lookup`. If the `mknod` *did* create the dentry, then `d_lookup` should find the dentry! But hang on... if `d_in_lookup` is *true*, the `dir_inode->i_op->lookup` is called. That should have happened in `mknod` already.
Wait, back to `mknod`. It uses `lookup_one_qstr_excl`, which *itself* calls `dir->i_op->lookup` on the parent. So by the time `creat` comes along, the dentry shouldn't be "in lookup" any more. So what *actually* happens next? Does `creat` call `dir_inode->i_op->create`?
Okay, let's step back and consider `lookup_open` again. I see this code block:
```c
if (d_in_lookup(dentry)) {
struct dentry *res = dir_inode->i_op->lookup(dir_inode, dentry,
nd->flags);
d_lookup_done(dentry);
...
}
```
If `d_in_lookup` is false, it skips that and goes to this:
```c
/* Negative dentry, just create the file */
if (!dentry->d_inode && (open_flag & O_CREAT)) {
...
error = dir_inode->i_op->create(idmap, dir_inode, dentry,
mode, open_flag & O_EXCL);
```
Right, because if the dentry is not present (negative dentry) and `O_CREAT` is set, then the file needs to be created, and this is where `create` gets called.
But if `mknod` is in progress, the *parent* directory should have a lock! How can `creat` even be in `lookup_open` at the same time? `lookup_open` is called from `open_last_lookups`. I should probably check that function's flow again to see where things might be getting interleaved or whether I'm making a flawed assumption. Gotta check `open_last_lookups`.
|
| 37/2 |
2026/05/22 20:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:open_last_lookups]
Results: map[SourceCode:4563: static const char *open_last_lookups(struct nameidata *nd,
4564: struct file *file, const struct open_flags *op)
4565: {
4566: struct delegated_inode delegated_inode = { };
4567: struct dentry *dir = nd->path.dentry;
4568: int open_flag = op->open_flag;
4569: bool got_write = false;
4570: struct dentry *dentry;
4571: const char *res;
4572:
4573: nd->flags |= op->intent;
4574:
4575: if (nd->last_type != LAST_NORM) {
4576: if (nd->depth)
4577: put_link(nd);
4578: return handle_dots(nd, nd->last_type);
4579: }
4580:
4581: /* We _can_ be in RCU mode here */
4582: dentry = lookup_fast_for_open(nd, open_flag);
4583: if (IS_ERR(dentry))
4584: return ERR_CAST(dentry);
4585:
4586: if (likely(dentry))
4587: goto finish_lookup;
4588:
4589: if (!(open_flag & O_CREAT)) {
4590: if (WARN_ON_ONCE(nd->flags & LOOKUP_RCU))
4591: return ERR_PTR(-ECHILD);
4592: } else {
4593: if (nd->flags & LOOKUP_RCU) {
4594: if (!try_to_unlazy(nd))
4595: return ERR_PTR(-ECHILD);
4596: }
4597: }
4598: retry:
4599: if (open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {
4600: got_write = !mnt_want_write(nd->path.mnt);
4601: /*
4602: * do _not_ fail yet - we might not need that or fail with
4603: * a different error; let lookup_open() decide; we'll be
4604: * dropping this one anyway.
4605: */
4606: }
4607: if (open_flag & O_CREAT)
4608: inode_lock(dir->d_inode);
4609: else
4610: inode_lock_shared(dir->d_inode);
4611: dentry = lookup_open(nd, file, op, got_write, &delegated_inode);
4612: if (!IS_ERR(dentry)) {
4613: if (file->f_mode & FMODE_CREATED)
4614: fsnotify_create(dir->d_inode, dentry);
4615: if (file->f_mode & FMODE_OPENED)
4616: fsnotify_open(file);
4617: }
4618: if (open_flag & O_CREAT)
4619: inode_unlock(dir->d_inode);
4620: else
4621: inode_unlock_shared(dir->d_inode);
4622:
4623: if (got_write)
4624: mnt_drop_write(nd->path.mnt);
4625:
4626: if (IS_ERR(dentry)) {
4627: if (is_delegated(&delegated_inode)) {
4628: int error = break_deleg_wait(&delegated_inode);
4629:
4630: if (!error)
4631: goto retry;
4632: return ERR_PTR(error);
4633: }
4634: return ERR_CAST(dentry);
4635: }
4636:
4637: if (file->f_mode & (FMODE_OPENED | FMODE_CREATED)) {
4638: dput(nd->path.dentry);
4639: nd->path.dentry = dentry;
4640: return NULL;
4641: }
4642:
4643: finish_lookup:
4644: if (nd->depth)
4645: put_link(nd);
4646: res = step_into(nd, WALK_TRAILING, dentry);
4647: if (unlikely(res))
4648: nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
4649: return res;
4650: }
SourceFile:fs/namei.c]
|
| 38/2 |
2026/05/22 20:59 |
llm |
expert |
0m
Model:
Tokens:
input: 33711
output: 41
thoughts: 227
Thoughts: **Concurreny Conundrum with `mknod` and `creat`**
Okay, hold on a second. This is making my head spin a little. I'm looking at how `open_last_lookups` handles things, and it hits me: if `O_CREAT` is set, that means `creat` is grabbing the parent directory lock with `inode_lock(dir->d_inode)`. But, I know `mknod` also needs to lock the parent directory. This isn't right. How are these two operations supposed to run concurrently if they're both vying for the same lock?
Ah, right, okay, I think I see it. `mknod` kicks things off with `__start_dirop`, which then uses `inode_lock_nested(dir, I_MUTEX_PARENT)`. So, `mknod` is setting that parent directory lock. Then `vfs_mknod` gets called. Now, `mknod` ends with a call to `end_creating_path` which then calls `end_dirop` and that calls `inode_unlock(dir)`. So, it seems like `mknod` *is* holding the lock.
Wait, if `mknod` has the lock, then `creat` absolutely *cannot* be inside `lookup_open` at the same time. The lock would prevent that. Unless… has `mknod` released the lock at some point? Could `vfs_mknod` possibly be dropping the lock? I need to go check the code for `vfs_mknod` right now. This is a potential race condition and I need to be sure.
|
| 39/2 |
2026/05/22 20:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:vfs_mknod]
Results: map[SourceCode:5073: /**
5074: * vfs_mknod - create device node or file
5075: * @idmap: idmap of the mount the inode was found from
5076: * @dir: inode of the parent directory
5077: * @dentry: dentry of the child device node
5078: * @mode: mode of the child device node
5079: * @dev: device number of device to create
5080: * @delegated_inode: returns parent inode, if the inode is delegated.
5081: *
5082: * Create a device node or file.
5083: *
5084: * If the inode has been found through an idmapped mount the idmap of
5085: * the vfsmount must be passed through @idmap. This function will then take
5086: * care to map the inode according to @idmap before checking permissions.
5087: * On non-idmapped mounts or if permission checking is to be performed on the
5088: * raw inode simply pass @nop_mnt_idmap.
5089: */
5090: int vfs_mknod(struct mnt_idmap *idmap, struct inode *dir,
5091: struct dentry *dentry, umode_t mode, dev_t dev,
5092: struct delegated_inode *delegated_inode)
5093: {
5094: bool is_whiteout = S_ISCHR(mode) && dev == WHITEOUT_DEV;
5095: int error = may_create_dentry(idmap, dir, dentry);
5096:
5097: if (error)
5098: return error;
5099:
5100: if ((S_ISCHR(mode) || S_ISBLK(mode)) && !is_whiteout &&
5101: !capable(CAP_MKNOD))
5102: return -EPERM;
5103:
5104: if (!dir->i_op->mknod)
5105: return -EPERM;
5106:
5107: mode = vfs_prepare_mode(idmap, dir, mode, mode, mode);
5108: error = devcgroup_inode_mknod(mode, dev);
5109: if (error)
5110: return error;
5111:
5112: error = security_inode_mknod(dir, dentry, mode, dev);
5113: if (error)
5114: return error;
5115:
5116: error = try_break_deleg(dir, delegated_inode);
5117: if (error)
5118: return error;
5119:
5120: error = dir->i_op->mknod(idmap, dir, dentry, mode, dev);
5121: if (!error)
5122: fsnotify_create(dir, dentry);
5123: return error;
5124: }
SourceFile:fs/namei.c]
|
| 40/2 |
2026/05/22 20:59 |
llm |
expert |
0m
Model:
Tokens:
input: 35215
output: 46
thoughts: 73
Thoughts: **Analyzing the Potential for Lock Drops in `ocfs2_mknod`**
Okay, so I'm digging into this `vfs_mknod` function, and it's clear that it's going to delegate the actual work to the filesystem-specific `mknod` operation via `dir->i_op->mknod`. The core question immediately popping into my head is whether this filesystem-specific `mknod` implementation has the potential to drop any locks it might be holding. I need to be absolutely sure about this; this is a critical aspect to scrutinize as part of the overall locking strategy and concurrency control.
The specific filesystem involved here is `ocfs2`. Therefore, the call flow will land in `ocfs2_mknod`, which is the function I need to focus on. I need to delve into `ocfs2_mknod`'s code. I need to understand its behavior thoroughly to identify any possibility of lock releases or changes to the lock state. I'll meticulously go through its locking mechanisms.
|
| 41/2 |
2026/05/22 20:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/namei.c IncludeLines:true Name:ocfs2_mknod]
Results: map[SourceCode: 230: static int ocfs2_mknod(struct mnt_idmap *idmap,
231: struct inode *dir,
232: struct dentry *dentry,
233: umode_t mode,
234: dev_t dev)
235: {
236: int status = 0;
237: struct buffer_head *parent_fe_bh = NULL;
238: handle_t *handle = NULL;
239: struct ocfs2_super *osb;
240: struct ocfs2_dinode *dirfe;
241: struct ocfs2_dinode *fe = NULL;
242: struct buffer_head *new_fe_bh = NULL;
243: struct inode *inode = NULL;
244: struct ocfs2_alloc_context *inode_ac = NULL;
245: struct ocfs2_alloc_context *data_ac = NULL;
246: struct ocfs2_alloc_context *meta_ac = NULL;
247: int want_clusters = 0;
248: int want_meta = 0;
249: int xattr_credits = 0;
250: struct ocfs2_security_xattr_info si = {
251: .name = NULL,
252: .enable = 1,
253: };
254: int did_quota_inode = 0;
255: struct ocfs2_dir_lookup_result lookup = { NULL, };
256: sigset_t oldset;
257: int did_block_signals = 0;
258: struct ocfs2_dentry_lock *dl = NULL;
259:
260: trace_ocfs2_mknod(dir, dentry, dentry->d_name.len, dentry->d_name.name,
261: (unsigned long long)OCFS2_I(dir)->ip_blkno,
262: (unsigned long)dev, mode);
263:
264: status = dquot_initialize(dir);
265: if (status) {
266: mlog_errno(status);
267: return status;
268: }
269:
270: /* get our super block */
271: osb = OCFS2_SB(dir->i_sb);
272:
273: status = ocfs2_inode_lock(dir, &parent_fe_bh, 1);
274: if (status < 0) {
275: if (status != -ENOENT)
276: mlog_errno(status);
277: return status;
278: }
279:
280: if (S_ISDIR(mode) && (dir->i_nlink >= ocfs2_link_max(osb))) {
281: status = -EMLINK;
282: goto leave;
283: }
284:
285: dirfe = (struct ocfs2_dinode *) parent_fe_bh->b_data;
286: if (!ocfs2_read_links_count(dirfe)) {
287: /* can't make a file in a deleted directory. */
288: status = -ENOENT;
289: goto leave;
290: }
291:
292: status = ocfs2_check_dir_for_entry(dir, dentry->d_name.name,
293: dentry->d_name.len);
294: if (status)
295: goto leave;
296:
297: /* get a spot inside the dir. */
298: status = ocfs2_prepare_dir_for_insert(osb, dir, parent_fe_bh,
299: dentry->d_name.name,
300: dentry->d_name.len, &lookup);
301: if (status < 0) {
302: mlog_errno(status);
303: goto leave;
304: }
305:
306: /* reserve an inode spot */
307: status = ocfs2_reserve_new_inode(osb, &inode_ac);
308: if (status < 0) {
309: if (status != -ENOSPC)
310: mlog_errno(status);
311: goto leave;
312: }
313:
314: inode = ocfs2_get_init_inode(dir, mode);
315: if (IS_ERR(inode)) {
316: status = PTR_ERR(inode);
317: inode = NULL;
318: mlog_errno(status);
319: goto leave;
320: }
321:
322: /* get security xattr */
323: status = ocfs2_init_security_get(inode, dir, &dentry->d_name, &si);
324: if (status) {
325: if (status == -EOPNOTSUPP)
326: si.enable = 0;
327: else {
328: mlog_errno(status);
329: goto leave;
330: }
331: }
332:
333: /* calculate meta data/clusters for setting security and acl xattr */
334: status = ocfs2_calc_xattr_init(dir, parent_fe_bh, mode,
335: &si, &want_clusters,
336: &xattr_credits, &want_meta);
337: if (status < 0) {
338: mlog_errno(status);
339: goto leave;
340: }
341:
342: /* Reserve a cluster if creating an extent based directory. */
343: if (S_ISDIR(mode) && !ocfs2_supports_inline_data(osb)) {
344: want_clusters += 1;
345:
346: /* Dir indexing requires extra space as well */
347: if (ocfs2_supports_indexed_dirs(osb))
348: want_meta++;
349: }
350:
351: status = ocfs2_reserve_new_metadata_blocks(osb, want_meta, &meta_ac);
352: if (status < 0) {
353: if (status != -ENOSPC)
354: mlog_errno(status);
355: goto leave;
356: }
357:
358: status = ocfs2_reserve_clusters(osb, want_clusters, &data_ac);
359: if (status < 0) {
360: if (status != -ENOSPC)
361: mlog_errno(status);
362: goto leave;
363: }
364:
365: handle = ocfs2_start_trans(osb, ocfs2_mknod_credits(osb->sb,
366: S_ISDIR(mode),
367: xattr_credits));
368: if (IS_ERR(handle)) {
369: status = PTR_ERR(handle);
370: handle = NULL;
371: mlog_errno(status);
372: goto leave;
373: }
374:
375: /* Starting to change things, restart is no longer possible. */
376: ocfs2_block_signals(&oldset);
377: did_block_signals = 1;
378:
379: status = dquot_alloc_inode(inode);
380: if (status)
381: goto leave;
382: did_quota_inode = 1;
383:
384: /* do the real work now. */
385: status = ocfs2_mknod_locked(osb, dir, inode, dev,
386: &new_fe_bh, parent_fe_bh, handle,
387: inode_ac);
388: if (status < 0) {
389: mlog_errno(status);
390: goto leave;
391: }
392:
393: fe = (struct ocfs2_dinode *) new_fe_bh->b_data;
394: if (S_ISDIR(mode)) {
395: status = ocfs2_fill_new_dir(osb, handle, dir, inode,
396: new_fe_bh, data_ac, meta_ac);
397: if (status < 0) {
398: mlog_errno(status);
399: goto leave;
400: }
401:
402: status = ocfs2_journal_access_di(handle, INODE_CACHE(dir),
403: parent_fe_bh,
404: OCFS2_JOURNAL_ACCESS_WRITE);
405: if (status < 0) {
406: mlog_errno(status);
407: goto leave;
408: }
409: ocfs2_add_links_count(dirfe, 1);
410: ocfs2_journal_dirty(handle, parent_fe_bh);
411: inc_nlink(dir);
412: }
413:
414: status = ocfs2_init_acl(handle, inode, dir, new_fe_bh, parent_fe_bh,
415: meta_ac, data_ac);
416:
417: if (status < 0) {
418: mlog_errno(status);
419: goto roll_back;
420: }
421:
422: if (si.enable) {
423: status = ocfs2_init_security_set(handle, inode, new_fe_bh, &si,
424: meta_ac, data_ac);
425: if (status < 0) {
426: mlog_errno(status);
427: goto roll_back;
428: }
429: }
430:
431: /*
432: * Do this before adding the entry to the directory. We add
433: * also set d_op after success so that ->d_iput() will cleanup
434: * the dentry lock even if ocfs2_add_entry() fails below.
435: */
436: status = ocfs2_dentry_attach_lock(dentry, inode,
437: OCFS2_I(dir)->ip_blkno);
438: if (status) {
439: mlog_errno(status);
440: goto roll_back;
441: }
442:
443: dl = dentry->d_fsdata;
444:
445: status = ocfs2_add_entry(handle, dentry, inode,
446: OCFS2_I(inode)->ip_blkno, parent_fe_bh,
447: &lookup);
448: if (status < 0) {
449: mlog_errno(status);
450: goto roll_back;
451: }
452:
453: insert_inode_hash(inode);
454: d_instantiate(dentry, inode);
455: status = 0;
456:
457: roll_back:
458: if (status < 0 && S_ISDIR(mode)) {
459: ocfs2_add_links_count(dirfe, -1);
460: drop_nlink(dir);
461: }
462:
463: leave:
464: if (status < 0 && did_quota_inode)
465: dquot_free_inode(inode);
466: if (handle) {
467: if (status < 0 && fe)
468: ocfs2_set_links_count(fe, 0);
469: ocfs2_commit_trans(osb, handle);
470: }
471:
472: ocfs2_inode_unlock(dir, 1);
473: if (did_block_signals)
474: ocfs2_unblock_signals(&oldset);
475:
476: brelse(new_fe_bh);
477: brelse(parent_fe_bh);
478: kfree(si.value);
479:
480: ocfs2_free_dir_lookup_result(&lookup);
481:
482: if (inode_ac)
483: ocfs2_free_alloc_context(inode_ac);
484:
485: if (data_ac)
486: ocfs2_free_alloc_context(data_ac);
487:
488: if (meta_ac)
489: ocfs2_free_alloc_context(meta_ac);
490:
491: /*
492: * We should call iput after the i_rwsem of the bitmap been
493: * unlocked in ocfs2_free_alloc_context, or the
494: * ocfs2_delete_inode will mutex_lock again.
495: */
496: if ((status < 0) && inode) {
497: if (dl)
498: ocfs2_cleanup_add_entry_failure(osb, dentry, inode);
499:
500: OCFS2_I(inode)->ip_flags |= OCFS2_INODE_SKIP_ORPHAN_DIR;
501: clear_nlink(inode);
502: iput(inode);
503: }
504:
505: if (status)
506: mlog_errno(status);
507:
508: return status;
509: }
SourceFile:fs/ocfs2/namei.c]
|
| 42/2 |
2026/05/22 20:59 |
llm |
expert |
0m
Model:
Tokens:
input: 39718
output: 44
thoughts: 1104
Thoughts: **Dentry Allocation and Freeing in `ocfs2_mknod` - A Deep Dive**
Okay, let me think this through. `ocfs2_mknod` *doesn't* release the parent directory lock immediately. It uses a cluster-wide lock (`ocfs2_inode_lock`) but the VFS lock (`dir->i_rwsem`) is still held by `vfs_mknod`. This is important. But then, `ocfs2_mknod` *does* call `ocfs2_dentry_attach_lock`, which uses `dput(alias)`. Wait a minute, what is `alias`?
The stack trace from the bug report gives us a clue: `ocfs2_dentry_attach_lock` called from `ocfs2_lookup` which called `dput`. So `ocfs2_lookup` is involved. Okay, the bug report says a dentry allocated by `mknod` was freed via `creat`, but how? The allocated stack is from `filename_mknodat`, so that seems clear.
Now, wait, the allocation stack from the bug report is: `filename_mknodat` -> `filename_create` -> ... . If `filename_mknodat` *fails*, where does it go? Ah! It calls `end_creating_path`, which in turn calls `end_dirop(dentry)`.
The bug report says the buggy address belongs to the cache dentry allocated by mknod but freed via `rcu`. How did `creat` get a reference to the dentry allocated by `mknod`? Did `mknod` block or drop a lock *after* allocating the dentry and adding it to the dcache?
`filename_mknodat` calls `filename_create`, then `security_path_mknod`, then `vfs_mknod`. If `security_path_mknod` fails, it jumps to `out2`, where it calls `end_creating_path`. But the `dentry` *is* in the dcache! Because it was added by `lookup_one_qstr_excl`.
If `dir->i_op->lookup` returns NULL, a *negative* dentry is left in the dcache. It's allocated via `d_alloc` from within `lookup_one_qstr_excl`, which then calls the filesystem's `lookup` operation. *Does* `lookup_one_qstr_excl` leave the dentry in the dcache if the dentry is new? I need to check `lookup_one_qstr_excl`.
|
| 43/2 |
2026/05/22 20:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:lookup_one_qstr_excl]
Results: map[SourceCode:1776: /*
1777: * Parent directory has inode locked exclusive. This is one
1778: * and only case when ->lookup() gets called on non in-lookup
1779: * dentries - as the matter of fact, this only gets called
1780: * when directory is guaranteed to have no in-lookup children
1781: * at all.
1782: * Will return -ENOENT if name isn't found and LOOKUP_CREATE wasn't passed.
1783: * Will return -EEXIST if name is found and LOOKUP_EXCL was passed.
1784: */
1785: static struct dentry *lookup_one_qstr_excl(const struct qstr *name,
1786: struct dentry *base, unsigned int flags)
1787: {
1788: struct dentry *dentry;
1789: struct dentry *old;
1790: struct inode *dir;
1791:
1792: dentry = lookup_dcache(name, base, flags);
1793: if (dentry)
1794: goto found;
1795:
1796: /* Don't create child dentry for a dead directory. */
1797: dir = base->d_inode;
1798: if (unlikely(IS_DEADDIR(dir)))
1799: return ERR_PTR(-ENOENT);
1800:
1801: dentry = d_alloc(base, name);
1802: if (unlikely(!dentry))
1803: return ERR_PTR(-ENOMEM);
1804:
1805: old = dir->i_op->lookup(dir, dentry, flags);
1806: if (unlikely(old)) {
1807: dput(dentry);
1808: dentry = old;
1809: }
1810: found:
1811: if (IS_ERR(dentry))
1812: return dentry;
1813: if (d_is_negative(dentry) && !(flags & LOOKUP_CREATE)) {
1814: dput(dentry);
1815: return ERR_PTR(-ENOENT);
1816: }
1817: if (d_is_positive(dentry) && (flags & LOOKUP_EXCL)) {
1818: dput(dentry);
1819: return ERR_PTR(-EEXIST);
1820: }
1821: return dentry;
1822: }
SourceFile:fs/namei.c]
|
| 44/2 |
2026/05/22 20:59 |
llm |
expert |
0m
Model:
Tokens:
input: 42177
output: 44
thoughts: 108
Thoughts: **Analyzing the `lookup_one_qstr_excl` Function**
Okay, so I'm tracing through this code, and it looks like `lookup_one_qstr_excl` eventually calls the `dir->i_op->lookup(dir, dentry, flags)` function. Makes sense, that's the standard entry point for looking up a dentry within a directory. The interesting part is what happens if `dir->i_op->lookup` returns NULL. The code indicates that the *`dentry`* is returned in that case. That's a key bit of logic.
Now I'm wondering… does the underlying `dir->i_op->lookup` function actually add this dentry to the dcache if the lookup is successful? We need to keep this in mind. It's crucial for understanding how the directory cache is maintained.
Ah, yes! Looking deeper, the `lookup` function appears to handle the dcache management directly. Specifically, I see `d_splice_alias` or `d_add` being called within the lookup function. That confirms my suspicion - the lookup routine *does* manage the dentry cache, which could have implications on performance and consistency.
Alright, let's zoom in and check what `ocfs2_lookup` actually does. That's the one we need to investigate.
|
| 45/2 |
2026/05/22 20:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/namei.c IncludeLines:true Name:ocfs2_lookup]
Results: map[SourceCode: 97: static struct dentry *ocfs2_lookup(struct inode *dir, struct dentry *dentry,
98: unsigned int flags)
99: {
100: int status;
101: u64 blkno;
102: struct inode *inode = NULL;
103: struct dentry *ret;
104: struct ocfs2_inode_info *oi;
105:
106: trace_ocfs2_lookup(dir, dentry, dentry->d_name.len,
107: dentry->d_name.name,
108: (unsigned long long)OCFS2_I(dir)->ip_blkno, 0);
109:
110: if (dentry->d_name.len > OCFS2_MAX_FILENAME_LEN) {
111: ret = ERR_PTR(-ENAMETOOLONG);
112: goto bail;
113: }
114:
115: status = ocfs2_inode_lock_nested(dir, NULL, 0, OI_LS_PARENT);
116: if (status < 0) {
117: if (status != -ENOENT)
118: mlog_errno(status);
119: ret = ERR_PTR(status);
120: goto bail;
121: }
122:
123: status = ocfs2_lookup_ino_from_name(dir, dentry->d_name.name,
124: dentry->d_name.len, &blkno);
125: if (status < 0)
126: goto bail_add;
127:
128: inode = ocfs2_iget(OCFS2_SB(dir->i_sb), blkno, 0, 0);
129: if (IS_ERR(inode)) {
130: ret = ERR_PTR(-EACCES);
131: goto bail_unlock;
132: }
133:
134: oi = OCFS2_I(inode);
135: /* Clear any orphaned state... If we were able to look up the
136: * inode from a directory, it certainly can't be orphaned. We
137: * might have the bad state from a node which intended to
138: * orphan this inode but crashed before it could commit the
139: * unlink. */
140: spin_lock(&oi->ip_lock);
141: oi->ip_flags &= ~OCFS2_INODE_MAYBE_ORPHANED;
142: spin_unlock(&oi->ip_lock);
143:
144: bail_add:
145: ret = d_splice_alias(inode, dentry);
146: if (IS_ERR(ret))
147: goto bail_unlock;
148:
149: if (inode) {
150: /*
151: * If d_splice_alias() finds a DCACHE_DISCONNECTED
152: * dentry, it will d_move() it on top of ourse. The
153: * return value will indicate this however, so in
154: * those cases, we switch them around for the locking
155: * code.
156: *
157: * NOTE: This dentry already has ->d_op set from
158: * ocfs2_get_parent() and ocfs2_get_dentry()
159: */
160: if (ret)
161: dentry = ret;
162:
163: status = ocfs2_dentry_attach_lock(dentry, inode,
164: OCFS2_I(dir)->ip_blkno);
165: if (status) {
166: mlog_errno(status);
167: if (ret)
168: dput(ret);
169: ret = ERR_PTR(status);
170: }
171: } else
172: ocfs2_dentry_attach_gen(dentry);
173:
174: bail_unlock:
175: /* Don't drop the cluster lock until *after* the d_add --
176: * unlink on another node will message us to remove that
177: * dentry under this lock so otherwise we can race this with
178: * the downconvert thread and have a stale dentry. */
179: ocfs2_inode_unlock(dir, 0);
180:
181: bail:
182:
183: trace_ocfs2_lookup_ret(ret);
184:
185: return ret;
186: }
SourceFile:fs/ocfs2/namei.c]
|
| 46/2 |
2026/05/22 20:59 |
llm |
expert |
0m
Model:
Tokens:
input: 44013
output: 44
thoughts: 376
Thoughts: **Negative Dentry Handling in OCFS2 - A Deep Dive**
Okay, so `ocfs2_lookup` is calling `d_splice_alias` with a specific `inode` and `dentry`, right? Hang on, if `inode` is NULL – meaning we're dealing with a negative dentry – `d_splice_alias` ends up calling `d_add(dentry, inode)`. Bingo! The negative dentry is getting added to the dcache! And `ocfs2_lookup` then just returns NULL. This means that `lookup_one_qstr_excl` hands back the negative dentry.
Now, `filename_mknodat` is subsequently called, and ultimately, if it's successful, it will call `vfs_mknod`. But, hold on a second... what if `vfs_mknod` *fails*? Then `filename_mknodat` calls `end_creating_path`, which leads to `end_dirop(dentry)`. And *that* calls `dput(dentry)`. Because we added that dentry to the dcache, its initial refcount is just 1 (from `d_alloc`).
Ah, let me confirm. `d_alloc` *does* set the refcount to 1, and `d_add` doesn't adjust it. So, `dput(dentry)` will drop it down to zero. Okay, but here's where it gets interesting... if that dentry is in the dcache, another task, perhaps doing a `lookup_open`, could find it! Let me think this through. `lookup_open` calls `d_lookup`, right? If `d_lookup` finds the negative dentry, it returns it. So, `lookup_open` then calls `dir_inode->i_op->create`, which is either `ocfs2_mknod` or `ocfs2_create` depending on the file type. And it looks like `ocfs2_create` might be calling `ocfs2_mknod`. Time to delve into `ocfs2_create`.
|
| 47/2 |
2026/05/22 20:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/namei.c IncludeLines:true Name:ocfs2_create]
Results: map[SourceCode: 667: static int ocfs2_create(struct mnt_idmap *idmap,
668: struct inode *dir,
669: struct dentry *dentry,
670: umode_t mode,
671: bool excl)
672: {
673: int ret;
674:
675: trace_ocfs2_create(dir, dentry, dentry->d_name.len, dentry->d_name.name,
676: (unsigned long long)OCFS2_I(dir)->ip_blkno, mode);
677: ret = ocfs2_mknod(&nop_mnt_idmap, dir, dentry, mode | S_IFREG, 0);
678: if (ret)
679: mlog_errno(ret);
680:
681: return ret;
682: }
SourceFile:fs/ocfs2/namei.c]
|
| 48/2 |
2026/05/22 20:59 |
llm |
expert |
0m
Model:
Tokens:
input: 45184
output: 46
thoughts: 4895
Thoughts: **Analyzing a File System Concurrency Bug**
Okay, here's what I've pieced together about this file system issue. `ocfs2_create` leads to `ocfs2_mknod`, which calls `ocfs2_dentry_attach_lock`. Aha! `ocfs2_dentry_attach_lock` calls `ocfs2_find_local_alias` which is the key. If this function doesn't find a matching alias for the new inode, it allocates `dl` and attaches to the dentry, but if it finds an alias, it'll increment the refcount and *then* call `dput`. Crucially, if that `dput` happens on the alias, it frees the dentry.
I looked at the stack trace - the issue seems to be during `ocfs2_lookup`, which calls `ocfs2_dentry_attach_lock`. `lookup_open` calls `ocfs2_lookup`, and `lookup_open` can call `dir_inode->i_op->lookup` i.e. `ocfs2_lookup` if the initial dentry is `d_in_lookup`. If there's an `inode`, `d_splice_alias` may return a different dentry and call `ocfs2_dentry_attach_lock`. Then there's an alias. If `ocfs2_find_local_alias` finds an alias, it increments the refcount, and *then* calls `dput(alias)`. If that `dput` drops the refcount to zero, the alias dentry is freed!
The bug report shows the dentry allocated by one task (`mknod`) and freed by another (`rcu`) after the creation of potentially related work (`creat`). Okay, the key here is concurrency. If `mknod` allocates the dentry, `creat` looks it up, and `ocfs2_lookup` finds the same name, then calls `d_splice_alias`, which might return the same dentry, so `ocfs2_dentry_attach_lock` will call `ocfs2_find_local_alias`, find the dentry and `dput`. No, `d_splice_alias` returns an existing dentry when a file is a directory or disconnected - not when a dentry is allocated by `mknod`, which is a negative dentry. That means `creat` won't call `dir_inode->i_op->lookup`.
Aha! We're dealing with concurrent `mknod` and `creat` calls. `mknod` calls `lookup_one_qstr_excl`, which allocates a dentry. `ocfs2_mknod` gets the inode and calls `ocfs2_dentry_attach_lock`, but `d_instantiate` comes *after* `ocfs2_dentry_attach_lock`. So the newly created inode hasn't been instantiated yet when the dentry is attached. But, `mknod` holds `dir->i_rwsem`, so `creat` can't run in `lookup_open` at the same time!
Unless... they're in *different* directories? But then, `ocfs2_find_local_alias` can find the same *inode*. If `mknod` fails *after* `d_instantiate`... no, that's impossible. If `ocfs2_mknod` fails *before* `d_instantiate`, the dentry is negative. `mknod` calls `end_dirop`, which calls `dput`. *Crucially*, `inode_unlock` is called *before* `dput`, releasing `dir->i_rwsem`. So, if `mknod` is preempted *between* `inode_unlock` and `dput`, `creat` can run in the *same* directory! `creat` will find that negative dentry, and then it is a call to `ocfs2_lookup`! `ocfs2_lookup` calls `d_alloc_parallel` - a NEW dentry. `mknod` could release the inode, *then* be freed by RCU.
If `creat` calls `ocfs2_lookup`, it's not finding a dentry. So `creat` allocated a new dentry. If `ocfs2_lookup` called `ocfs2_dentry_attach_lock`, it called `ocfs2_find_local_alias` and found an *alias* - the dentry that `mknod` allocated! How? If `mknod` succeeded, the dentry is
|
| 49/2 |
2026/05/22 21:00 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/namei.c IncludeLines:true Name:ocfs2_mknod]
Results: map[SourceCode: 230: static int ocfs2_mknod(struct mnt_idmap *idmap,
231: struct inode *dir,
232: struct dentry *dentry,
233: umode_t mode,
234: dev_t dev)
235: {
236: int status = 0;
237: struct buffer_head *parent_fe_bh = NULL;
238: handle_t *handle = NULL;
239: struct ocfs2_super *osb;
240: struct ocfs2_dinode *dirfe;
241: struct ocfs2_dinode *fe = NULL;
242: struct buffer_head *new_fe_bh = NULL;
243: struct inode *inode = NULL;
244: struct ocfs2_alloc_context *inode_ac = NULL;
245: struct ocfs2_alloc_context *data_ac = NULL;
246: struct ocfs2_alloc_context *meta_ac = NULL;
247: int want_clusters = 0;
248: int want_meta = 0;
249: int xattr_credits = 0;
250: struct ocfs2_security_xattr_info si = {
251: .name = NULL,
252: .enable = 1,
253: };
254: int did_quota_inode = 0;
255: struct ocfs2_dir_lookup_result lookup = { NULL, };
256: sigset_t oldset;
257: int did_block_signals = 0;
258: struct ocfs2_dentry_lock *dl = NULL;
259:
260: trace_ocfs2_mknod(dir, dentry, dentry->d_name.len, dentry->d_name.name,
261: (unsigned long long)OCFS2_I(dir)->ip_blkno,
262: (unsigned long)dev, mode);
263:
264: status = dquot_initialize(dir);
265: if (status) {
266: mlog_errno(status);
267: return status;
268: }
269:
270: /* get our super block */
271: osb = OCFS2_SB(dir->i_sb);
272:
273: status = ocfs2_inode_lock(dir, &parent_fe_bh, 1);
274: if (status < 0) {
275: if (status != -ENOENT)
276: mlog_errno(status);
277: return status;
278: }
279:
280: if (S_ISDIR(mode) && (dir->i_nlink >= ocfs2_link_max(osb))) {
281: status = -EMLINK;
282: goto leave;
283: }
284:
285: dirfe = (struct ocfs2_dinode *) parent_fe_bh->b_data;
286: if (!ocfs2_read_links_count(dirfe)) {
287: /* can't make a file in a deleted directory. */
288: status = -ENOENT;
289: goto leave;
290: }
291:
292: status = ocfs2_check_dir_for_entry(dir, dentry->d_name.name,
293: dentry->d_name.len);
294: if (status)
295: goto leave;
296:
297: /* get a spot inside the dir. */
298: status = ocfs2_prepare_dir_for_insert(osb, dir, parent_fe_bh,
299: dentry->d_name.name,
300: dentry->d_name.len, &lookup);
301: if (status < 0) {
302: mlog_errno(status);
303: goto leave;
304: }
305:
306: /* reserve an inode spot */
307: status = ocfs2_reserve_new_inode(osb, &inode_ac);
308: if (status < 0) {
309: if (status != -ENOSPC)
310: mlog_errno(status);
311: goto leave;
312: }
313:
314: inode = ocfs2_get_init_inode(dir, mode);
315: if (IS_ERR(inode)) {
316: status = PTR_ERR(inode);
317: inode = NULL;
318: mlog_errno(status);
319: goto leave;
320: }
321:
322: /* get security xattr */
323: status = ocfs2_init_security_get(inode, dir, &dentry->d_name, &si);
324: if (status) {
325: if (status == -EOPNOTSUPP)
326: si.enable = 0;
327: else {
328: mlog_errno(status);
329: goto leave;
330: }
331: }
332:
333: /* calculate meta data/clusters for setting security and acl xattr */
334: status = ocfs2_calc_xattr_init(dir, parent_fe_bh, mode,
335: &si, &want_clusters,
336: &xattr_credits, &want_meta);
337: if (status < 0) {
338: mlog_errno(status);
339: goto leave;
340: }
341:
342: /* Reserve a cluster if creating an extent based directory. */
343: if (S_ISDIR(mode) && !ocfs2_supports_inline_data(osb)) {
344: want_clusters += 1;
345:
346: /* Dir indexing requires extra space as well */
347: if (ocfs2_supports_indexed_dirs(osb))
348: want_meta++;
349: }
350:
351: status = ocfs2_reserve_new_metadata_blocks(osb, want_meta, &meta_ac);
352: if (status < 0) {
353: if (status != -ENOSPC)
354: mlog_errno(status);
355: goto leave;
356: }
357:
358: status = ocfs2_reserve_clusters(osb, want_clusters, &data_ac);
359: if (status < 0) {
360: if (status != -ENOSPC)
361: mlog_errno(status);
362: goto leave;
363: }
364:
365: handle = ocfs2_start_trans(osb, ocfs2_mknod_credits(osb->sb,
366: S_ISDIR(mode),
367: xattr_credits));
368: if (IS_ERR(handle)) {
369: status = PTR_ERR(handle);
370: handle = NULL;
371: mlog_errno(status);
372: goto leave;
373: }
374:
375: /* Starting to change things, restart is no longer possible. */
376: ocfs2_block_signals(&oldset);
377: did_block_signals = 1;
378:
379: status = dquot_alloc_inode(inode);
380: if (status)
381: goto leave;
382: did_quota_inode = 1;
383:
384: /* do the real work now. */
385: status = ocfs2_mknod_locked(osb, dir, inode, dev,
386: &new_fe_bh, parent_fe_bh, handle,
387: inode_ac);
388: if (status < 0) {
389: mlog_errno(status);
390: goto leave;
391: }
392:
393: fe = (struct ocfs2_dinode *) new_fe_bh->b_data;
394: if (S_ISDIR(mode)) {
395: status = ocfs2_fill_new_dir(osb, handle, dir, inode,
396: new_fe_bh, data_ac, meta_ac);
397: if (status < 0) {
398: mlog_errno(status);
399: goto leave;
400: }
401:
402: status = ocfs2_journal_access_di(handle, INODE_CACHE(dir),
403: parent_fe_bh,
404: OCFS2_JOURNAL_ACCESS_WRITE);
405: if (status < 0) {
406: mlog_errno(status);
407: goto leave;
408: }
409: ocfs2_add_links_count(dirfe, 1);
410: ocfs2_journal_dirty(handle, parent_fe_bh);
411: inc_nlink(dir);
412: }
413:
414: status = ocfs2_init_acl(handle, inode, dir, new_fe_bh, parent_fe_bh,
415: meta_ac, data_ac);
416:
417: if (status < 0) {
418: mlog_errno(status);
419: goto roll_back;
420: }
421:
422: if (si.enable) {
423: status = ocfs2_init_security_set(handle, inode, new_fe_bh, &si,
424: meta_ac, data_ac);
425: if (status < 0) {
426: mlog_errno(status);
427: goto roll_back;
428: }
429: }
430:
431: /*
432: * Do this before adding the entry to the directory. We add
433: * also set d_op after success so that ->d_iput() will cleanup
434: * the dentry lock even if ocfs2_add_entry() fails below.
435: */
436: status = ocfs2_dentry_attach_lock(dentry, inode,
437: OCFS2_I(dir)->ip_blkno);
438: if (status) {
439: mlog_errno(status);
440: goto roll_back;
441: }
442:
443: dl = dentry->d_fsdata;
444:
445: status = ocfs2_add_entry(handle, dentry, inode,
446: OCFS2_I(inode)->ip_blkno, parent_fe_bh,
447: &lookup);
448: if (status < 0) {
449: mlog_errno(status);
450: goto roll_back;
451: }
452:
453: insert_inode_hash(inode);
454: d_instantiate(dentry, inode);
455: status = 0;
456:
457: roll_back:
458: if (status < 0 && S_ISDIR(mode)) {
459: ocfs2_add_links_count(dirfe, -1);
460: drop_nlink(dir);
461: }
462:
463: leave:
464: if (status < 0 && did_quota_inode)
465: dquot_free_inode(inode);
466: if (handle) {
467: if (status < 0 && fe)
468: ocfs2_set_links_count(fe, 0);
469: ocfs2_commit_trans(osb, handle);
470: }
471:
472: ocfs2_inode_unlock(dir, 1);
473: if (did_block_signals)
474: ocfs2_unblock_signals(&oldset);
475:
476: brelse(new_fe_bh);
477: brelse(parent_fe_bh);
478: kfree(si.value);
479:
480: ocfs2_free_dir_lookup_result(&lookup);
481:
482: if (inode_ac)
483: ocfs2_free_alloc_context(inode_ac);
484:
485: if (data_ac)
486: ocfs2_free_alloc_context(data_ac);
487:
488: if (meta_ac)
489: ocfs2_free_alloc_context(meta_ac);
490:
491: /*
492: * We should call iput after the i_rwsem of the bitmap been
493: * unlocked in ocfs2_free_alloc_context, or the
494: * ocfs2_delete_inode will mutex_lock again.
495: */
496: if ((status < 0) && inode) {
497: if (dl)
498: ocfs2_cleanup_add_entry_failure(osb, dentry, inode);
499:
500: OCFS2_I(inode)->ip_flags |= OCFS2_INODE_SKIP_ORPHAN_DIR;
501: clear_nlink(inode);
502: iput(inode);
503: }
504:
505: if (status)
506: mlog_errno(status);
507:
508: return status;
509: }
SourceFile:fs/ocfs2/namei.c]
|
| 50/2 |
2026/05/22 21:00 |
llm |
expert |
0m
Model:
Tokens:
input: 55309
output: 50
thoughts: 156
Thoughts: **Analyzing the OCFS2 Mknod Failure Path**
Okay, let's break this down. If `ocfs2_mknod` hits a snag during the `ocfs2_add_entry` phase, it seems like the system initiates a rollback and returns a negative status, which makes sense. Then, the code calls `iput(inode)`. But, I'm a bit confused because it appears that `ocfs2_dentry_attach_lock(dentry, inode, ...)` was already invoked *before* the potential failure.
Now I remember, `ocfs2_dentry_attach_lock` does set `dentry->d_fsdata` to `dl`, which is important for later. And if there *is* a failure, the code goes on to execute `ocfs2_cleanup_add_entry_failure(osb, dentry, inode)`.
Alright, I should probably take a closer look at what that `ocfs2_cleanup_add_entry_failure` function actually *does*. That's where the real clean up will happen. I need to understand what state it's trying to restore and ensure no resources are leaked.
|
| 51/2 |
2026/05/22 21:00 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/namei.c IncludeLines:true Name:ocfs2_cleanup_add_entry_failure]
Results: map[SourceCode: 215: static void ocfs2_cleanup_add_entry_failure(struct ocfs2_super *osb,
216: struct dentry *dentry, struct inode *inode)
217: {
218: struct ocfs2_dentry_lock *dl = dentry->d_fsdata;
219:
220: ocfs2_simple_drop_lockres(osb, &dl->dl_lockres);
221: ocfs2_lock_res_free(&dl->dl_lockres);
222: BUG_ON(dl->dl_count != 1);
223: spin_lock(&dentry_attach_lock);
224: dentry->d_fsdata = NULL;
225: spin_unlock(&dentry_attach_lock);
226: kfree(dl);
227: iput(inode);
228: }
SourceFile:fs/ocfs2/namei.c]
|
| 52/2 |
2026/05/22 21:00 |
llm |
expert |
0m
Model:
Tokens:
input: 56055
output: 48
thoughts: 1488
Thoughts: **Analyzing a Tricky dentry/inode Race Condition**
Okay, so I'm digging into this issue and here's what I'm seeing. It starts with a `mknod` call that, after some setup, apparently cleans up the `dentry->d_fsdata`. But then, there's a return of `status < 0` from `ocfs2_mknod` and later from `vfs_mknod`. This makes me think there's an error. The control flow eventually reaches `end_dirop(dentry)` after going through `filename_mknodat`. `end_dirop` does an unlock and `dput(dentry)`.
Since the `dentry` was allocated within `mknod`, its initial refcount is 1. `dput(dentry)` will drop it to 0. It's now a negative dentry, meaning it *could* be cached. Now, a subsequent `creat` call on the same name comes along, potentially finding this cached negative dentry. Then `creat` internally ends up calling `ocfs2_create`, which internally calls `ocfs2_mknod`, so it creates a new inode, calls `ocfs2_dentry_attach_lock`, and then `d_instantiate(dentry, inode)`.
Okay, so `creat` *should* finish up, but something's going awry. If `creat` finishes cleanly, the dentry should be positive and the refcount should still be > 0. Why is it being freed? The bug report states: "Allocated by task 12158:" (mknod) "Freed by task 20:" (rcu) "Last potentially related work creation:" (creat -> ocfs2_lookup -> dput). This makes me think `creat` is calling `ocfs2_lookup` and not `ocfs2_create`! So `creat` calls `open(O_CREAT)`, which calls `lookup_open` internally!
If that is true, then `lookup_open` calls `d_lookup`, and *if* that fails, it calls `d_alloc_parallel`. Eventually, it calls `dir_inode->i_op->lookup`, which in this case is `ocfs2_lookup`. Thus the dentry *was not* found initially. Why not? Because the names in `mknod` and `creat` are *different*!
Okay, so `ocfs2_lookup` calls `ocfs2_dentry_attach_lock`, which calls `ocfs2_find_local_alias(inode, parent_blkno, 0)`. Then it finds the alias. So `mknod` and `creat` are for different names, but point to the same inode! How? `mknod` is creating a *new* inode! That means the inode number *could* have been reused! If `mknod` failed and the inode was freed, then `creat` might be getting the *same* inode number.
However, if `mknod` failed it should call `iput(inode)` which frees the inode and removes it from the inode cache. When `creat` allocates a *new* inode, it should be a new `struct inode`. If it's new, `i_dentry` should be empty, and `ocfs2_find_local_alias` wouldn't find anything. So that doesn't track.
But, what if `mknod` *succeeded*? It created the inode, and `d_instantiate` added `dentry` to `inode->i_dentry`. Then, after the `mknod` created dentry, there is an `end_dirop(dentry)` call. Wait, `end_dirop` unlocks the directory and calls `dput(dentry)`. If `mknod` got preempted *before* the `dput(dentry)`, the dentry would have refcount 1. Then `creat` could be looking up a *different* name, which happens to be a hard link to the same inode. `ocfs2_lookup` would find the inode and call `ocfs2_find_local_alias`, and *then* the `dentry` would be found.
However, `ocfs2_find_local_alias` *increments* the refcount. `ocfs2_dentry_attach_lock` calls `dput(alias)`. So that *should* only drop the refcount back to 1. Not free it.
But... the stack trace: `call_rcu+0xee/0x890
|
| 53/2 |
2026/05/22 21:01 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/ocfs2/dcache.c IncludeLines:true Name:ocfs2_find_local_alias]
Results: map[SourceCode: 134: /*
135: * Walk the inode alias list, and find a dentry which has a given
136: * parent. ocfs2_dentry_attach_lock() wants to find _any_ alias as it
137: * is looking for a dentry_lock reference. The downconvert thread is
138: * looking to unhash aliases, so we allow it to skip any that already
139: * have that property.
140: */
141: struct dentry *ocfs2_find_local_alias(struct inode *inode,
142: u64 parent_blkno,
143: int skip_unhashed)
144: {
145: struct dentry *dentry;
146:
147: spin_lock(&inode->i_lock);
148: hlist_for_each_entry(dentry, &inode->i_dentry, d_u.d_alias) {
149: spin_lock(&dentry->d_lock);
150: if (ocfs2_match_dentry(dentry, parent_blkno, skip_unhashed)) {
151: trace_ocfs2_find_local_alias(dentry->d_name.len,
152: dentry->d_name.name);
153:
154: dget_dlock(dentry);
155: spin_unlock(&dentry->d_lock);
156: spin_unlock(&inode->i_lock);
157: return dentry;
158: }
159: spin_unlock(&dentry->d_lock);
160: }
161: spin_unlock(&inode->i_lock);
162: return NULL;
163: }
SourceFile:fs/ocfs2/dcache.c]
|
| 54/2 |
2026/05/22 21:01 |
llm |
expert |
2m
Model:
Tokens:
input: 59147
output: 38
thoughts: 17801
Thoughts: **The Mystery of the Double dput in ocfs2**
Alright, let's break this down. We've got a crash, and it looks like a `dput` call on a dentry that's already been freed. My initial thought process revolves around tracking the dentry's lifetime and figuring out how it's getting prematurely killed, and why `mknod` is trying to free it *again*.
First, let's remember `ocfs2_find_local_alias` uses `dget_dlock(dentry)`: this *increments* the refcount. If the dentry refcount goes to zero within `ocfs2_dentry_attach_lock`, it means the refcount was one *before* `dput`, which means the dentry was already on the LRU (refcount zero) before `ocfs2_find_local_alias`. The first thought, is that `ocfs2_lookup` called `dput`, and it does! But if that dentry is found by another thread, `ocfs2_lookup` will call `ocfs2_dentry_attach_lock`, which uses `dget_dlock`. That's where it's all going wrong.
The critical issue is the refcount. If a dentry's refcount is *zero* before `ocfs2_find_local_alias`, it has to have been put on the LRU list, and means `mknod` itself has already called `dput(de)`. If that were the case, why is `mknod` calling it *again*? Is it possible for a `dput(de)` to occur from two completely separate paths?
Looking at `filename_mknodat`, the code *should* call `dput(dentry)` once, at the end of `end_dirop`. The key point is to see if another function is calling `dput`. I looked through `vfs_mknod` and `ocfs2_mknod`, but they don't seem to have a second `dput`. But, something calls `end_creating_path` and `end_dirop`! This `dput(dentry)` is the problem!
Now, the crash stack trace points to `mknod` trying to `dput` a dentry that's *already* been freed. So *someone* else must have freed it. The KASAN report, tells us that another task called `__dentry_kill`. This stack trace tells us where the dentry was freed: `ocfs2_dentry_attach_lock`, called during a `creat` operation, calling `dput(alias)`. The dentry being freed is the one allocated by `mknod`!
The crucial point is this: why is the *same* dentry being freed by `creat`, when `mknod` still has it? The refcount is key! In the correct path, the `ocfs2_dentry_attach_lock` does the following: finds an alias (calls `dget_dlock`), attaches it, then calls `dput`. If the dentry is on the LRU list (refcount 0), it goes up to 1 when calling `dget_dlock`, and goes down when calling `dput`. `__dentry_kill` *must* be getting called! How can a crash in `dput` of task 12158 be related to the `__dentry_kill` of task 20?
So, if `ocfs2_dentry_attach_lock` calls `dput(alias)` and this `__dentry_kill` call is recorded, the refcount *must* have dropped to zero. What if the `mknod` somehow lost its reference? `d_alloc` can't return a zero-refcount dentry.
I'm back to thinking about the flow in `lookup_one_qstr_excl`. If the `lookup` returns a valid dentry, it calls `dput(dentry)` on a dentry that was just allocated. If it returns NULL, then `lookup_one_qstr_excl` returns `dentry`, which is later passed to `filename_mknodat`. But, in either case, the crash in `dput` is not expected. Then, what if the crash is in `inode_unlock`? And, if the crash is inside `dput` as shown in the stack trace, and is related to `wait_lock` of the lock, why it's not at offset 80 as expected in the dentry, but 208? It's not the `de->
|
| 55/2 |
2026/05/22 21:03 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c IncludeLines:true Name:filename_create]
Results: map[SourceCode:4920: static struct dentry *filename_create(int dfd, struct filename *name,
4921: struct path *path, unsigned int lookup_flags)
4922: {
4923: struct dentry *dentry = ERR_PTR(-EEXIST);
4924: struct qstr last;
4925: bool want_dir = lookup_flags & LOOKUP_DIRECTORY;
4926: unsigned int reval_flag = lookup_flags & LOOKUP_REVAL;
4927: unsigned int create_flags = LOOKUP_CREATE | LOOKUP_EXCL;
4928: int type;
4929: int error;
4930:
4931: error = filename_parentat(dfd, name, reval_flag, path, &last, &type);
4932: if (error)
4933: return ERR_PTR(error);
4934:
4935: /*
4936: * Yucky last component or no last component at all?
4937: * (foo/., foo/.., /////)
4938: */
4939: if (unlikely(type != LAST_NORM))
4940: goto out;
4941:
4942: /* don't fail immediately if it's r/o, at least try to report other errors */
4943: error = mnt_want_write(path->mnt);
4944: /*
4945: * Do the final lookup. Suppress 'create' if there is a trailing
4946: * '/', and a directory wasn't requested.
4947: */
4948: if (last.name[last.len] && !want_dir)
4949: create_flags &= ~LOOKUP_CREATE;
4950: dentry = start_dirop(path->dentry, &last, reval_flag | create_flags);
4951: if (IS_ERR(dentry))
4952: goto out_drop_write;
4953:
4954: if (unlikely(error))
4955: goto fail;
4956:
4957: return dentry;
4958: fail:
4959: end_dirop(dentry);
4960: dentry = ERR_PTR(error);
4961: out_drop_write:
4962: if (!error)
4963: mnt_drop_write(path->mnt);
4964: out:
4965: path_put(path);
4966: return dentry;
4967: }
SourceFile:fs/namei.c]
|
| 56/2 |
2026/05/22 21:03 |
llm |
expert |
1m
Model:
Tokens:
input: 78751
output: 66
thoughts: 5629
Thoughts: **Bug Analysis: UAF in OCFS2 mknod/creat Interaction**
Alright, let's break this down. We're looking at a use-after-free (UAF) in the OCFS2 filesystem, specifically related to the interplay between `mknod` and potentially `creat` (or concurrent file operations). The core problem seems to be an unexpected `dput` on a `dentry` that's still in use, leading to a crash in `rt_mutex_slowunlock`, which stems from the `d_lock` spinlock within the freed `dentry`.
Initially, the code flow of `filename_create` and `mknod` seems sound. The issue arises when we consider the interaction with the OCFS2-specific `ocfs2_dentry_attach_lock` and `ocfs2_lookup` functions. I'm focusing on scenarios where `d_splice_alias` might return an *existing* dentry, or `ocfs2_find_local_alias` returns a dentry which is potentially a dentry that we also have from `mknod`, thus potentially leading to double `dput` calls.
The hypothesis is that something is dropping a reference to the `dentry` allocated by `mknod` prematurely. The analysis centers on `ocfs2_dentry_attach_lock` as it seems to be calling `dput` on a dentry aliased with the allocated dentry.
The code reveals that if `d_splice_alias` returns an existing, disconnected dentry (`alias`), `ocfs2_lookup` might return it. `ocfs2_dentry_attach_lock` then *potentially* calls `dput` on this `alias` after finding and incrementing refcount of the dentry via `ocfs2_find_local_alias`. However, the refcount is balanced when `ocfs2_lookup` is called by `ocfs2_mknod`.
The key to the crash appears to be the *timing*. The key is if `d_splice_alias` returns an existing dentry AND the `old` value set in `lookup_one_qstr_excl` is already freed.
In this specific case, the `mknod` code allocates the dentry, but if there's an existing `dentry`, or if some other concurrent process (like `creat`, but not necessarily that one in particular) is messing with the dentry, then the reference count could become corrupted and the allocated dentry could get freed while a reference is still held.
The bug is reproducible by a combination of calls between `ocfs2` internals. This is a local denial-of-service vulnerability and may potentially allow memory corruption. While OCFS2 filesystem mounting requires root, any user with write access to the mounted OCFS2 filesystem can trigger this bug via standard file operations like `mknod` and `creat`.
|
| 57/2 |
2026/05/22 21:04 |
tool |
set-results |
0m
Args: map[DenialOfService:true Exploitable:true FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:true Exploitable:true FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 58/2 |
2026/05/22 21:04 |
llm |
expert |
0m
Model:
Tokens:
input: 85146
output: 627
thoughts: 372
Thoughts: **Final Thoughts on the OCFS2 Use-After-Free Bug**
Okay, the analysis is complete, and the results are clear. Time to put together the final assessment. This is a classic use-after-free scenario within the `ocfs2` filesystem, specifically targeting a `dentry` object. The crash manifests within `rt_mutex_slowunlock`, which, in PREEMPT_RT kernels, essentially means a spinlock unlock operation. This unlock originates from `dput(dentry)` called in `end_dirop`, a function involved in the `mknod` system call.
What's happening is that the `dentry`, initially allocated by `mknod`, is being prematurely released. A race condition with another concurrent operation—think `creat` or `unlink` potentially, that are also triggering `ocfs2_dentry_attach_lock` and the associated `dput` calls —causes the `dentry`'s reference count to hit zero and get freed through RCU. Later, during the `mknod` cleanup in `end_dirop`, it tries to interact with this now-freed `dentry`, resulting in the inevitable KASAN slab-use-after-free report.
Given that we're talking about a kernel object (`struct dentry`) with function pointers (`d_op`) and spinlocks, this is ripe for exploitation. We're looking at a plausible path to either local privilege escalation or, at a minimum, a kernel panic. The good news (or bad news, depending on perspective) is that triggering this doesn't require any fancy malicious filesystem image. An unprivileged user with write access to a directory on a mounted `ocfs2` filesystem can trigger this through standard operations like `mknod`, `creat`, and `unlink`.
|
| 59/1 |
2026/05/22 21:04 |
action |
format-explanation |
0m
Results: map[Explanation:Based on the analysis of the bug report and the kernel source code, here is the
assessment of the vulnerability:
**Bug Mechanism:**
The bug is a slab-use-after-free of a `struct dentry` object in the `ocfs2`
filesystem.
The crash occurs in `rt_mutex_slowunlock`, which is called from
`spin_unlock(&dentry->d_lock)` (on `PREEMPT_RT` kernels, spinlocks are
implemented as `rt_mutex`es). This happens during `dput(dentry)` called from
`end_dirop()` in the cleanup path of the `mknod` system call
(`filename_mknodat`).
The `dentry` is initially allocated by the `mknod` task. However, due to a race
condition with another concurrent filesystem operation (such as `creat` or
`unlink`), the `ocfs2` cluster locking code (`ocfs2_dentry_attach_lock` ->
`ocfs2_find_local_alias` -> `dput`) prematurely drops the reference count of
this `dentry` to 0. This causes the `dentry` to be killed and freed via RCU
(`__dentry_kill` -> `call_rcu`).
When the original `mknod` task resumes and reaches its cleanup path
(`end_dirop`), it calls `dput(dentry)` on the already freed object. The `dput`
function attempts to acquire and release the `dentry->d_lock` (which resides at
offset 208 inside the `d_lockref` field), leading to the KASAN use-after-free
report.
**Security Impact:**
* **Exploitable:** **Yes**. Use-after-free vulnerabilities on heavily used
kernel structures like `struct dentry` are generally exploitable. The structure
contains spinlocks and pointers (including `d_op` function pointers). An
attacker could potentially spray the heap to reallocate the freed `dentry` with
controlled data, leading to arbitrary code execution or memory corruption when
the kernel attempts to unlock the overwritten spinlock.
* **Denial Of Service:** **Yes**. Triggering the bug will cause a kernel panic
and crash the system.
* **Accessible From Unprivileged Processes:** **Yes**. The bug is triggered by
standard filesystem system calls (`mknod` for regular files/FIFOs, `creat`,
`unlink`). As long as an unprivileged user has write access to a directory
within an `ocfs2` mount, they can trigger the race condition.
* **Malicious Filesystem Trigger:** **No**. The bug is a concurrency issue in
the filesystem's runtime operations (specifically the distributed lock
management `dentry_attach_lock`), not a vulnerability in parsing a maliciously
crafted filesystem image.
**Conclusion:**
This is a local privilege escalation and denial-of-service vulnerability
reachable by unprivileged users interacting with an `ocfs2` filesystem.]
|