| 0/0 |
2026/09/04 05:12 |
flow |
assessment-security |
25m
Results: map[DenialOfService:false Explanation:An analysis of the bug report and the corresponding kernel source code reveals
the following:
**Bug Mechanism and Impact:**
The bug is a `WARN_ON(!reader)` in `ring_buffer_map_get_reader()` located in
`kernel/trace/ring_buffer.c`. It is triggered via the
`TRACE_MMAP_IOCTL_GET_READER` ioctl on tracefs ring buffer files (e.g.,
`per_cpu/cpuX/trace_pipe_raw`).
The warning is hit when `rb_get_reader_page(cpu_buffer)` returns `NULL`. This
can happen due to race conditions with concurrent writers. For instance,
`ring_buffer_map_get_reader()` first checks if the buffer is empty using
`rb_per_cpu_empty()`. If a writer subsequently overwrites events (increasing the
overrun count), the number of entries can drop to zero before
`rb_get_reader_page()` is called. `rb_get_reader_page()` will then return `NULL`
because it sees an empty buffer or fails to swap the head page.
When `reader` is `NULL`, the code hits the `WARN_ON(!reader)` and safely bails
out by jumping to the `out` label. At the `out` label, it flushes the cache,
updates the meta page, releases the reader lock, and returns `0`. There is no
invalid memory access, memory corruption, or information leak. The kernel state
remains consistent, and execution continues normally. Therefore, this is not a
memory safety issue and is not exploitable. It also does not cause a Denial of
Service (DoS) in production builds where `panic_on_warn` is disabled.
**Accessibility:**
The ioctl is exposed through tracefs (typically mounted at
`/sys/kernel/tracing`). Tracefs is restricted to root (requires capabilities
like `CAP_SYS_ADMIN` or `CAP_TRACING`) and its files are not accessible to
unprivileged users. Furthermore, tracefs does not have the `FS_USERNS_MOUNT`
flag, meaning it cannot be mounted inside an unprivileged user namespace. Thus,
the bug cannot be triggered by unprivileged users or from within user
namespaces.
**Conclusion:**
The bug is a benign warning caused by a race condition in the tracing subsystem.
It is only reachable by highly privileged users (root) and does not lead to any
memory corruption, information leak, or denial of service. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/09/04 05:12 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/09/04 05:13 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/297d41c12fba07000f4cde269f3580a719860dd9]
|
| 3/1 |
2026/09/04 05:13 |
action |
kernel-builder |
0m
Results: map[KernelObj:/app/workdir/cache/build/9011d17c7e9067b0246a98528ec14a126164b380]
|
| 4/1 |
2026/09/04 05:13 |
action |
codesearch-prepare |
18m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/09/04 05:31 |
agent |
expert |
7m
Model:
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false 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 grepping
".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, elevated privileges, or an information leak.
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 completely 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 transfers, that 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 exploitability depends on how exactly the uninit value
is used in the code, and what it affects.
Information leaks are exploitable on their own and should be classified as such. A bug that copies kernel
memory contents to userspace (e.g. an out-of-bounds read whose result is returned to the caller, or
uninitialized stack/heap bytes written to a user buffer) is exploitable: it can reveal kernel pointer
values and defeat KASLR, expose sensitive data such as cryptographic keys or other processes' memory, and
serves as a necessary building block in most modern kernel privilege-escalation exploit chains. Do not classify
an information leak as non-exploitable solely because it does not directly cause a memory write or control-flow
hijack; the leak itself is the exploit primitive.
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 Confidential 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:
Cannot find set identified by id 0 to match
------------[ cut here ]------------
!reader
WARNING: kernel/trace/ring_buffer.c:7927 at ring_buffer_map_get_reader+0x5f3/0x930 kernel/trace/ring_buffer.c:7927, CPU#0: syz.2.1764/13608
Modules linked in:
CPU: 0 UID: 0 PID: 13608 Comm: syz.2.1764 Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:ring_buffer_map_get_reader+0x5f3/0x930 kernel/trace/ring_buffer.c:7927
Code: ff 48 89 df e8 2e 21 fe ff 48 89 c5 e8 46 8b fb ff 48 85 ed 0f 85 9f fc ff ff 4c 8b 74 24 58 4c 8b 7c 24 60 e8 2e 8b fb ff 90 <0f> 0b 90 e8 25 8b fb ff 49 8d bf d0 01 00 00 48 b8 00 00 00 00 00
RSP: 0018:ffffc90006b57d98 EFLAGS: 00010087
RAX: 000000000000086c RBX: ffff88801c0e5000 RCX: ffffc90024800000
RDX: 0000000000080000 RSI: ffffffff821064e2 RDI: ffff888027852580
RBP: 0000000000000000 R08: 0000000000000007 R09: 0000000000000000
R10: 0000000000000000 R11: ffff88801c0e50b0 R12: ffff88801c0e50b8
R13: dffffc0000000000 R14: ffff88801c0e5198 R15: ffff88801c0ab800
FS: 00007fa41bfdb6c0(0000) GS:ffff8880d5b61000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f60453c8ff8 CR3: 0000000035d0b000 CR4: 0000000000352ef0
Call Trace:
<TASK>
tracing_buffers_ioctl+0x2e6/0x3d0 kernel/trace/trace.c:7392
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa41b19e159
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:00007fa41bfdb028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fa41b425fa0 RCX: 00007fa41b19e159
RDX: 0000000000000000 RSI: 0000000000005220 RDI: 0000000000000007
RBP: 00007fa41b235024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa41b426038 R14: 00007fa41b425fa0 R15: 00007ffd22388a08
</TASK>
Reply: An analysis of the bug report and the corresponding kernel source code reveals the following:
**Bug Mechanism and Impact:**
The bug is a `WARN_ON(!reader)` in `ring_buffer_map_get_reader()` located in `kernel/trace/ring_buffer.c`. It is triggered via the `TRACE_MMAP_IOCTL_GET_READER` ioctl on tracefs ring buffer files (e.g., `per_cpu/cpuX/trace_pipe_raw`).
The warning is hit when `rb_get_reader_page(cpu_buffer)` returns `NULL`. This can happen due to race conditions with concurrent writers. For instance, `ring_buffer_map_get_reader()` first checks if the buffer is empty using `rb_per_cpu_empty()`. If a writer subsequently overwrites events (increasing the overrun count), the number of entries can drop to zero before `rb_get_reader_page()` is called. `rb_get_reader_page()` will then return `NULL` because it sees an empty buffer or fails to swap the head page.
When `reader` is `NULL`, the code hits the `WARN_ON(!reader)` and safely bails out by jumping to the `out` label. At the `out` label, it flushes the cache, updates the meta page, releases the reader lock, and returns `0`. There is no invalid memory access, memory corruption, or information leak. The kernel state remains consistent, and execution continues normally. Therefore, this is not a memory safety issue and is not exploitable. It also does not cause a Denial of Service (DoS) in production builds where `panic_on_warn` is disabled.
**Accessibility:**
The ioctl is exposed through tracefs (typically mounted at `/sys/kernel/tracing`). Tracefs is restricted to root (requires capabilities like `CAP_SYS_ADMIN` or `CAP_TRACING`) and its files are not accessible to unprivileged users. Furthermore, tracefs does not have the `FS_USERNS_MOUNT` flag, meaning it cannot be mounted inside an unprivileged user namespace. Thus, the bug cannot be triggered by unprivileged users or from within user namespaces.
**Conclusion:**
The bug is a benign warning caused by a race condition in the tracing subsystem. It is only reachable by highly privileged users (root) and does not lead to any memory corruption, information leak, or denial of service.
|
| 6/2 |
2026/09/04 05:31 |
llm |
expert |
0m
Model:
Tokens:
input: 7492
output: 38
thoughts: 182
Thoughts: **Investigating a Ring Buffer Warning**
Okay, I've got a WARNING flagged in `ring_buffer_map_get_reader` at `kernel/trace/ring_buffer.c:7927`. Looks like there's an issue with the ring buffer mechanism in the tracing subsystem. The call trace seems straightforward enough; it originates from `tracing_buffers_ioctl` and ripples down through the standard `vfs_ioctl` path. No surprises there in terms of where the call is coming from. Now, the real meat of the problem is happening inside that `ring_buffer_map_get_reader` function, so let's zoom in on that specific section of `kernel/trace/ring_buffer.c` around line 7927. Hopefully the warning message contains enough detail to pinpoint the root cause quickly, but at the very least, looking at the surrounding code should provide some valuable context. I'm going to want to understand what conditions trigger this warning and what implications it has for the ring buffer's integrity or performance.
|
| 7/2 |
2026/09/04 05:31 |
tool |
read-file |
0m
Args: map[File:kernel/trace/ring_buffer.c FirstLine:7900 LineCount:50]
Results: map[Contents:7900: if (IS_ERR(cpu_buffer))
7901: return (int)PTR_ERR(cpu_buffer);
7902:
7903: raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
7904:
7905: consume:
7906: if (rb_per_cpu_empty(cpu_buffer))
7907: goto out;
7908:
7909: reader_size = rb_page_size(cpu_buffer->reader_page);
7910:
7911: /*
7912: * There are data to be read on the current reader page, we can
7913: * return to the caller. But before that, we assume the latter will read
7914: * everything. Let's update the kernel reader accordingly.
7915: */
7916: if (cpu_buffer->reader_page->read < reader_size) {
7917: while (cpu_buffer->reader_page->read < reader_size)
7918: rb_advance_reader(cpu_buffer);
7919: goto out;
7920: }
7921:
7922: /* Did the reader catch up with the writer? */
7923: if (cpu_buffer->reader_page == cpu_buffer->commit_page)
7924: goto out;
7925:
7926: reader = rb_get_reader_page(cpu_buffer);
7927: if (WARN_ON(!reader))
7928: goto out;
7929:
7930: /* Check if any events were dropped */
7931: missed_events = cpu_buffer->lost_events;
7932:
7933: if (missed_events) {
7934: if (cpu_buffer->reader_page != cpu_buffer->commit_page) {
7935: struct buffer_data_page *dpage = reader->page;
7936: unsigned int commit;
7937: /*
7938: * Use the real_end for the data size,
7939: * This gives us a chance to store the lost events
7940: * on the page.
7941: */
7942: if (reader->real_end)
7943: local_set(&dpage->commit, reader->real_end);
7944: /*
7945: * If there is room at the end of the page to save the
7946: * missed events, then record it there.
7947: */
7948: commit = rb_page_size(reader);
7949: if (rb_page_capacity(reader) - commit >= sizeof(missed_events)) {
]
|
| 8/2 |
2026/09/04 05:31 |
llm |
expert |
0m
Model:
Tokens:
input: 8700
output: 36
thoughts: 50
Thoughts: Here's my summary, as though I'm thinking through this:
**Analyzing a Kernel Trace Warning**
Okay, so I've got a `WARN_ON(!reader)` warning popping up in `ring_buffer_map_get_reader`. My immediate thought is to trace the root of this. A null reader is usually a symptom, not the cause. The key will be to figure out where that `reader` is being assigned, or failing to be assigned.
Let's start by backing up a bit. Since this is likely related to tracing functionality, I'm going to jump to `tracing_buffers_ioctl` in `kernel/trace/trace.c:7392`. It's a common area for interacting with ring buffers. I'll need to see how the reader is being created, initialized, and potentially, if the function is even working as expected. If I can understand the context of this ioctl call, I can probably determine under which conditions a null reader might be possible. From there, I'll trace through the code paths and identify the most likely places where the `reader` variable is at risk. It could be a simple race condition, a bug in the allocation logic, or some other unexpected edge case. Time to dig in and do some deep code review.
|
| 9/2 |
2026/09/04 05:31 |
tool |
read-file |
0m
Args: map[File:kernel/trace/trace.c FirstLine:7370 LineCount:50]
Results: map[Contents:7370: out:
7371: splice_shrink_spd(&spd);
7372:
7373: return ret;
7374: }
7375:
7376: static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
7377: {
7378: struct ftrace_buffer_info *info = file->private_data;
7379: struct trace_iterator *iter = &info->iter;
7380: int err;
7381:
7382: if (cmd == TRACE_MMAP_IOCTL_GET_READER) {
7383: if (!(file->f_flags & O_NONBLOCK)) {
7384: err = ring_buffer_wait(iter->array_buffer->buffer,
7385: iter->cpu_file,
7386: iter->tr->buffer_percent,
7387: NULL, NULL);
7388: if (err)
7389: return err;
7390: }
7391:
7392: return ring_buffer_map_get_reader(iter->array_buffer->buffer,
7393: iter->cpu_file);
7394: } else if (cmd) {
7395: return -ENOTTY;
7396: }
7397:
7398: /*
7399: * An ioctl call with cmd 0 to the ring buffer file will wake up all
7400: * waiters
7401: */
7402: guard(mutex)(&trace_types_lock);
7403:
7404: /* Make sure the waiters see the new wait_index */
7405: (void)atomic_fetch_inc_release(&iter->wait_index);
7406:
7407: ring_buffer_wake_waiters(iter->array_buffer->buffer, iter->cpu_file);
7408:
7409: return 0;
7410: }
7411:
7412: /*
7413: * This is called when a VMA is duplicated (e.g., on fork()) to increment
7414: * the user_mapped counter without remapping pages.
7415: */
7416: static void tracing_buffers_mmap_open(struct vm_area_struct *vma)
7417: {
7418: struct ftrace_buffer_info *info = vma->vm_file->private_data;
7419: struct trace_iterator *iter = &info->iter;
]
|
| 10/2 |
2026/09/04 05:31 |
llm |
expert |
0m
Model:
Tokens:
input: 9784
output: 23
thoughts: 63
Thoughts: **Investigating the Trace Pipe's Reader Acquisition**
Okay, so I'm looking into this `TRACE_MMAP_IOCTL_GET_READER` ioctl. It seems to be calling `ring_buffer_map_get_reader` – which makes sense given the function name. I know this ioctl operates on the `trace_pipe_raw` file, or a similar trace pipe file. Now, I want to trace the execution path and identify any potential issues or nuances, or perhaps just to understand its behavior more fully, I'll check where `tracing_buffers_ioctl` is used within the kernel. Let's see how this ioctl integrates with the broader tracing infrastructure.
|
| 11/2 |
2026/09/04 05:31 |
tool |
codesearch-find-references |
0m
Args: map[Name:tracing_buffers_ioctl]
Results: map[References:[map[ReferenceKind:takes-address-of ReferencingEntityKind:global_variable ReferencingEntityName:tracing_buffers_fops SourceFile:kernel/trace/trace.c SourceLine:7478]] TruncatedOutput:false]
|
| 12/2 |
2026/09/04 05:31 |
llm |
expert |
0m
Model:
Tokens:
input: 9799
output: 36
thoughts: 0
|
| 13/2 |
2026/09/04 05:31 |
tool |
read-file |
0m
Args: map[File:kernel/trace/trace.c FirstLine:7470 LineCount:20]
Results: map[Contents:7470:
7471: static const struct file_operations tracing_buffers_fops = {
7472: .open = tracing_buffers_open,
7473: .read = tracing_buffers_read,
7474: .poll = tracing_buffers_poll,
7475: .release = tracing_buffers_release,
7476: .flush = tracing_buffers_flush,
7477: .splice_read = tracing_buffers_splice_read,
7478: .unlocked_ioctl = tracing_buffers_ioctl,
7479: .mmap = tracing_buffers_mmap,
7480: };
7481:
7482: static ssize_t
7483: tracing_stats_read(struct file *filp, char __user *ubuf,
7484: size_t count, loff_t *ppos)
7485: {
7486: struct inode *inode = file_inode(filp);
7487: struct trace_array *tr = inode->i_private;
7488: struct array_buffer *trace_buf = &tr->array_buffer;
7489: int cpu = tracing_get_cpu(inode);
]
|
| 14/2 |
2026/09/04 05:31 |
llm |
expert |
0m
Model:
Tokens:
input: 10476
output: 35
thoughts: 93
Thoughts: **My Investigation into Tracefs and User Namespaces**
Okay, so I'm digging into how `tracing_buffers_fops` works within the context of tracefs. Specifically, I'm thinking about those `per_cpu/cpu%d/trace_pipe_raw` files that show up there. Tracefs, of course, usually lives under `/sys/kernel/tracing`, and I know access to that is locked down pretty tight – root privileges or specific capabilities like `CAP_SYS_ADMIN` or `CAP_PERFMON` are usually required, or membership in the tracing group.
But now I'm wondering, can we mount tracefs *within* a user namespace? That's the real question I need to answer. It could open up some interesting possibilities if we could isolate tracing functionality in this way. I need to figure out if the design allows for this type of nested or isolated environment. It would be valuable for certain use-cases.
|
| 15/2 |
2026/09/04 05:31 |
tool |
grepper |
0m
Error:
Args: map[Expression:fs_flags.*FS_USERNS_MOUNT PathPrefix:fs/tracefs]
Results:
|
| 16/2 |
2026/09/04 05:31 |
llm |
expert |
0m
Model:
Tokens:
input: 10821
output: 25
thoughts: 162
Thoughts: **Investigating Tracefs Access**
Okay, so Tracefs doesn't support `FS_USERNS_MOUNT`, meaning I can't just slap it into a user namespace like I'd hoped. Hmm, but what if a privileged user mounts it, and I then fiddle with the permissions after the fact? Potentially. It seems it's locked down by default, though.
Right, let's pivot. Are there *any* other avenues to get at `tracing_buffers_ioctl`? Thinking strategically here...
Wait a minute, this bug report says: `WARNING: kernel/trace/ring_buffer.c:7927 at ring_buffer_map_get_reader+0x5f3/0x930`. I need to parse that. Is this a real memory corruption issue, or just a benign warning?
It's a `WARN_ON(!reader)`. Okay, so it's a warning, specifically. Now, I should dive into what `rb_get_reader_page` is doing and what its implications are in this scenario.
|
| 17/2 |
2026/09/04 05:31 |
tool |
codesearch-definition-source |
0m
Args: map[Name:rb_get_reader_page]
Results: map[SourceCode:6034: static struct buffer_page *
6035: rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
6036: {
6037: return cpu_buffer->remote ? __rb_get_reader_page_from_remote(cpu_buffer) :
6038: __rb_get_reader_page(cpu_buffer);
6039: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 18/2 |
2026/09/04 05:31 |
llm |
expert |
0m
Model:
Tokens:
input: 11383
output: 26
thoughts: 15
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience and written in the first person:
**Diving into `__rb_get_reader_page`**
Alright, let's zero in on `__rb_get_reader_page`. I need to understand this function's role in the larger picture. My initial thought is it's likely a key component within the reader subsystem, maybe a page management function. What data structures is it operating on? Specifically, I need to know the context: what kind of reader is this tied to? Is it buffering, caching, or something else entirely? The name itself suggests we are reading some form of "page". Is that a hardware page, a virtual page within a file, or an application-defined concept? I'm going to quickly skim the surrounding code to determine how this function fits into the overall reading process. I want to look for call sites and data flows; I need to form a mental model of this page retrieval and how it impacts system performance. It's time to dig in and determine if this is a potential performance bottleneck, or perhaps it’s part of a critical error path that could be a source of future issues. Let's see what's actually going on.
|
| 19/2 |
2026/09/04 05:31 |
tool |
codesearch-definition-source |
0m
Args: map[Name:__rb_get_reader_page]
Results: map[SourceCode:5853: static struct buffer_page *
5854: __rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
5855: {
5856: int max_loops = cpu_buffer->ring_meta ? cpu_buffer->nr_pages : 3;
5857: struct buffer_page *reader = NULL;
5858: unsigned long overwrite;
5859: unsigned long flags;
5860: int missed_events = 0;
5861: int nr_loops = 0;
5862: bool ret;
5863:
5864: local_irq_save(flags);
5865: arch_spin_lock(&cpu_buffer->lock);
5866:
5867: again:
5868: /*
5869: * This should normally only loop twice. But because the
5870: * start of the reader inserts an empty page, it causes a
5871: * case where we will loop three times. There should be no
5872: * reason to loop four times unless the ring buffer is a
5873: * recovered persistent ring buffer. For persistent ring buffers,
5874: * invalid pages are reset during recovery, so there may be more
5875: * than 3 contiguous pages can be empty, but less than nr_pages.
5876: */
5877: if (RB_WARN_ON(cpu_buffer, ++nr_loops > max_loops)) {
5878: reader = NULL;
5879: goto out;
5880: }
5881:
5882: reader = cpu_buffer->reader_page;
5883:
5884: /* If there's more to read, return this page */
5885: if (cpu_buffer->reader_page->read < rb_page_size(reader))
5886: goto out;
5887:
5888: /* Never should we have an index greater than the size */
5889: if (RB_WARN_ON(cpu_buffer,
5890: cpu_buffer->reader_page->read > rb_page_size(reader)))
5891: goto out;
5892:
5893: /* check if we caught up to the tail */
5894: reader = NULL;
5895: if (cpu_buffer->commit_page == cpu_buffer->reader_page)
5896: goto out;
5897:
5898: /* Don't bother swapping if the ring buffer is empty */
5899: if (rb_num_of_entries(cpu_buffer) == 0)
5900: goto out;
5901:
5902: /*
5903: * Reset the reader page to size zero.
5904: */
5905: local_set(&cpu_buffer->reader_page->write, 0);
5906: local_set(&cpu_buffer->reader_page->entries, 0);
5907: rb_init_data_page(cpu_buffer->reader_page->page);
5908: cpu_buffer->reader_page->real_end = 0;
5909:
5910: spin:
5911: /*
5912: * Splice the empty reader page into the list around the head.
5913: */
5914: reader = rb_set_head_page(cpu_buffer);
5915: if (!reader)
5916: goto out;
5917: cpu_buffer->reader_page->list.next = rb_list_head(reader->list.next);
5918: cpu_buffer->reader_page->list.prev = reader->list.prev;
5919:
5920: /*
5921: * cpu_buffer->pages just needs to point to the buffer, it
5922: * has no specific buffer page to point to. Lets move it out
5923: * of our way so we don't accidentally swap it.
5924: */
5925: cpu_buffer->pages = reader->list.prev;
5926:
5927: /* The reader page will be pointing to the new head */
5928: rb_set_list_to_head(&cpu_buffer->reader_page->list);
5929:
5930: /*
5931: * We want to make sure we read the overruns after we set up our
5932: * pointers to the next object. The writer side does a
5933: * cmpxchg to cross pages which acts as the mb on the writer
5934: * side. Note, the reader will constantly fail the swap
5935: * while the writer is updating the pointers, so this
5936: * guarantees that the overwrite recorded here is the one we
5937: * want to compare with the last_overrun.
5938: */
5939: smp_mb();
5940: overwrite = local_read(&(cpu_buffer->overrun));
5941:
5942: /*
5943: * Here's the tricky part.
5944: *
5945: * We need to move the pointer past the header page.
5946: * But we can only do that if a writer is not currently
5947: * moving it. The page before the header page has the
5948: * flag bit '1' set if it is pointing to the page we want.
5949: * but if the writer is in the process of moving it
5950: * then it will be '2' or already moved '0'.
5951: */
5952:
5953: ret = rb_head_page_replace(reader, cpu_buffer->reader_page);
5954:
5955: /*
5956: * If we did not convert it, then we must try again.
5957: */
5958: if (!ret)
5959: goto spin;
5960:
5961: if (rb_page_commit(reader) & RB_MISSED_EVENTS)
5962: missed_events = -1;
5963:
5964: if (cpu_buffer->ring_meta)
5965: rb_update_meta_reader(cpu_buffer, reader);
5966:
5967: /*
5968: * Yay! We succeeded in replacing the page.
5969: *
5970: * Now make the new head point back to the reader page.
5971: */
5972: rb_list_head(reader->list.next)->prev = &cpu_buffer->reader_page->list;
5973: rb_inc_page(&cpu_buffer->head_page);
5974:
5975: cpu_buffer->cnt++;
5976: local_inc(&cpu_buffer->pages_read);
5977:
5978: /* Finally update the reader page to the new head */
5979: cpu_buffer->reader_page = reader;
5980: cpu_buffer->reader_page->read = 0;
5981:
5982: if (overwrite != cpu_buffer->last_overrun) {
5983: cpu_buffer->lost_events = overwrite - cpu_buffer->last_overrun;
5984: cpu_buffer->last_overrun = overwrite;
5985: }
5986:
5987: goto again;
5988:
5989: out:
5990: /* Update the read_stamp on the first event */
5991: if (reader && reader->read == 0)
5992: cpu_buffer->read_stamp = reader->page->time_stamp;
5993:
5994: arch_spin_unlock(&cpu_buffer->lock);
5995: local_irq_restore(flags);
5996:
5997: /*
5998: * The writer has preempt disable, wait for it. But not forever
5999: * Although, 1 second is pretty much "forever"
6000: */
6001: #define USECS_WAIT 1000000
6002: for (nr_loops = 0; nr_loops < USECS_WAIT; nr_loops++) {
6003: /* If the write is past the end of page, a writer is still updating it */
6004: if (likely(!reader || rb_page_write(reader) <= rb_page_capacity(reader)))
6005: break;
6006:
6007: udelay(1);
6008:
6009: /* Get the latest version of the reader write value */
6010: smp_rmb();
6011: }
6012:
6013: /* The writer is not moving forward? Something is wrong */
6014: if (RB_WARN_ON(cpu_buffer, nr_loops == USECS_WAIT))
6015: reader = NULL;
6016:
6017: /*
6018: * Make sure we see any padding after the write update
6019: * (see rb_reset_tail()).
6020: *
6021: * In addition, a writer may be writing on the reader page
6022: * if the page has not been fully filled, so the read barrier
6023: * is also needed to make sure we see the content of what is
6024: * committed by the writer (see rb_set_commit_to_write()).
6025: */
6026: smp_rmb();
6027:
6028: if (!cpu_buffer->lost_events)
6029: cpu_buffer->lost_events = missed_events;
6030:
6031: return reader;
6032: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 20/2 |
2026/09/04 05:31 |
llm |
expert |
0m
Model:
Tokens:
input: 14425
output: 38
thoughts: 467
Thoughts: **Analyzing Potential User-Triggerable Bugs in Ring Buffer Reader Retrieval**
Okay, let's break this down. The core function of interest is `__rb_get_reader_page`. I'm seeing a few conditions that can cause it to return NULL, which immediately flags a potential issue:
1. A warning is triggered if the number of loops exceeds `max_loops`.
2. Another warning happens if the reader's read pointer exceeds the page size.
3. The `commit_page` and `reader_page` point to the same location. This suggests the reader might be catching up and hasn't advanced.
4. The number of entries in the `cpu_buffer` is zero; effectively, the buffer is empty.
5. `rb_set_head_page` returns NULL, which would be a more fundamental issue with buffer setup.
6. A warning is triggered if the number of loops equals `USECS_WAIT`.
The next point of interest is the function `ring_buffer_map_get_reader`. It calls `__rb_get_reader_page`, and if that returns NULL, it results in a `WARN_ON`. This is the critical juncture. Now, the question is whether user-space can trigger this NULL return.
I see that `tracing_buffers_ioctl` makes a call to `ring_buffer_map_get_reader`. This is where it gets interesting. Inside `tracing_buffers_ioctl`, there's a conditional check on `O_NONBLOCK` file flag. If `O_NONBLOCK` is *not* set, the code calls `ring_buffer_wait` which likely includes a wait condition. However, if `O_NONBLOCK` *is* set, `ring_buffer_map_get_reader` is called directly.
Crucially, in the non-blocking case (`O_NONBLOCK` set), if the ring buffer is empty, `__rb_get_reader_page` *will* return NULL due to condition 4 I mentioned earlier. This leads to the WARN_ON. Therefore, yes, it looks like a user-space application *can* trigger this NULL return and, consequently, the warning, by setting the `O_NONBLOCK` flag and attempting to read from an empty ring buffer. This means a user can potentially trigger this `WARN_ON` within the kernel. I need to investigate further the implications of this warning, as it could be indicative of a more significant underlying issue, especially in a tracing context.
|
| 21/2 |
2026/09/04 05:31 |
tool |
read-file |
0m
Args: map[File:kernel/trace/ring_buffer.c FirstLine:7900 LineCount:50]
Results: map[Contents:7900: if (IS_ERR(cpu_buffer))
7901: return (int)PTR_ERR(cpu_buffer);
7902:
7903: raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
7904:
7905: consume:
7906: if (rb_per_cpu_empty(cpu_buffer))
7907: goto out;
7908:
7909: reader_size = rb_page_size(cpu_buffer->reader_page);
7910:
7911: /*
7912: * There are data to be read on the current reader page, we can
7913: * return to the caller. But before that, we assume the latter will read
7914: * everything. Let's update the kernel reader accordingly.
7915: */
7916: if (cpu_buffer->reader_page->read < reader_size) {
7917: while (cpu_buffer->reader_page->read < reader_size)
7918: rb_advance_reader(cpu_buffer);
7919: goto out;
7920: }
7921:
7922: /* Did the reader catch up with the writer? */
7923: if (cpu_buffer->reader_page == cpu_buffer->commit_page)
7924: goto out;
7925:
7926: reader = rb_get_reader_page(cpu_buffer);
7927: if (WARN_ON(!reader))
7928: goto out;
7929:
7930: /* Check if any events were dropped */
7931: missed_events = cpu_buffer->lost_events;
7932:
7933: if (missed_events) {
7934: if (cpu_buffer->reader_page != cpu_buffer->commit_page) {
7935: struct buffer_data_page *dpage = reader->page;
7936: unsigned int commit;
7937: /*
7938: * Use the real_end for the data size,
7939: * This gives us a chance to store the lost events
7940: * on the page.
7941: */
7942: if (reader->real_end)
7943: local_set(&dpage->commit, reader->real_end);
7944: /*
7945: * If there is room at the end of the page to save the
7946: * missed events, then record it there.
7947: */
7948: commit = rb_page_size(reader);
7949: if (rb_page_capacity(reader) - commit >= sizeof(missed_events)) {
]
|
| 22/2 |
2026/09/04 05:31 |
llm |
expert |
0m
Model:
Tokens:
input: 16237
output: 25
thoughts: 134
Thoughts: **Analyzing the Ring Buffer Reader Acquisition**
Okay, so I'm looking at `ring_buffer_map_get_reader`. My initial thought is that it seems to be doing the right checks. It's properly checking if the CPU's buffer is empty using `rb_per_cpu_empty(cpu_buffer)` and also verifying that the reader and commit pages are the same. That seems sensible, so far so good. Then, the code calls `rb_get_reader_page`.
But, wait a minute... I know that `rb_get_reader_page` can return a NULL pointer. And I know that it will do so specifically when `rb_num_of_entries(cpu_buffer) == 0`. This triggers a concern. Is it actually possible that `rb_per_cpu_empty` could return false (meaning the buffer *isn't* empty), while simultaneously `rb_num_of_entries` is actually zero? That would be a race condition, or a logic error, depending on the implementation!
I need to dig into `rb_per_cpu_empty`. I'm going to have to check that function’s implementation, because that's the key to understanding the potential problem. This discrepancy has me intrigued.
|
| 23/2 |
2026/09/04 05:31 |
tool |
codesearch-definition-source |
0m
Args: map[Name:rb_per_cpu_empty]
Results: map[SourceCode:5219: static bool rb_per_cpu_empty(struct ring_buffer_per_cpu *cpu_buffer)
5220: {
5221: return !rb_num_of_entries(cpu_buffer);
5222: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 24/2 |
2026/09/04 05:31 |
llm |
expert |
0m
Model:
Tokens:
input: 16765
output: 19
thoughts: 499
Thoughts: **Debugging a Ring Buffer Reader**
Okay, so I'm trying to figure out why `rb_get_reader_page` might return NULL, even when `rb_per_cpu_empty` (which, remember, is just the inverse of the number of entries being non-zero) suggests there *should* be data. The core of this issue seems to be how the read pointer is being handled in relation to the ring buffer's state.
First, I need to jog my memory. `rb_get_reader_page` returns NULL under several conditions, and I have them listed here. Let's start with the likely culprit.
1. A warning is triggered if we loop too many times, indicating an unusual condition. This is possible if there is a race-condition.
2. If the reader's read position is beyond the page size.
3. If the reader and commit pages are the same, indicating the ring is either empty or in an unusual state.
4. If the number of entries is zero.
5. If `rb_set_head_page` returns NULL, which would point to something going wrong with the ring's setup.
6. A warning is triggered if we loop to many times.
But, hang on a second. `ring_buffer_map_get_reader` calls `rb_get_reader_page`, and it *seems* to check if `cpu_buffer->reader_page == cpu_buffer->commit_page` *before* calling `rb_get_reader_page`. That should catch case 3, so why am I getting a NULL?
Wait, I need to check something... `rb_get_reader_page` is *not* called with a lock held. *However*, `ring_buffer_map_get_reader` holds `cpu_buffer->reader_lock`. Okay, that's crucial. `__rb_get_reader_page` *does* take `cpu_buffer->lock`.
So, the question is: could something change *between* the check using `cpu_buffer->reader_lock` and the call to `__rb_get_reader_page`? No, because `rb_per_cpu_empty` is called inside the protected region. I need to make sure I am thinking clearly.
Aha! It just hit me. `cpu_buffer->lock` and `cpu_buffer->reader_lock` are *different* locks! That explains it! Now I need to figure out who is using the different locks. Are writers taking `cpu_buffer->lock`? If they are lockless, the writers could be writing at any time. Or maybe they are taking the lock as well. I need to find out what `cpu_buffer->lock` *actually* is. I'm going to have to dive into the definition of this struct.
|
| 25/2 |
2026/09/04 05:32 |
tool |
codesearch-find-references |
0m
Args:
Results: map[References:<nil> TruncatedOutput:false]
|
| 26/2 |
2026/09/04 05:32 |
llm |
expert |
0m
Model:
Tokens:
input: 17918
output: 42
thoughts: 30
Thoughts: Okay, here's my take on those thoughts, framed as my own:
**Reframing the Search Strategy**
Hmm, hold on. Searching for just "lock" is way too broad. I'll drown in irrelevant results. That's not the path to efficiency. I need to be more precise. Instead of a generic term, I should focus on the *specific* lock I'm interested in, right? Let's use the code itself to guide the search. The real question is how it interacts with the buffer. I need to zero in. So, forget the general term, instead of "lock", let's be crystal clear about the target. I'll search for the *exact* call: `arch_spin_lock(&cpu_buffer->lock)`. That should give me a much more focused and actionable set of results. This is the way.
|
| 27/2 |
2026/09/04 05:32 |
tool |
grepper |
0m
Args: map[Expression:arch_spin_lock\(&cpu_buffer->lock\) PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:kernel/trace/ring_buffer.c=5854=__rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-5864- local_irq_save(flags);
kernel/trace/ring_buffer.c:5865: arch_spin_lock(&cpu_buffer->lock);
kernel/trace/ring_buffer.c-5866-
--
kernel/trace/ring_buffer.c=6435=ring_buffer_read_start(struct trace_buffer *buffer, int cpu, gfp_t flags)
--
kernel/trace/ring_buffer.c-6469- guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
kernel/trace/ring_buffer.c:6470: arch_spin_lock(&cpu_buffer->lock);
kernel/trace/ring_buffer.c-6471- rb_iter_reset(iter);
--
kernel/trace/ring_buffer.c=6685=static void reset_disabled_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-6691-
kernel/trace/ring_buffer.c:6692: arch_spin_lock(&cpu_buffer->lock);
kernel/trace/ring_buffer.c-6693-
--
kernel/trace/ring_buffer.c=7009=ring_buffer_alloc_read_page(struct trace_buffer *buffer, int cpu)
--
kernel/trace/ring_buffer.c-7024- local_irq_save(flags);
kernel/trace/ring_buffer.c:7025: arch_spin_lock(&cpu_buffer->lock);
kernel/trace/ring_buffer.c-7026-
--
kernel/trace/ring_buffer.c=7057=void ring_buffer_free_read_page(struct trace_buffer *buffer, int cpu,
--
kernel/trace/ring_buffer.c-7078- local_irq_save(flags);
kernel/trace/ring_buffer.c:7079: arch_spin_lock(&cpu_buffer->lock);
kernel/trace/ring_buffer.c-7080-
--
kernel/trace/ring_buffer.c=7378=int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
--
kernel/trace/ring_buffer.c-7493-
kernel/trace/ring_buffer.c:7494: arch_spin_lock(&cpu_buffer->lock);
kernel/trace/ring_buffer.c-7495- old_free_data_page = cpu_buffer->free_page;
]
|
| 28/2 |
2026/09/04 05:32 |
llm |
expert |
0m
Model:
Tokens:
input: 18878
output: 25
thoughts: 172
Thoughts: **Analyzing the Race Conditions in This Concurrency Scenario**
Okay, so the writers here are lockless, meaning they don't take `cpu_buffer->lock`. That immediately flags `rb_num_of_entries` as a potential concurrency hotspot. My initial thought is that `rb_num_of_entries` can be altered concurrently by a writer adding an event. Makes sense, it's just incrementing in that situation. But wait, it's *only* supposed to increase, right?
Alright, let's play this out: if `rb_num_of_entries` is zero, that definitively signals an empty buffer. If a writer comes along, it will add an event, meaning `rb_num_of_entries` goes above zero. So, logically, if `rb_per_cpu_empty` is false, then `rb_num_of_entries` *has* to be greater than zero.
Hold on, could `rb_num_of_entries` *decrease*? That's the real question. Readers are the ones decreasing the counter. But readers are serialized by `cpu_buffer->reader_lock`. So, that shouldn't be possible. But, wait, what if the value becomes negative? That is a state I do not currently understand how it happens, so let's check `rb_num_of_entries`.
|
| 29/2 |
2026/09/04 05:32 |
tool |
codesearch-definition-source |
0m
Args: map[Name:rb_num_of_entries]
Results: map[SourceCode:5206: /*
5207: * The total entries in the ring buffer is the running counter
5208: * of entries entered into the ring buffer, minus the sum of
5209: * the entries read from the ring buffer and the number of
5210: * entries that were overwritten.
5211: */
5212: static inline unsigned long
5213: rb_num_of_entries(struct ring_buffer_per_cpu *cpu_buffer)
5214: {
5215: return local_read(&cpu_buffer->entries) -
5216: (local_read(&cpu_buffer->overrun) + cpu_buffer->read);
5217: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 30/2 |
2026/09/04 05:32 |
llm |
expert |
0m
Model:
Tokens:
input: 19587
output: 25
thoughts: 584
Thoughts: **Debugging a Ring Buffer Concurrency Issue**
Okay, so I'm digging into this `rb_num_of_entries` calculation: `entries - (overrun + read)`. I'm starting to think about concurrent access. If `overrun` increases at the same time, this `rb_num_of_entries` value could *decrease* unexpectedly. Could this value become zero between the calls to `rb_per_cpu_empty` and `rb_get_reader_page`? The more I think about it the more I believe, yes, if a writer is concurrently overwriting the *last* event, that is, when the ring is full, it's possible.
But wait, if the last event is overwritten, wouldn't `cpu_buffer->commit_page` also move forward? I'm getting ahead of myself, let me think this through. Right, if `rb_num_of_entries` hits zero, `__rb_get_reader_page` *should* return NULL. And if that happens, `ring_buffer_map_get_reader` will trigger a `WARN_ON(!reader)`. Okay, is this the root of the problem? Let's go back and examine `__rb_get_reader_page` more closely:
```c
/* check if we caught up to the tail */
reader = NULL;
if (cpu_buffer->commit_page == cpu_buffer->reader_page)
goto out;
/* Don't bother swapping if the ring buffer is empty */
if (rb_num_of_entries(cpu_buffer) == 0)
goto out;
```
Yup, if `rb_num_of_entries` is zero, the function returns NULL. Okay, so now I am seeing this. `ring_buffer_map_get_reader` does:
```c
/* Did the reader catch up with the writer? */
if (cpu_buffer->reader_page == cpu_buffer->commit_page)
goto out;
reader = rb_get_reader_page(cpu_buffer);
if (WARN_ON(!reader))
goto out;
```
Aha! If `rb_num_of_entries` goes to zero because of an overrun, meaning the writer has overwritten the entire buffer, the second `if` fails, because the writer overwrote the whole buffer. But if the writer overwrites the *whole* buffer, `cpu_buffer->commit_page` *should* advance, which makes the check pass. But wait, if the writer overwrites everything, `cpu_buffer->commit_page` *will not* equal `cpu_buffer->reader_page` because the reader page is not actually in the ring, it's swapped out! This is the key.
Wait, the `reader_page` is swapped out? And the writer never touches the `reader_page` directly. Yes, that makes sense. So, `cpu_buffer->commit_page` can *never* equal `cpu_buffer->reader_page` if the `reader_page` is swapped out. Wait, is the `reader_page` initially within the ring buffer? No, that's wrong. It is always *outside* the ring unless it is swapped in. So, let me check the `rb_set_head_page` function again. I am starting to see the issue...
|
| 31/2 |
2026/09/04 05:32 |
tool |
codesearch-definition-source |
0m
Args: map[Name:rb_set_head_page]
Results: map[SourceCode:1460: static struct buffer_page *
1461: rb_set_head_page(struct ring_buffer_per_cpu *cpu_buffer)
1462: {
1463: struct buffer_page *head;
1464: struct buffer_page *page;
1465: struct list_head *list;
1466: int i;
1467:
1468: if (RB_WARN_ON(cpu_buffer, !cpu_buffer->head_page))
1469: return NULL;
1470:
1471: /* sanity check */
1472: list = cpu_buffer->pages;
1473: if (RB_WARN_ON(cpu_buffer, rb_list_head(list->prev->next) != list))
1474: return NULL;
1475:
1476: page = head = cpu_buffer->head_page;
1477: /*
1478: * It is possible that the writer moves the header behind
1479: * where we started, and we miss in one loop.
1480: * A second loop should grab the header, but we'll do
1481: * three loops just because I'm paranoid.
1482: */
1483: for (i = 0; i < 3; i++) {
1484: do {
1485: if (rb_is_head_page(page, page->list.prev)) {
1486: cpu_buffer->head_page = page;
1487: return page;
1488: }
1489: rb_inc_page(&page);
1490: } while (page != head);
1491: }
1492:
1493: RB_WARN_ON(cpu_buffer, 1);
1494:
1495: return NULL;
1496: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 32/2 |
2026/09/04 05:32 |
llm |
expert |
0m
Model:
Tokens:
input: 21495
output: 26
thoughts: 1343
Thoughts: **Debugging a Ring Buffer Reader Issue**
Okay, let's break this down. I'm looking at `ring_buffer_map_get_reader` and tracing its logic. I see that it first checks if the buffer is empty using `rb_per_cpu_empty(cpu_buffer)`. If it's empty, it bails. If not, it checks if the reader has read everything in the current page, and if it hasn't, it advances the reader and returns. If it has, and if the reader hasn't caught up to the commit page, it calls `rb_get_reader_page` to get the next page. If `rb_get_reader_page` returns NULL, it warns, which is the crux of the issue reported by syzbot.
So, why would `rb_get_reader_page` return NULL? I'm going back into the definition of `__rb_get_reader_page` to check. The code shows several conditions where it could return NULL: if the current page isn't fully read, if the reader has reached the commit page, or if the buffer is empty. But I'm going to dig deeper into the last one.
It checks `rb_num_of_entries(cpu_buffer) == 0`. Could this happen even after `rb_per_cpu_empty` returned false? It *seems* like overwriting might cause that. But after thinking about it, if a writer overwrites, `overrun` increases, but `entries` and the actual amount of entries in the buffer also goes up, meaning `entries - overrun` remains constant. So `rb_num_of_entries` only decreases when the reader reads an event. And, readers are serialized by `cpu_buffer->reader_lock`. So, `rb_num_of_entries` cannot decrease concurrently! So how is `rb_num_of_entries` 0 in `__rb_get_reader_page` if it was non-zero earlier?
Wait, hold on a second. `rb_per_cpu_empty` is actually the inverse of `rb_num_of_entries`, so if it wasn't empty, then `rb_num_of_entries` cannot be 0! Since the reader is serialized, there is no way for it to become 0.
Okay, let's explore if it could return NULL for another reason. It does have a few `RB_WARN_ON` checks, for things like loop counts and invalid read sizes. But wait, if any of those `RB_WARN_ON` conditions were triggered, it would print a warning. The syzbot report only shows ONE warning from the line of `ring_buffer_map_get_reader`. I am going to check the definition of `RB_WARN_ON`. I see that if it triggers, the kernel will print the statement `RB_WARN_ON(...)`. So, if an `RB_WARN_ON` condition *did* trigger, there should be an indicator in the log, but there isn't.
So, `__rb_get_reader_page` must be returning NULL WITHOUT triggering any of the `RB_WARN_ON` checks! How is that possible? I have to re-examine the code to figure out how `__rb_get_reader_page` can return NULL without triggering a warning.
|
| 33/2 |
2026/09/04 05:32 |
tool |
codesearch-definition-source |
0m
Args: map[Name:__rb_get_reader_page]
Results: map[SourceCode:5853: static struct buffer_page *
5854: __rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
5855: {
5856: int max_loops = cpu_buffer->ring_meta ? cpu_buffer->nr_pages : 3;
5857: struct buffer_page *reader = NULL;
5858: unsigned long overwrite;
5859: unsigned long flags;
5860: int missed_events = 0;
5861: int nr_loops = 0;
5862: bool ret;
5863:
5864: local_irq_save(flags);
5865: arch_spin_lock(&cpu_buffer->lock);
5866:
5867: again:
5868: /*
5869: * This should normally only loop twice. But because the
5870: * start of the reader inserts an empty page, it causes a
5871: * case where we will loop three times. There should be no
5872: * reason to loop four times unless the ring buffer is a
5873: * recovered persistent ring buffer. For persistent ring buffers,
5874: * invalid pages are reset during recovery, so there may be more
5875: * than 3 contiguous pages can be empty, but less than nr_pages.
5876: */
5877: if (RB_WARN_ON(cpu_buffer, ++nr_loops > max_loops)) {
5878: reader = NULL;
5879: goto out;
5880: }
5881:
5882: reader = cpu_buffer->reader_page;
5883:
5884: /* If there's more to read, return this page */
5885: if (cpu_buffer->reader_page->read < rb_page_size(reader))
5886: goto out;
5887:
5888: /* Never should we have an index greater than the size */
5889: if (RB_WARN_ON(cpu_buffer,
5890: cpu_buffer->reader_page->read > rb_page_size(reader)))
5891: goto out;
5892:
5893: /* check if we caught up to the tail */
5894: reader = NULL;
5895: if (cpu_buffer->commit_page == cpu_buffer->reader_page)
5896: goto out;
5897:
5898: /* Don't bother swapping if the ring buffer is empty */
5899: if (rb_num_of_entries(cpu_buffer) == 0)
5900: goto out;
5901:
5902: /*
5903: * Reset the reader page to size zero.
5904: */
5905: local_set(&cpu_buffer->reader_page->write, 0);
5906: local_set(&cpu_buffer->reader_page->entries, 0);
5907: rb_init_data_page(cpu_buffer->reader_page->page);
5908: cpu_buffer->reader_page->real_end = 0;
5909:
5910: spin:
5911: /*
5912: * Splice the empty reader page into the list around the head.
5913: */
5914: reader = rb_set_head_page(cpu_buffer);
5915: if (!reader)
5916: goto out;
5917: cpu_buffer->reader_page->list.next = rb_list_head(reader->list.next);
5918: cpu_buffer->reader_page->list.prev = reader->list.prev;
5919:
5920: /*
5921: * cpu_buffer->pages just needs to point to the buffer, it
5922: * has no specific buffer page to point to. Lets move it out
5923: * of our way so we don't accidentally swap it.
5924: */
5925: cpu_buffer->pages = reader->list.prev;
5926:
5927: /* The reader page will be pointing to the new head */
5928: rb_set_list_to_head(&cpu_buffer->reader_page->list);
5929:
5930: /*
5931: * We want to make sure we read the overruns after we set up our
5932: * pointers to the next object. The writer side does a
5933: * cmpxchg to cross pages which acts as the mb on the writer
5934: * side. Note, the reader will constantly fail the swap
5935: * while the writer is updating the pointers, so this
5936: * guarantees that the overwrite recorded here is the one we
5937: * want to compare with the last_overrun.
5938: */
5939: smp_mb();
5940: overwrite = local_read(&(cpu_buffer->overrun));
5941:
5942: /*
5943: * Here's the tricky part.
5944: *
5945: * We need to move the pointer past the header page.
5946: * But we can only do that if a writer is not currently
5947: * moving it. The page before the header page has the
5948: * flag bit '1' set if it is pointing to the page we want.
5949: * but if the writer is in the process of moving it
5950: * then it will be '2' or already moved '0'.
5951: */
5952:
5953: ret = rb_head_page_replace(reader, cpu_buffer->reader_page);
5954:
5955: /*
5956: * If we did not convert it, then we must try again.
5957: */
5958: if (!ret)
5959: goto spin;
5960:
5961: if (rb_page_commit(reader) & RB_MISSED_EVENTS)
5962: missed_events = -1;
5963:
5964: if (cpu_buffer->ring_meta)
5965: rb_update_meta_reader(cpu_buffer, reader);
5966:
5967: /*
5968: * Yay! We succeeded in replacing the page.
5969: *
5970: * Now make the new head point back to the reader page.
5971: */
5972: rb_list_head(reader->list.next)->prev = &cpu_buffer->reader_page->list;
5973: rb_inc_page(&cpu_buffer->head_page);
5974:
5975: cpu_buffer->cnt++;
5976: local_inc(&cpu_buffer->pages_read);
5977:
5978: /* Finally update the reader page to the new head */
5979: cpu_buffer->reader_page = reader;
5980: cpu_buffer->reader_page->read = 0;
5981:
5982: if (overwrite != cpu_buffer->last_overrun) {
5983: cpu_buffer->lost_events = overwrite - cpu_buffer->last_overrun;
5984: cpu_buffer->last_overrun = overwrite;
5985: }
5986:
5987: goto again;
5988:
5989: out:
5990: /* Update the read_stamp on the first event */
5991: if (reader && reader->read == 0)
5992: cpu_buffer->read_stamp = reader->page->time_stamp;
5993:
5994: arch_spin_unlock(&cpu_buffer->lock);
5995: local_irq_restore(flags);
5996:
5997: /*
5998: * The writer has preempt disable, wait for it. But not forever
5999: * Although, 1 second is pretty much "forever"
6000: */
6001: #define USECS_WAIT 1000000
6002: for (nr_loops = 0; nr_loops < USECS_WAIT; nr_loops++) {
6003: /* If the write is past the end of page, a writer is still updating it */
6004: if (likely(!reader || rb_page_write(reader) <= rb_page_capacity(reader)))
6005: break;
6006:
6007: udelay(1);
6008:
6009: /* Get the latest version of the reader write value */
6010: smp_rmb();
6011: }
6012:
6013: /* The writer is not moving forward? Something is wrong */
6014: if (RB_WARN_ON(cpu_buffer, nr_loops == USECS_WAIT))
6015: reader = NULL;
6016:
6017: /*
6018: * Make sure we see any padding after the write update
6019: * (see rb_reset_tail()).
6020: *
6021: * In addition, a writer may be writing on the reader page
6022: * if the page has not been fully filled, so the read barrier
6023: * is also needed to make sure we see the content of what is
6024: * committed by the writer (see rb_set_commit_to_write()).
6025: */
6026: smp_rmb();
6027:
6028: if (!cpu_buffer->lost_events)
6029: cpu_buffer->lost_events = missed_events;
6030:
6031: return reader;
6032: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 34/2 |
2026/09/04 05:32 |
llm |
expert |
0m
Model:
Tokens:
input: 26327
output: 23
thoughts: 370
Thoughts: **Analyzing Ring Buffer Reader Logic**
Okay, here we go. I'm looking at this snippet of code and something feels off. First, I see this check: if `cpu_buffer->commit_page` equals `cpu_buffer->reader_page`, we return `NULL`. Seems straightforward, an empty buffer scenario. But wait a second, isn't that same check done in `ring_buffer_map_get_reader` *before* the call to `rb_get_reader_page`?
```c
if (cpu_buffer->reader_page == cpu_buffer->commit_page)
goto out;
```
Ah, I see... but `ring_buffer_map_get_reader` only holds the `cpu_buffer->reader_lock`, not the main `cpu_buffer->lock`. Meaning writers could be updating `cpu_buffer->commit_page` concurrently! Okay, now the gears are turning. If a writer is updating `commit_page` concurrently, is it possible for `commit_page` to become equal to `reader_page` in this unprotected window? No, that's not possible. The `reader_page` is separate from the ring structure.
So then how can `commit_page` ever *equal* `reader_page`? That's what's bothering me. Oh! Now I see. When the buffer is *empty*, `commit_page` and `reader_page` *can* be equal. It's when the buffer is empty, and then the reader catches up to the writer. Okay, I think I'm starting to get this. Let's see what happens during the reset. Time to examine the `rb_reset_cpu` function and make sure this is all correct.
|
| 35/2 |
2026/09/04 05:32 |
tool |
codesearch-definition-source |
0m
Args:
Results: map[SourceCode:6612: static void
6613: rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
6614: {
6615: struct buffer_page *page;
6616:
6617: if (cpu_buffer->remote) {
6618: if (!cpu_buffer->remote->reset)
6619: return;
6620:
6621: cpu_buffer->remote->reset(cpu_buffer->cpu, cpu_buffer->remote->priv);
6622: rb_read_remote_meta_page(cpu_buffer);
6623:
6624: /* Read related values, not covered by the meta-page */
6625: local_set(&cpu_buffer->pages_read, 0);
6626: cpu_buffer->read = 0;
6627: cpu_buffer->read_bytes = 0;
6628: cpu_buffer->last_overrun = 0;
6629: cpu_buffer->reader_page->read = 0;
6630:
6631: return;
6632: }
6633:
6634: rb_head_page_deactivate(cpu_buffer);
6635:
6636: cpu_buffer->head_page
6637: = list_entry(cpu_buffer->pages, struct buffer_page, list);
6638: rb_clear_buffer_page(cpu_buffer->head_page);
6639: list_for_each_entry(page, cpu_buffer->pages, list) {
6640: rb_clear_buffer_page(page);
6641: }
6642:
6643: cpu_buffer->tail_page = cpu_buffer->head_page;
6644: cpu_buffer->commit_page = cpu_buffer->head_page;
6645:
6646: INIT_LIST_HEAD(&cpu_buffer->reader_page->list);
6647: INIT_LIST_HEAD(&cpu_buffer->new_pages);
6648: rb_clear_buffer_page(cpu_buffer->reader_page);
6649:
6650: local_set(&cpu_buffer->entries_bytes, 0);
6651: local_set(&cpu_buffer->overrun, 0);
6652: local_set(&cpu_buffer->commit_overrun, 0);
6653: local_set(&cpu_buffer->dropped_events, 0);
6654: local_set(&cpu_buffer->entries, 0);
6655: local_set(&cpu_buffer->committing, 0);
6656: local_set(&cpu_buffer->commits, 0);
6657: local_set(&cpu_buffer->pages_touched, 0);
6658: local_set(&cpu_buffer->pages_lost, 0);
6659: local_set(&cpu_buffer->pages_read, 0);
6660: cpu_buffer->last_pages_touch = 0;
6661: cpu_buffer->shortest_full = 0;
6662: cpu_buffer->read = 0;
6663: cpu_buffer->read_bytes = 0;
6664:
6665: rb_time_set(&cpu_buffer->write_stamp, 0);
6666: rb_time_set(&cpu_buffer->before_stamp, 0);
6667:
6668: memset(cpu_buffer->event_stamp, 0, sizeof(cpu_buffer->event_stamp));
6669:
6670: cpu_buffer->lost_events = 0;
6671: cpu_buffer->last_overrun = 0;
6672:
6673: rb_head_page_activate(cpu_buffer);
6674: cpu_buffer->pages_removed = 0;
6675:
6676: rb_update_meta_page(cpu_buffer);
6677: if (cpu_buffer->ring_meta) {
6678: struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
6679:
6680: meta->commit_buffer = meta->head_buffer;
6681: }
6682: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 36/2 |
2026/09/04 05:32 |
llm |
expert |
0m
Model:
Tokens:
input: 28303
output: 25
thoughts: 730
Thoughts: **Analyzing Ring Buffer Reader/Writer Pointers**
Okay, so I'm diving into this ring buffer code and trying to get a solid grasp on how the `reader_page` and `commit_page` pointers interact. Initially, it looked like `commit_page` would always be distinct from `reader_page` because `rb_reset_cpu` sets `commit_page` to `head_page`, and then clears and initializes `reader_page`. I was thinking, "never the twain shall meet," right?
But then I hit a snag in `__rb_get_reader_page`. There's this check: `if (cpu_buffer->commit_page == cpu_buffer->reader_page) goto out;`. Wait, if they *are* equal, that means the writer is currently, actively, writing on the `reader_page`? How does that even happen? My first thought was a wrap-around scenario, where the writer had looped back. But, wait, `reader_page` is supposed to be swapped *out* of the ring!
Aha! `rb_set_head_page` swaps the `reader_page` *into* the ring, and the old `head_page` becomes the new `reader_page` – that's the key. So, `reader_page` is consistently the page that *was* swapped out. That clarifies things.
But I got hung up on a one-page buffer possibility. Could the reader and commit page ever overlap if the buffer was extremely small? No, the minimum buffer size must be two pages; phew. That removes the ambiguity.
Okay, let's step back a moment and look at `ring_buffer_map_get_reader`, and that check that's essentially the same: `if (cpu_buffer->reader_page == cpu_buffer->commit_page) goto out;`. I was getting tripped up because it looked redundant, but now it makes sense. It's a critical check. But wait, is it possible for the writer to *still* be writing on the `reader_page`?
And then I circle back to `rb_get_reader_page` which returns `NULL` if `rb_num_of_entries(cpu_buffer) == 0`. `rb_num_of_entries` is `entries - (overrun + read)`. So if the number of entries is zero, we get null. But wait, `ring_buffer_map_get_reader` checks `rb_per_cpu_empty(cpu_buffer)` which is equivalent to `!rb_num_of_entries(cpu_buffer)`. If it was false, `rb_num_of_entries` was > 0. So, can the value of `rb_num_of_entries` go to zero between the check of `rb_per_cpu_empty` and `rb_get_reader_page`?
We have to consider the math of this. How can the value of `rb_num_of_entries` decrease? `entries` only increases. `overrun` only increases. `read` only increases. So, can `entries - overrun` decrease?
Oh, I think I see it. The writer overwrites an event, which *increments* `entries` and also *increments* `overrun`. But is `entries` incremented *before* or *after* `overrun`? We need to look at `rb_handle_head_page` to understand the sequence here.
|
| 37/2 |
2026/09/04 05:33 |
tool |
codesearch-definition-source |
0m
Args: map[Name:rb_handle_head_page]
Results: map[SourceCode:3643: /*
3644: * rb_handle_head_page - writer hit the head page
3645: *
3646: * Returns: +1 to retry page
3647: * 0 to continue
3648: * -1 on error
3649: */
3650: static int
3651: rb_handle_head_page(struct ring_buffer_per_cpu *cpu_buffer,
3652: struct buffer_page *tail_page,
3653: struct buffer_page *next_page)
3654: {
3655: struct buffer_page *new_head;
3656: int entries;
3657: int type;
3658: int ret;
3659:
3660: entries = rb_page_entries(next_page);
3661:
3662: /*
3663: * The hard part is here. We need to move the head
3664: * forward, and protect against both readers on
3665: * other CPUs and writers coming in via interrupts.
3666: */
3667: type = rb_head_page_set_update(cpu_buffer, next_page, tail_page,
3668: RB_PAGE_HEAD);
3669:
3670: /*
3671: * type can be one of four:
3672: * NORMAL - an interrupt already moved it for us
3673: * HEAD - we are the first to get here.
3674: * UPDATE - we are the interrupt interrupting
3675: * a current move.
3676: * MOVED - a reader on another CPU moved the next
3677: * pointer to its reader page. Give up
3678: * and try again.
3679: */
3680:
3681: switch (type) {
3682: case RB_PAGE_HEAD:
3683: /*
3684: * We changed the head to UPDATE, thus
3685: * it is our responsibility to update
3686: * the counters.
3687: */
3688: local_add(entries, &cpu_buffer->overrun);
3689: local_sub(rb_page_commit(next_page), &cpu_buffer->entries_bytes);
3690: local_inc(&cpu_buffer->pages_lost);
3691:
3692: if (cpu_buffer->ring_meta)
3693: rb_update_meta_head(cpu_buffer, next_page);
3694: /*
3695: * The entries will be zeroed out when we move the
3696: * tail page.
3697: */
3698:
3699: /* still more to do */
3700: break;
3701:
3702: case RB_PAGE_UPDATE:
3703: /*
3704: * This is an interrupt that interrupt the
3705: * previous update. Still more to do.
3706: */
3707: break;
3708: case RB_PAGE_NORMAL:
3709: /*
3710: * An interrupt came in before the update
3711: * and processed this for us.
3712: * Nothing left to do.
3713: */
3714: return 1;
3715: case RB_PAGE_MOVED:
3716: /*
3717: * The reader is on another CPU and just did
3718: * a swap with our next_page.
3719: * Try again.
3720: */
3721: return 1;
3722: default:
3723: RB_WARN_ON(cpu_buffer, 1); /* WTF??? */
3724: return -1;
3725: }
3726:
3727: /*
3728: * Now that we are here, the old head pointer is
3729: * set to UPDATE. This will keep the reader from
3730: * swapping the head page with the reader page.
3731: * The reader (on another CPU) will spin till
3732: * we are finished.
3733: *
3734: * We just need to protect against interrupts
3735: * doing the job. We will set the next pointer
3736: * to HEAD. After that, we set the old pointer
3737: * to NORMAL, but only if it was HEAD before.
3738: * otherwise we are an interrupt, and only
3739: * want the outer most commit to reset it.
3740: */
3741: new_head = next_page;
3742: rb_inc_page(&new_head);
3743:
3744: ret = rb_head_page_set_head(cpu_buffer, new_head, next_page,
3745: RB_PAGE_NORMAL);
3746:
3747: /*
3748: * Valid returns are:
3749: * HEAD - an interrupt came in and already set it.
3750: * NORMAL - One of two things:
3751: * 1) We really set it.
3752: * 2) A bunch of interrupts came in and moved
3753: * the page forward again.
3754: */
3755: switch (ret) {
3756: case RB_PAGE_HEAD:
3757: case RB_PAGE_NORMAL:
3758: /* OK */
3759: break;
3760: default:
3761: RB_WARN_ON(cpu_buffer, 1);
3762: return -1;
3763: }
3764:
3765: /*
3766: * It is possible that an interrupt came in,
3767: * set the head up, then more interrupts came in
3768: * and moved it again. When we get back here,
3769: * the page would have been set to NORMAL but we
3770: * just set it back to HEAD.
3771: *
3772: * How do you detect this? Well, if that happened
3773: * the tail page would have moved.
3774: */
3775: if (ret == RB_PAGE_NORMAL) {
3776: struct buffer_page *buffer_tail_page;
3777:
3778: buffer_tail_page = READ_ONCE(cpu_buffer->tail_page);
3779: /*
3780: * If the tail had moved passed next, then we need
3781: * to reset the pointer.
3782: */
3783: if (buffer_tail_page != tail_page &&
3784: buffer_tail_page != next_page)
3785: rb_head_page_set_normal(cpu_buffer, new_head,
3786: next_page,
3787: RB_PAGE_HEAD);
3788: }
3789:
3790: /*
3791: * If this was the outer most commit (the one that
3792: * changed the original pointer from HEAD to UPDATE),
3793: * then it is up to us to reset it to NORMAL.
3794: */
3795: if (type == RB_PAGE_HEAD) {
3796: ret = rb_head_page_set_normal(cpu_buffer, next_page,
3797: tail_page,
3798: RB_PAGE_UPDATE);
3799: if (RB_WARN_ON(cpu_buffer,
3800: ret != RB_PAGE_UPDATE))
3801: return -1;
3802: }
3803:
3804: return 0;
3805: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 38/2 |
2026/09/04 05:33 |
llm |
expert |
0m
Model:
Tokens:
input: 32073
output: 19
thoughts: 56
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and presented as if it were my own internal monologue:
**Investigating a Data Overrun Scenario**
Alright, I'm digging into `rb_handle_head_page`. Something's not quite right. I see that the `overrun` counter is being bumped up by `entries`. That's the expected behavior, typically. The problem is that I'm seeing an unexpected overrun. Okay, wait a second... `entries`. That's incremented, but *where*? I need to track down the points of origin where we're actually increasing the value of `entries`. That's likely where the root cause lies. If `entries` is being incremented incorrectly, it's going to lead to the `overrun` counter not reflecting the actual data being handled properly. I need to trace this variable's usage now.
|
| 39/2 |
2026/09/04 05:33 |
tool |
codesearch-find-references |
0m
Error:
Args:
Results: map[References:<nil> TruncatedOutput:false]
|
| 40/2 |
2026/09/04 05:33 |
llm |
expert |
0m
Model:
Tokens:
input: 32365
output: 54
thoughts: 31
Thoughts: Okay, here's my interpretation of those thoughts, framed as a first-person summary, keeping in mind my expertise:
**Targeting the `cpu_buffer->entries` Data Structure**
Right, let's zero in on how we're manipulating the `cpu_buffer->entries` data structure. I need to understand the fundamental operations being performed on it. My immediate inclination is to search for two key patterns. First, I'll look for calls to a function that looks like `local_inc(&cpu_buffer->entries)`. This suggests an atomic increment, likely managing counters or indexing within that buffer. Seeing the address being passed strongly reinforces that assumption, I'm expecting some kind of in-place modification. Secondly, I'll be searching for a pattern that uses an addition function: `local_add(..., &cpu_buffer->entries)`. Again, the address of the `cpu_buffer` entries is the argument, so this is probably managing an atomic addition, likely to an entry in the buffer, but the arguments will give important clues about the data being updated. These calls are going to be key to tracing data flow and identifying potential bottlenecks or race conditions in any concurrent access to the `cpu_buffer`. I'll be able to work backward from these calls to see how the buffer is populated and used.
|
| 41/2 |
2026/09/04 05:33 |
tool |
grepper |
0m
Args: map[Expression:local_inc\(&cpu_buffer->entries\)|local_add\(.*, &cpu_buffer->entries\) PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:kernel/trace/ring_buffer.c=4323=static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer)
kernel/trace/ring_buffer.c-4324-{
kernel/trace/ring_buffer.c:4325: local_inc(&cpu_buffer->entries);
kernel/trace/ring_buffer.c-4326- rb_end_commit(cpu_buffer);
]
|
| 42/2 |
2026/09/04 05:33 |
llm |
expert |
0m
Model:
Tokens:
input: 32845
output: 44
thoughts: 2190
Thoughts: **Race Condition in Ring Buffer: A Deep Dive**
Okay, so I've been tracing the logic here in the ring buffer code, and I think I've found a concurrency issue. It all hinges on how `entries` and `overrun` are updated relative to each other, particularly when the writer wraps around.
Initially, I noted that `entries` is incremented *after* an event is written, while `overrun` is incremented *before* a new event is added (during the wrap-around process via `rb_handle_head_page`). This immediately raised a flag: because `overrun` is incremented by the number of events being overwritten, and it does so *before* `entries` is updated for the new events, `entries - overrun` can potentially *decrease* dramatically.
My initial thought was, could this result in a negative value? If so, the calculation of available entries (`entries - overrun - read`) would be flawed. I realized, though, that with unsigned integers, it wraps around. So, it can *become* zero, but not negative.
Let's break it down further. I checked the `rb_num_of_entries` calculation, which reads `entries - (overrun + read)`. When the writer wraps, `overrun` jumps up by the number of events in the head page. This means that, *concurrently*, the reader might see a sudden drop in available entries. If the reader is just behind the writer, and the writer wraps exactly onto the reader, then `entries - (overrun + read)` can become *exactly* 0. That's where the problem shows up.
When this happens, `rb_num_of_entries` returns 0, and `__rb_get_reader_page` returns NULL. This means that when `ring_buffer_map_get_reader` subsequently calls `rb_get_reader_page`, and it finds the reader has completely read the current page but it is not the current commit page, it will trigger the `WARN_ON(!reader)`. And this is where the race happens. It is possible for the reader to hit this scenario.
I confirmed that `ring_buffer_map_get_reader` only holds the `reader_lock` but not the `cpu_buffer->lock`. Meanwhile, the writers do *not* take the `reader_lock`. This allows a writer to wrap around and increment `overrun` *concurrently* with the reader. So, with no synchronization, the writer can cause `rb_num_of_entries` to drop to zero while the reader is in the middle of reading events.
Then I checked if a negative number could be possible. While `entries - overrun` can wrap around, that doesn't trigger the warning. Only when the writer overwrites EXACTLY the unread events, leading to `rb_num_of_entries` being 0, does the warning happen.
Now, why the `WARN_ON(!reader)`? The code *assumes* that if the buffer isn't empty, if the current reader page has been fully read, and the reader page isn't the commit page, then a valid reader page *must* be available. But the assumption is wrong because the concurrent writer overwriting the head page can make `rb_num_of_entries` become 0!
Basically, the reader is being told it has data, but the writer has *already* overwritten it. So, the reader gets a NULL reader. I think the fix would be to have the reader check the return and not `WARN_ON`, but to simply retry or indicate a "missed event" or "no events" condition.
Finally, I checked the case where `commit_page` could be `reader_page`. I'm confident now that this isn't possible in the current implementation.
So, in short, there's a clear race condition where a concurrent writer can invalidate the reader's view of available events, leading to a potentially spurious warning. The crucial point is the timing of `overrun` updates and the lack of proper synchronization between reader and writer when calculating the number of available entries. This must be addressed.
|
| 43/2 |
2026/09/04 05:33 |
tool |
grepper |
0m
Args: map[Expression:cpu_buffer->commit_page = cpu_buffer->reader_page PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:kernel/trace/ring_buffer.c=2092=static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-2168- if (meta->commit_buffer == (unsigned long)cpu_buffer->reader_page->page) {
kernel/trace/ring_buffer.c:2169: cpu_buffer->commit_page = cpu_buffer->reader_page;
kernel/trace/ring_buffer.c-2170- /* Nothing more to do, the only page is the reader page */
]
|
| 44/2 |
2026/09/04 05:33 |
llm |
expert |
0m
Model:
Tokens:
input: 36132
output: 37
thoughts: 97
Thoughts: **My Deep Dive into Persistent Ring Buffer Initialization**
Okay, so I'm really digging into this `rb_meta_validate_events` function and something just clicked. It looks like it can actually set `cpu_buffer->commit_page` to the same as `cpu_buffer->reader_page`. And this is specifically for those persistent ring buffers, the ones that are either mapped into userspace or designed to survive system reboots.
I'm following the logic, and it seems like if `meta->commit_buffer` is pointing to the same memory page as the reader, the function will assign `commit_page` to that `reader_page`. I get it – during the initialization and validation phase of a persistent buffer, this sort of synchronization is crucial. But this has me thinking: is this *the* only place this assignment happens? I need to be absolutely sure this is the only spot this critical assignment is occurring.
|
| 45/2 |
2026/09/04 05:33 |
tool |
grepper |
0m
Args: map[Expression:commit_page = .*reader_page PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:kernel/trace/ring_buffer.c=2092=static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-2168- if (meta->commit_buffer == (unsigned long)cpu_buffer->reader_page->page) {
kernel/trace/ring_buffer.c:2169: cpu_buffer->commit_page = cpu_buffer->reader_page;
kernel/trace/ring_buffer.c-2170- /* Nothing more to do, the only page is the reader page */
]
|
| 46/2 |
2026/09/04 05:33 |
llm |
expert |
0m
Model:
Tokens:
input: 36620
output: 37
thoughts: 12
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Considering Reader Page Synchronization**
Okay, so I'm chewing on this `reader_page = commit_page` line of code. It's got me thinking about the implications for our read-committed or potentially snapshot isolation scenarios. Specifically, I'm trying to decide if directly assigning the `commit_page` to `reader_page` is the right move for establishing a consistent view. Does this assignment truly guarantee that the reader will only ever see data committed at or before that `commit_page`? Are we handling all the necessary fencing mechanisms and memory barriers correctly here? I need to analyze how we're managing concurrent access to these pages, especially if other transactions might be writing.
Is there a risk of a read operation inadvertently observing intermediate or incomplete writes if the assignment happens too early? Alternatively, could there be a performance bottleneck or other negative side effects if we delay the assignment? I need to consider scenarios where we are working with an optimistic concurrency model. Perhaps it will make sense to copy the page content so we maintain a more stable reader view. I also have to keep in mind the underlying storage structure, and how versioning is managed. Is `commit_page` the immutable final view, or is it a placeholder for an eventual commit, and this assignment introduces a race condition? We have to examine this line in the context of the overall transaction management strategy to fully understand the impact. The implications for concurrency control are paramount.
|
| 47/2 |
2026/09/04 05:34 |
tool |
grepper |
0m
Error:
Args: map[Expression:reader_page = .*commit_page PathPrefix:kernel/trace/ring_buffer.c]
Results:
|
| 48/2 |
2026/09/04 05:34 |
llm |
expert |
0m
Model:
Tokens:
input: 29717
output: 33
thoughts: 0
|
| 49/2 |
2026/09/04 05:34 |
tool |
grepper |
0m
Args: map[Expression:reader_page = PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:kernel/trace/ring_buffer.c=943=static bool rb_watermark_hit(struct trace_buffer *buffer, int cpu, int full)
--
kernel/trace/ring_buffer.c-961- raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
kernel/trace/ring_buffer.c:962: pagebusy = cpu_buffer->reader_page == cpu_buffer->commit_page;
kernel/trace/ring_buffer.c-963- ret = !pagebusy && full_hit(buffer, cpu, full);
--
kernel/trace/ring_buffer.c=2047=static void rb_meta_inject_reader_page(struct ring_buffer_per_cpu *cpu_buffer,
--
kernel/trace/ring_buffer.c-2066- /* Make the head_page the reader page */
kernel/trace/ring_buffer.c:2067: cpu_buffer->reader_page = head_page;
kernel/trace/ring_buffer.c-2068- bpage = head_page;
--
kernel/trace/ring_buffer.c=2548=rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
--
kernel/trace/ring_buffer.c-2577-
kernel/trace/ring_buffer.c:2578: cpu_buffer->reader_page = bpage;
kernel/trace/ring_buffer.c-2579-
--
kernel/trace/ring_buffer.c=4347=rb_wakeups(struct trace_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-4363-
kernel/trace/ring_buffer.c:4364: if (cpu_buffer->reader_page == cpu_buffer->commit_page)
kernel/trace/ring_buffer.c-4365- return;
--
kernel/trace/ring_buffer.c=5632=static void rb_iter_reset(struct ring_buffer_iter *iter)
--
kernel/trace/ring_buffer.c-5646-
kernel/trace/ring_buffer.c:5647: iter->cache_reader_page = iter->head_page;
kernel/trace/ring_buffer.c-5648- iter->cache_read = cpu_buffer->read;
--
kernel/trace/ring_buffer.c=5792=__rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-5843- cpu_buffer->head_page = new_head;
kernel/trace/ring_buffer.c:5844: cpu_buffer->reader_page = new_reader;
kernel/trace/ring_buffer.c-5845- cpu_buffer->reader_page->read = 0;
--
kernel/trace/ring_buffer.c=5854=__rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-5978- /* Finally update the reader page to the new head */
kernel/trace/ring_buffer.c:5979: cpu_buffer->reader_page = reader;
kernel/trace/ring_buffer.c-5980- cpu_buffer->reader_page->read = 0;
--
kernel/trace/ring_buffer.c=7129=int ring_buffer_read_page(struct trace_buffer *buffer,
--
kernel/trace/ring_buffer.c-7187- if (read || (len < (size - read)) ||
kernel/trace/ring_buffer.c:7188: cpu_buffer->reader_page == cpu_buffer->commit_page ||
kernel/trace/ring_buffer.c-7189- rb_is_static(cpu_buffer)) {
--
kernel/trace/ring_buffer.c-7203- (!read || (len < (size - read)) ||
kernel/trace/ring_buffer.c:7204: cpu_buffer->reader_page == cpu_buffer->commit_page))
kernel/trace/ring_buffer.c-7205- return -1;
--
kernel/trace/ring_buffer.c=7378=int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
--
kernel/trace/ring_buffer.c-7477- /* One page was allocated for the reader page */
kernel/trace/ring_buffer.c:7478: cpu_buffer->reader_page = list_entry(cpu_buffer->new_pages.next,
kernel/trace/ring_buffer.c-7479- struct buffer_page, list);
--
kernel/trace/ring_buffer.c=7891=int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
--
kernel/trace/ring_buffer.c-7922- /* Did the reader catch up with the writer? */
kernel/trace/ring_buffer.c:7923: if (cpu_buffer->reader_page == cpu_buffer->commit_page)
kernel/trace/ring_buffer.c-7924- goto out;
--
kernel/trace/ring_buffer.c-7954- local_add(RB_MISSED_EVENTS, &dpage->commit);
kernel/trace/ring_buffer.c:7955: } else if (!WARN_ONCE(cpu_buffer->reader_page == cpu_buffer->tail_page,
kernel/trace/ring_buffer.c-7956- "Reader on commit with %ld missed events",
]
|
| 50/2 |
2026/09/04 05:34 |
llm |
expert |
0m
Model:
Tokens:
input: 38480
output: 23
thoughts: 211
Thoughts: **Analyzing the Kernel Tracepoint Bug**
Okay, so I'm diving into this kernel tracepoint issue. The key line in the backtrace is `rb_meta_validate_events` assigning `commit_page` to `reader_page`. That's the core of the problem. My immediate thought is, "Wait, why is this happening here?" It's the *only* place this assignment takes place.
I know `rb_meta_validate_events` is *specifically* used for persistent ring buffers. The bug report is pointing to `tracing_buffers_ioctl+0x2e6/0x3d0` and `TRACE_MMAP_IOCTL_GET_READER`. This gets called on `trace_pipe_raw`. Okay, is `trace_pipe_raw` a persistent ring buffer? I don't *think* so. Persistent ring buffers are created during `trace_mmap` on a specific file; `trace_pipe_raw` doesn't fit that pattern.
Ah, I'm remembering – `TRACE_MMAP_IOCTL_GET_READER` is used for memory-mapped ring buffers in general. So this isn't necessarily persistent, it just means the ring buffer is being memory mapped. And memory-mapped buffers use `ring_buffer_map`, right? Okay, *that's* the connection. I need to understand what's happening within `ring_buffer_map`. That's where I need to focus my attention. Let's dig into that function.
|
| 51/2 |
2026/09/04 05:34 |
tool |
codesearch-definition-source |
0m
Args: map[Name:ring_buffer_map]
Results: map[SourceCode:7772: int ring_buffer_map(struct trace_buffer *buffer, int cpu,
7773: struct vm_area_struct *vma)
7774: {
7775: struct ring_buffer_per_cpu *cpu_buffer;
7776: struct buffer_page **subbuf_ids;
7777: unsigned long flags;
7778: int err;
7779:
7780: if (!cpumask_test_cpu(cpu, buffer->cpumask) || buffer->remote)
7781: return -EINVAL;
7782:
7783: cpu_buffer = buffer->buffers[cpu];
7784:
7785: guard(mutex)(&cpu_buffer->mapping_lock);
7786:
7787: if (cpu_buffer->user_mapped) {
7788: err = __rb_map_vma(cpu_buffer, vma);
7789: if (!err)
7790: err = __rb_inc_dec_mapped(cpu_buffer, true);
7791: return err;
7792: }
7793:
7794: /* prevent another thread from changing buffer/sub-buffer sizes */
7795: guard(mutex)(&buffer->mutex);
7796:
7797: err = rb_alloc_meta_page(cpu_buffer);
7798: if (err)
7799: return err;
7800:
7801: /* subbuf_ids includes the reader while nr_pages does not */
7802: subbuf_ids = kcalloc(cpu_buffer->nr_pages + 1, sizeof(*subbuf_ids), GFP_KERNEL);
7803: if (!subbuf_ids) {
7804: rb_free_meta_page(cpu_buffer);
7805: return -ENOMEM;
7806: }
7807:
7808: atomic_inc(&cpu_buffer->resize_disabled);
7809:
7810: /*
7811: * Lock all readers to block any subbuf swap until the subbuf IDs are
7812: * assigned.
7813: */
7814: raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
7815: rb_setup_ids_meta_page(cpu_buffer, subbuf_ids);
7816:
7817: raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
7818:
7819: err = __rb_map_vma(cpu_buffer, vma);
7820: if (!err) {
7821: raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
7822: /* This is the first time it is mapped by user */
7823: cpu_buffer->user_mapped = 1;
7824: raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
7825: } else {
7826: kfree(cpu_buffer->subbuf_ids);
7827: cpu_buffer->subbuf_ids = NULL;
7828: rb_free_meta_page(cpu_buffer);
7829: atomic_dec(&cpu_buffer->resize_disabled);
7830: }
7831:
7832: return err;
7833: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 52/2 |
2026/09/04 05:34 |
llm |
expert |
0m
Model:
Tokens:
input: 32525
output: 27
thoughts: 0
|
| 53/2 |
2026/09/04 05:34 |
tool |
codesearch-definition-source |
0m
Args: map[Name:rb_setup_ids_meta_page]
Results: map[SourceCode:7562: static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
7563: struct buffer_page **subbuf_ids)
7564: {
7565: struct trace_buffer_meta *meta = cpu_buffer->meta_page;
7566: unsigned int nr_subbufs = cpu_buffer->nr_pages + 1;
7567: struct buffer_page *first_subbuf, *subbuf;
7568: int cnt = 0;
7569: int id = 0;
7570:
7571: id = rb_page_id(cpu_buffer, cpu_buffer->reader_page, id);
7572: subbuf_ids[id++] = cpu_buffer->reader_page;
7573: cnt++;
7574:
7575: first_subbuf = subbuf = rb_set_head_page(cpu_buffer);
7576: do {
7577: id = rb_page_id(cpu_buffer, subbuf, id);
7578:
7579: if (WARN_ON(id >= nr_subbufs))
7580: break;
7581:
7582: subbuf_ids[id] = subbuf;
7583:
7584: rb_inc_page(&subbuf);
7585: id++;
7586: cnt++;
7587: } while (subbuf != first_subbuf);
7588:
7589: WARN_ON(cnt != nr_subbufs);
7590:
7591: /* install subbuf ID to bpage translation */
7592: cpu_buffer->subbuf_ids = subbuf_ids;
7593:
7594: meta->meta_struct_len = sizeof(*meta);
7595: meta->nr_subbufs = nr_subbufs;
7596: meta->subbuf_size = rb_subbuf_size(cpu_buffer->buffer);
7597: meta->meta_page_size = meta->subbuf_size;
7598:
7599: rb_update_meta_page(cpu_buffer);
7600: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 54/2 |
2026/09/04 05:34 |
llm |
expert |
0m
Model:
Tokens:
input: 40682
output: 25
thoughts: 2192
Thoughts: **Analyzing a Ring Buffer Concurrency Issue**
Okay, here's the situation as I see it. I'm looking at the `rb_setup_ids_meta_page` call, which ultimately uses `rb_set_head_page`. This function, at its core, retrieves and returns the head page of the ring buffer. I'm keying into `ring_buffer_map_get_reader`. It calls `rb_get_reader_page`, and if that fails, it WARNs. The heart of the problem looks to be why `rb_get_reader_page` could return NULL.
Tracing, the function `__rb_get_reader_page` immediately returns if `rb_num_of_entries` is zero. `rb_num_of_entries` is the difference of events written, overrun, and events read. However, `ring_buffer_map_get_reader` calls `rb_per_cpu_empty`, and if this call is false, `rb_num_of_entries` must be greater than zero.
So, how can `rb_num_of_entries` be zero *after* `rb_per_cpu_empty` reports it wasn't? The clue is in the concurrency; ring buffer is concurrent, with writers and readers. I know that when a writer wraps around, `overrun` increases. If only the head page contained data, then the calculation of entries minus overrun minus read could result in zero.
Is there a lock here? No, writers and readers do not hold the same lock. It is possible for the writer to cause an overrun concurrently.
So the sequence is: writer wraps, then the writer overwrites, and the data is lost. `rb_num_of_entries` now becomes zero.
The code assumes that if `rb_per_cpu_empty` is false then the data is present; but a concurrent writer can break this assumption. It's a kernel bug.
Okay, let's nail this down. Suppose the writer *does* overwrite. Can `rb_num_of_entries` *actually* become zero? Let's check: If we have a buffer with full pages, the total number of unread events is `entries - overrun - read`.
When the writer wraps around, `overrun` increases by events in the overwritten head page. But what if the buffer has other pages? In a scenario with two pages, the entries minus overrun minus read becomes events in page2. Therefore the number of events *cannot* be zero.
If the reader reads events in page2, then the writer overwrites the head page, the entries minus overrun minus read is zero! The unread events are now zero.
If the reader fully reads the reader page, the reader is about to swap the reader page with the head page, so the reader calls `rb_get_reader_page` to swap. But if the writer wraps around before the reader swaps, it will increment the `overrun` count for the *old* head page. If the old head page was the *only* one with unread events, `rb_num_of_entries` becomes 0.
But for the writer to wrap around, other pages must be full. The `entries` should *include* events from *all* full pages, as well. Entries are incremented for every event written. Thus, `entries - overrun - read` would then be greater than zero, since entries would at least have the events that were written to the non-head pages. Thus, it cannot be zero.
Let me go deeper. The number of unread events is exactly `entries - overrun - read`, which are the events from all pages, plus the `reader_page`. Since at least one page exists (really at least two), and the writer is writing, then it *cannot* be zero.
Here's an edge case: if the writer is in the process of writing, `overrun` gets incremented, but `entries` has *not* yet been incremented for *that* event. Looking at `rb_handle_head_page` shows `overrun` is updated *before* `entries`. If this wrap-around causes overrun, even then the other pages had already incremented `entries`. For the writer to wrap, the other pages must be full. So `entries` includes the other pages! Thus, `entries - overrun - read` cannot be zero.
Wait, is a single-page buffer possible? No, `nr_pages` is at least two.
The writer will fill and overflow the head page, and `overrun` will increase. The writer will have to write to other pages. Therefore `entries` will always be larger than `overrun + read`. The math doesn't work out.
So, the issue
|
| 55/2 |
2026/09/04 05:34 |
tool |
codesearch-definition-source |
0m
Args: map[Name:rb_allocate_cpu_buffer]
Results: map[SourceCode:2547: static struct ring_buffer_per_cpu *
2548: rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
2549: {
2550: struct ring_buffer_per_cpu *cpu_buffer __free(kfree) =
2551: alloc_cpu_buffer(cpu);
2552: struct ring_buffer_cpu_meta *meta;
2553: struct buffer_page *bpage;
2554: int ret;
2555:
2556: if (!cpu_buffer)
2557: return NULL;
2558:
2559: cpu_buffer->cpu = cpu;
2560: cpu_buffer->buffer = buffer;
2561: raw_spin_lock_init(&cpu_buffer->reader_lock);
2562: lockdep_set_class(&cpu_buffer->reader_lock, buffer->reader_lock_key);
2563: cpu_buffer->lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
2564: INIT_WORK(&cpu_buffer->update_pages_work, update_pages_handler);
2565: init_completion(&cpu_buffer->update_done);
2566: init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters);
2567: init_waitqueue_head(&cpu_buffer->irq_work.waiters);
2568: init_waitqueue_head(&cpu_buffer->irq_work.full_waiters);
2569: mutex_init(&cpu_buffer->mapping_lock);
2570:
2571: bpage = alloc_cpu_page(cpu);
2572: if (!bpage)
2573: return NULL;
2574: bpage->order = cpu_buffer->buffer->subbuf_order;
2575:
2576: rb_check_bpage(cpu_buffer, bpage);
2577:
2578: cpu_buffer->reader_page = bpage;
2579:
2580: if (buffer->range_addr_start) {
2581: /*
2582: * Range mapped buffers have the same restrictions as memory
2583: * mapped ones do.
2584: */
2585: cpu_buffer->ring_meta = rb_range_meta(buffer, nr_pages, cpu);
2586: bpage->page = rb_range_buffer(cpu_buffer, 0);
2587: if (!bpage->page)
2588: goto fail_free_reader;
2589: if (cpu_buffer->ring_meta->head_buffer)
2590: rb_meta_buffer_update(cpu_buffer, bpage);
2591: bpage->range = 1;
2592:
2593: atomic_inc(&cpu_buffer->resize_disabled);
2594: } else if (buffer->remote) {
2595: struct ring_buffer_desc *desc = ring_buffer_desc(buffer->remote->desc, cpu);
2596:
2597: if (!desc)
2598: goto fail_free_reader;
2599:
2600: cpu_buffer->remote = buffer->remote;
2601: cpu_buffer->meta_page = (struct trace_buffer_meta *)(void *)desc->meta_va;
2602: cpu_buffer->nr_pages = nr_pages;
2603: cpu_buffer->subbuf_ids = kcalloc(cpu_buffer->nr_pages + 1,
2604: sizeof(*cpu_buffer->subbuf_ids), GFP_KERNEL);
2605: if (!cpu_buffer->subbuf_ids)
2606: goto fail_free_reader;
2607:
2608: /* Remote buffers are read-only and immutable */
2609: atomic_inc(&cpu_buffer->record_disabled);
2610: atomic_inc(&cpu_buffer->resize_disabled);
2611:
2612: bpage->page = ring_buffer_desc_page(desc, cpu_buffer->meta_page->reader.id);
2613: if (!bpage->page)
2614: goto fail_free_reader;
2615:
2616: bpage->range = 1;
2617: cpu_buffer->subbuf_ids[0] = bpage;
2618: } else {
2619: bpage->page = alloc_cpu_data(cpu, bpage->order);
2620: if (!bpage->page)
2621: goto fail_free_reader;
2622: }
2623:
2624: INIT_LIST_HEAD(&cpu_buffer->reader_page->list);
2625: INIT_LIST_HEAD(&cpu_buffer->new_pages);
2626:
2627: ret = rb_allocate_pages(cpu_buffer, nr_pages);
2628: if (ret < 0)
2629: goto fail_free_reader;
2630:
2631: rb_meta_validate_events(cpu_buffer);
2632:
2633: /* If the boot meta was valid then this has already been updated */
2634: meta = cpu_buffer->ring_meta;
2635: if (!meta || !meta->head_buffer ||
2636: !cpu_buffer->head_page || !cpu_buffer->commit_page || !cpu_buffer->tail_page) {
2637: if (meta && meta->head_buffer &&
2638: (cpu_buffer->head_page || cpu_buffer->commit_page || cpu_buffer->tail_page)) {
2639: pr_warn("Ring buffer meta buffers not all mapped\n");
2640: if (!cpu_buffer->head_page)
2641: pr_warn(" Missing head_page\n");
2642: if (!cpu_buffer->commit_page)
2643: pr_warn(" Missing commit_page\n");
2644: if (!cpu_buffer->tail_page)
2645: pr_warn(" Missing tail_page\n");
2646: }
2647:
2648: cpu_buffer->head_page
2649: = list_entry(cpu_buffer->pages, struct buffer_page, list);
2650: cpu_buffer->tail_page = cpu_buffer->commit_page = cpu_buffer->head_page;
2651:
2652: rb_head_page_activate(cpu_buffer);
2653:
2654: if (cpu_buffer->ring_meta)
2655: meta->commit_buffer = meta->head_buffer;
2656: } else {
2657: /* The valid meta buffer still needs to activate the head page */
2658: rb_head_page_activate(cpu_buffer);
2659: }
2660:
2661: return_ptr(cpu_buffer);
2662:
2663: fail_free_reader:
2664: kfree(cpu_buffer->subbuf_ids);
2665: free_buffer_page(cpu_buffer->reader_page);
2666:
2667: return NULL;
2668: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 56/2 |
2026/09/04 05:34 |
llm |
expert |
0m
Model:
Tokens:
input: 45974
output: 23
thoughts: 72
Thoughts: **Page Allocation Quandary**
Okay, so I'm looking at `rb_allocate_pages`. The code tells me it allocates `nr_pages`, which seems straightforward enough. One page? `nr_pages` is 1, okay, I get that. But then my brain catches... wait a minute, isn't `nr_pages` actually the number of pages that comprise the ring? Now I'm questioning myself. Does the design allow for a ring with only a single page? That feels... unusual, doesn't it? Let me just double check `rb_allocate_pages` itself. Gotta make sure the assumptions are sound here.
|
| 57/2 |
2026/09/04 05:34 |
tool |
codesearch-definition-source |
0m
Args: map[Name:rb_allocate_pages]
Results: map[SourceCode:2522: static int rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
2523: unsigned long nr_pages)
2524: {
2525: LIST_HEAD(pages);
2526:
2527: WARN_ON(!nr_pages);
2528:
2529: if (__rb_allocate_pages(cpu_buffer, nr_pages, &pages))
2530: return -ENOMEM;
2531:
2532: /*
2533: * The ring buffer page list is a circular list that does not
2534: * start and end with a list head. All page list items point to
2535: * other pages.
2536: */
2537: cpu_buffer->pages = pages.next;
2538: list_del(&pages);
2539:
2540: cpu_buffer->nr_pages = nr_pages;
2541:
2542: rb_check_pages(cpu_buffer);
2543:
2544: return 0;
2545: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 58/2 |
2026/09/04 05:34 |
llm |
expert |
0m
Model:
Tokens:
input: 46552
output: 23
thoughts: 255
Thoughts: **Analyzing a Potential Ring Buffer Issue**
Okay, so I'm diving into this ring buffer logic, and something's bugging me. If `nr_pages` is set to 1, I allocate just a single page, right? Makes sense. Then my ring buffer *only* has that one page. That means `head_page` is the same, it's the *only* page in the entire ring.
Now, if the writer gets going and wraps around, it's going to overwrite `head_page`. And when that happens, `overrun` increases based on the events packed into `head_page`. Since that same `head_page` is the *only* page in the ring, `entries - overrun - read` will become zero.
Wait, if `nr_pages` is set to 1, could `rb_num_of_entries` become zero? Yes!
Because with `nr_pages` at 1, the buffer has one page in the ring and a separate `reader_page` outside the ring. When the writer wraps around, it overwrites the ring's *only* page. Suddenly, `rb_num_of_entries` becomes zero.
And then, if that happens, `rb_get_reader_page` returns NULL! Which then triggers the `WARN_ON(!reader)` call.
So, the real question is: Is `nr_pages` *supposed* to be allowed to be 1 in the first place? Time to go investigate `ring_buffer_resize` and find out.
|
| 59/2 |
2026/09/04 05:34 |
tool |
codesearch-definition-source |
0m
Args: map[Name:ring_buffer_resize]
Results: map[SourceCode:3253: /**
3254: * ring_buffer_resize - resize the ring buffer
3255: * @buffer: the buffer to resize.
3256: * @size: the new size.
3257: * @cpu_id: the cpu buffer to resize
3258: *
3259: * Minimum size is 2 * rb_subbuf_capacity(buffer).
3260: *
3261: * Returns 0 on success and < 0 on failure.
3262: */
3263: int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size,
3264: int cpu_id)
3265: {
3266: struct ring_buffer_per_cpu *cpu_buffer;
3267: unsigned long nr_pages;
3268: int cpu, err;
3269:
3270: /*
3271: * Always succeed at resizing a non-existent buffer:
3272: */
3273: if (!buffer)
3274: return 0;
3275:
3276: /* Make sure the requested buffer exists */
3277: if (cpu_id != RING_BUFFER_ALL_CPUS &&
3278: !cpumask_test_cpu(cpu_id, buffer->cpumask))
3279: return 0;
3280:
3281: /*
3282: * Keep CPUs from coming online while resizing to synchronize
3283: * with new per CPU buffers being created.
3284: */
3285: guard(cpus_read_lock)();
3286:
3287: /* prevent another thread from changing buffer sizes */
3288: mutex_lock(&buffer->mutex);
3289: atomic_inc(&buffer->resizing);
3290:
3291: nr_pages = DIV_ROUND_UP(size, rb_subbuf_capacity(buffer));
3292:
3293: /* we need a minimum of two pages */
3294: if (nr_pages < 2)
3295: nr_pages = 2;
3296:
3297: if (cpu_id == RING_BUFFER_ALL_CPUS) {
3298: /*
3299: * Don't succeed if resizing is disabled, as a reader might be
3300: * manipulating the ring buffer and is expecting a sane state while
3301: * this is true.
3302: */
3303: for_each_buffer_cpu(buffer, cpu) {
3304: cpu_buffer = buffer->buffers[cpu];
3305: if (atomic_read(&cpu_buffer->resize_disabled)) {
3306: err = -EBUSY;
3307: goto out_err_unlock;
3308: }
3309: }
3310:
3311: /* calculate the pages to update */
3312: for_each_buffer_cpu(buffer, cpu) {
3313: cpu_buffer = buffer->buffers[cpu];
3314:
3315: cpu_buffer->nr_pages_to_update = nr_pages -
3316: cpu_buffer->nr_pages;
3317: /*
3318: * nothing more to do for removing pages or no update
3319: */
3320: if (cpu_buffer->nr_pages_to_update <= 0)
3321: continue;
3322: /*
3323: * to add pages, make sure all new pages can be
3324: * allocated without receiving ENOMEM
3325: */
3326: INIT_LIST_HEAD(&cpu_buffer->new_pages);
3327: if (__rb_allocate_pages(cpu_buffer, cpu_buffer->nr_pages_to_update,
3328: &cpu_buffer->new_pages)) {
3329: /* not enough memory for new pages */
3330: err = -ENOMEM;
3331: goto out_err;
3332: }
3333:
3334: cond_resched();
3335: }
3336:
3337: /*
3338: * Fire off all the required work handlers
3339: * We can't schedule on offline CPUs, but it's not necessary
3340: * since we can change their buffer sizes without any race.
3341: */
3342: for_each_buffer_cpu(buffer, cpu) {
3343: cpu_buffer = buffer->buffers[cpu];
3344: if (!cpu_buffer->nr_pages_to_update)
3345: continue;
3346:
3347: /* Can't run something on an offline CPU. */
3348: if (!cpu_online(cpu)) {
3349: rb_update_pages(cpu_buffer);
3350: cpu_buffer->nr_pages_to_update = 0;
3351: } else {
3352: /* Run directly if possible. */
3353: migrate_disable();
3354: if (cpu != smp_processor_id()) {
3355: migrate_enable();
3356: schedule_work_on(cpu,
3357: &cpu_buffer->update_pages_work);
3358: } else {
3359: update_pages_handler(&cpu_buffer->update_pages_work);
3360: migrate_enable();
3361: }
3362: }
3363: }
3364:
3365: /* wait for all the updates to complete */
3366: for_each_buffer_cpu(buffer, cpu) {
3367: cpu_buffer = buffer->buffers[cpu];
3368: if (!cpu_buffer->nr_pages_to_update)
3369: continue;
3370:
3371: if (cpu_online(cpu))
3372: wait_for_completion(&cpu_buffer->update_done);
3373: cpu_buffer->nr_pages_to_update = 0;
3374: }
3375:
3376: } else {
3377: cpu_buffer = buffer->buffers[cpu_id];
3378:
3379: if (nr_pages == cpu_buffer->nr_pages)
3380: goto out;
3381:
3382: /*
3383: * Don't succeed if resizing is disabled, as a reader might be
3384: * manipulating the ring buffer and is expecting a sane state while
3385: * this is true.
3386: */
3387: if (atomic_read(&cpu_buffer->resize_disabled)) {
3388: err = -EBUSY;
3389: goto out_err_unlock;
3390: }
3391:
3392: cpu_buffer->nr_pages_to_update = nr_pages -
3393: cpu_buffer->nr_pages;
3394:
3395: INIT_LIST_HEAD(&cpu_buffer->new_pages);
3396: if (cpu_buffer->nr_pages_to_update > 0 &&
3397: __rb_allocate_pages(cpu_buffer, cpu_buffer->nr_pages_to_update,
3398: &cpu_buffer->new_pages)) {
3399: err = -ENOMEM;
3400: goto out_err;
3401: }
3402:
3403: /* Can't run something on an offline CPU. */
3404: if (!cpu_online(cpu_id))
3405: rb_update_pages(cpu_buffer);
3406: else {
3407: /* Run directly if possible. */
3408: migrate_disable();
3409: if (cpu_id == smp_processor_id()) {
3410: rb_update_pages(cpu_buffer);
3411: migrate_enable();
3412: } else {
3413: migrate_enable();
3414: schedule_work_on(cpu_id,
3415: &cpu_buffer->update_pages_work);
3416: wait_for_completion(&cpu_buffer->update_done);
3417: }
3418: }
3419:
3420: cpu_buffer->nr_pages_to_update = 0;
3421: }
3422:
3423: out:
3424: /*
3425: * The ring buffer resize can happen with the ring buffer
3426: * enabled, so that the update disturbs the tracing as little
3427: * as possible. But if the buffer is disabled, we do not need
3428: * to worry about that, and we can take the time to verify
3429: * that the buffer is not corrupt.
3430: */
3431: if (atomic_read(&buffer->record_disabled)) {
3432: atomic_inc(&buffer->record_disabled);
3433: /*
3434: * Even though the buffer was disabled, we must make sure
3435: * that it is truly disabled before calling rb_check_pages.
3436: * There could have been a race between checking
3437: * record_disable and incrementing it.
3438: */
3439: synchronize_rcu();
3440: for_each_buffer_cpu(buffer, cpu) {
3441: cpu_buffer = buffer->buffers[cpu];
3442: rb_check_pages(cpu_buffer);
3443: }
3444: atomic_dec(&buffer->record_disabled);
3445: }
3446:
3447: atomic_dec(&buffer->resizing);
3448: mutex_unlock(&buffer->mutex);
3449: return 0;
3450:
3451: out_err:
3452: for_each_buffer_cpu(buffer, cpu) {
3453: struct buffer_page *bpage, *tmp;
3454:
3455: cpu_buffer = buffer->buffers[cpu];
3456: cpu_buffer->nr_pages_to_update = 0;
3457:
3458: if (list_empty(&cpu_buffer->new_pages))
3459: continue;
3460:
3461: list_for_each_entry_safe(bpage, tmp, &cpu_buffer->new_pages,
3462: list) {
3463: list_del_init(&bpage->list);
3464: free_buffer_page(bpage);
3465:
3466: cond_resched();
3467: }
3468: }
3469: out_err_unlock:
3470: atomic_dec(&buffer->resizing);
3471: mutex_unlock(&buffer->mutex);
3472: return err;
3473: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 60/2 |
2026/09/04 05:34 |
llm |
expert |
0m
Model:
Tokens:
input: 50292
output: 25
thoughts: 2878
Thoughts: **Analyzing a Ring Buffer Race Condition**
Okay, so I'm diving into this ring buffer logic, specifically trying to understand a `WARN_ON(!reader)` triggered within `ring_buffer_map_get_reader`. The core issue seems to be a scenario where the reader page becomes unavailable, even though we shouldn't have an empty buffer.
First, I'm establishing that the minimum size of the ring is two pages, which ensures that `rb_num_of_entries` (the number of valid entries in the ring buffer) can't *immediately* become zero due to a writer wrap-around. However, there's this edge case: a writer wrapping around and overwriting the head page *after* the reader has read all other pages but *before* the reader has swapped the pages. This can lead to `rb_num_of_entries` becoming zero temporarily, which is a key observation. In this case, `rb_num_of_entries` becoming zero *is* possible.
I'm methodically tracing this scenario, step by step: events being written to two pages, the reader moving, reading all events, the writer wrapping around. Now I'm looking for where `entries - overrun - read` would become 0:
* I can see that `entries` increases with EVERY event, so they should always be in the ring, and therefore > 0.
So it MUST be a race condition.
I'm drilling down into the possible causes for `__rb_get_reader_page` to return NULL. It can return NULL if:
1. The reader has caught up to the tail (if `commit_page` == `reader_page`). This is a false lead, because it's only possible during persistent buffer init, which isn't the case here.
2. `rb_num_of_entries` is zero.
3. Error conditions in `rb_set_head_page`.
I'm ruling out scenarios, especially those that trigger warnings via `RB_WARN_ON`. The lack of such warnings suggests they aren't the root cause. This leads to the only possible explanation being: the reader page is null because the ring is *empty* (though this should NOT be possible).
I'm now back to the question of *how* the ring buffer might be empty. If `rb_num_of_entries` is 0 but `rb_per_cpu_empty` is FALSE, it would return NULL. The reader lock *should* prevent a concurrent read making it empty.
Aha! **`rb_reset_cpu`**. This can reset the entire ring. If this gets called concurrently with the reader, it can set all important fields to zero. This could very well lead to this. If `rb_reset_cpu` clears the buffer CONCURRENTLY, then `rb_num_of_entries` can be zero! Yes, `ring_buffer_reset_cpu` and `ring_buffer_reset` are triggered by writes, which are concurrent! So `rb_reset_cpu` *must* be called concurrently. This is the race condition.
|
| 61/2 |
2026/09/04 05:35 |
tool |
codesearch-definition-source |
0m
Args: map[Name:ring_buffer_reset_cpu]
Results: map[SourceCode:6699: /**
6700: * ring_buffer_reset_cpu - reset a ring buffer per CPU buffer
6701: * @buffer: The ring buffer to reset a per cpu buffer of
6702: * @cpu: The CPU buffer to be reset
6703: */
6704: void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu)
6705: {
6706: struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
6707:
6708: if (!cpumask_test_cpu(cpu, buffer->cpumask))
6709: return;
6710:
6711: /* prevent another thread from changing buffer sizes */
6712: mutex_lock(&buffer->mutex);
6713:
6714: atomic_inc(&cpu_buffer->resize_disabled);
6715: atomic_inc(&cpu_buffer->record_disabled);
6716:
6717: /* Make sure all commits have finished */
6718: synchronize_rcu();
6719:
6720: reset_disabled_cpu_buffer(cpu_buffer);
6721:
6722: atomic_dec(&cpu_buffer->record_disabled);
6723: atomic_dec(&cpu_buffer->resize_disabled);
6724:
6725: mutex_unlock(&buffer->mutex);
6726: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 62/2 |
2026/09/04 05:35 |
llm |
expert |
0m
Model:
Tokens:
input: 54297
output: 25
thoughts: 13
Thoughts: Okay, here's my attempt at summarizing that thought process, tailored for an expert audience:
**Checking `reset_disabled_cpu_buffer`**
Alright, let's dive into this `reset_disabled_cpu_buffer` function. My initial thought is to quickly verify what this is actually doing. Is it just clearing a memory region, potentially zeroing it? Or is it more involved, maybe invalidating cache lines or orchestrating some sort of hardware-level reset on the disabled CPU's memory controller? I need to understand the *why* behind this reset. Are we dealing with a specific error condition? Is it part of a graceful shutdown procedure? Or is it a more general cleanup routine after a core has been taken offline for some reason?
Depending on the context, I'll need to look at a few things: the data structures involved, what exactly is being considered "disabled," and the architectural implications. Is this multi-threaded? If so, are there any synchronization mechanisms at play to prevent race conditions during the reset? I'm already anticipating potential issues with data coherency if we're dealing with shared memory. Need to confirm that this is handled correctly.
Ultimately, I want to confirm that this function is both *correct* and *efficient*. Correct in that it does what it's supposed to do, and doesn't introduce any new bugs or data corruption. Efficient in that it minimizes any performance impact or unnecessary operations. Let's see what's what.
|
| 63/2 |
2026/09/04 05:35 |
tool |
codesearch-definition-source |
0m
Args: map[Name:reset_disabled_cpu_buffer]
Results: map[SourceCode:6684: /* Must have disabled the cpu buffer then done a synchronize_rcu */
6685: static void reset_disabled_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
6686: {
6687: guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
6688:
6689: if (RB_WARN_ON(cpu_buffer, local_read(&cpu_buffer->committing)))
6690: return;
6691:
6692: arch_spin_lock(&cpu_buffer->lock);
6693:
6694: rb_reset_cpu(cpu_buffer);
6695:
6696: arch_spin_unlock(&cpu_buffer->lock);
6697: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 64/2 |
2026/09/04 05:35 |
llm |
expert |
0m
Model:
Tokens:
input: 54893
output: 30
thoughts: 3279
Thoughts: **Deep Dive into Potential NULL Returns in Ring Buffer Reader Function**
Okay, so I've been wrestling with this `ring_buffer` code and trying to chase down a possible NULL return. The core of the issue revolves around `ring_buffer_map_get_reader` and its interaction with `rb_get_reader_page`, specifically when that latter function could return NULL. The goal is to figure out why a `reader` can return NULL, and how concurrent operations might cause it.
First, I identified that `reset_disabled_cpu_buffer` *takes* `cpu_buffer->reader_lock`. That means it *cannot* run concurrently with `ring_buffer_map_get_reader`, which *also* takes the same lock. This seemed like a promising avenue, but is eliminated.
Next, I looked at edge cases for `rb_num_of_entries`. What if it's negative due to a wrap-around? The code handles that, as negative values effectively become huge positive ones. However, this path also did not present the source of the issue.
I then considered what if overrun occurs. Overrun is increased *before* the `entries` field is incremented, meaning `entries - overrun - read` could be negative. A negative count of `rb_num_of_entries` is a problem. However, I concluded this cannot occur, and the calculation of `rb_num_of_entries` is sound.
I've carefully examined conditions where `rb_num_of_entries` could be 0, even transiently. I've eliminated concurrent resets, and the race-condition of multiple writers.
Then I considered `rb_per_cpu_empty`. I've determined that `rb_per_cpu_empty` and `rb_num_of_entries` MUST be correct.
The most promising area seems to be within `__rb_get_reader_page` itself. The code has several checks for various conditions. The key is to see under what *legitimate* scenarios can this return NULL.
The `reader_page->read == reader_size` check in `ring_buffer_map_get_reader` will call `rb_get_reader_page`. The question is if a reader will get to the point of calling this function. However, the conditions for calling this function depend on the previous conditional being FALSE.
I've carefully evaluated all possible scenarios. The buffer will never return NULL here, except if something unexpected occurs.
Finally, I also looked at the remote ring buffers, but these seem to have the same constraints as local ones.
|
| 65/2 |
2026/09/04 05:35 |
tool |
codesearch-definition-source |
0m
Args: map[Name:__rb_get_reader_page_from_remote]
Results: map[SourceCode:5791: static struct buffer_page *
5792: __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer)
5793: {
5794: struct buffer_page *new_reader, *prev_reader, *prev_head, *new_head, *last;
5795:
5796: if (!rb_read_remote_meta_page(cpu_buffer))
5797: return NULL;
5798:
5799: /* More to read on the reader page */
5800: if (cpu_buffer->reader_page->read < rb_page_size(cpu_buffer->reader_page)) {
5801: if (!cpu_buffer->reader_page->read)
5802: cpu_buffer->read_stamp = cpu_buffer->reader_page->page->time_stamp;
5803: return cpu_buffer->reader_page;
5804: }
5805:
5806: prev_reader = cpu_buffer->subbuf_ids[cpu_buffer->meta_page->reader.id];
5807:
5808: if (cpu_buffer->remote->swap_reader_page(cpu_buffer->cpu,
5809: cpu_buffer->remote->priv)) {
5810: pr_warn_ratelimited("Remote reader page swap failed\n");
5811: return NULL;
5812: }
5813: /* nr_pages doesn't include the reader page */
5814: if (WARN_ON_ONCE(cpu_buffer->meta_page->reader.id > cpu_buffer->nr_pages))
5815: return NULL;
5816:
5817: new_reader = cpu_buffer->subbuf_ids[cpu_buffer->meta_page->reader.id];
5818:
5819: WARN_ON_ONCE(prev_reader == new_reader);
5820:
5821: prev_head = new_reader; /* New reader was also the previous head */
5822: new_head = prev_head;
5823: rb_inc_page(&new_head);
5824: last = prev_head;
5825: rb_dec_page(&last);
5826:
5827: /* Clear the old HEAD flag */
5828: rb_list_head_clear(cpu_buffer->head_page->list.prev);
5829:
5830: prev_reader->list.next = prev_head->list.next;
5831: prev_reader->list.prev = prev_head->list.prev;
5832:
5833: /* Swap prev_reader with new_reader */
5834: last->list.next = &prev_reader->list;
5835: new_head->list.prev = &prev_reader->list;
5836:
5837: new_reader->list.prev = &new_reader->list;
5838: new_reader->list.next = &new_head->list;
5839:
5840: /* Reactivate the HEAD flag */
5841: rb_set_list_to_head(&last->list);
5842:
5843: cpu_buffer->head_page = new_head;
5844: cpu_buffer->reader_page = new_reader;
5845: cpu_buffer->reader_page->read = 0;
5846: cpu_buffer->pages = &new_head->list;
5847: cpu_buffer->read_stamp = new_reader->page->time_stamp;
5848: cpu_buffer->lost_events = cpu_buffer->meta_page->reader.lost_events;
5849:
5850: return rb_page_size(cpu_buffer->reader_page) ? cpu_buffer->reader_page : NULL;
5851: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 66/2 |
2026/09/04 05:35 |
llm |
expert |
0m
Model:
Tokens:
input: 59848
output: 21
thoughts: 1112
Thoughts: **Debugging a Potential NULL Pointer in Ring Buffer Reader Page Retrieval**
Okay, so I'm looking at this `__rb_get_reader_page_from_remote` function, and it seems like it can potentially return NULL. I see several places where this can occur: `rb_read_remote_meta_page` failing, `cpu_buffer->remote->swap_reader_page` failing, or that `rb_page_size` returning 0. The bug report doesn't mention the swap failing or `WARN_ON_ONCE` so it seems like its neither.
Let's trace this back to the `tracing_buffers_ioctl` call on `trace_pipe_raw`. Is `trace_pipe_raw` using a remote buffer? My understanding is that remote buffers are typically used for guest tracing, but `trace_pipe_raw` is the standard ring buffer, so no, it shouldn't be remote. Therefore we are dealing with a standard ring buffer.
Crucially, `__rb_get_reader_page_from_remote` is only called if `cpu_buffer->remote` is set. Since we are not a remote buffer, this function is not the problem.
Now, let's look at the standard version: `__rb_get_reader_page`. I see that if `rb_num_of_entries(cpu_buffer)` is 0, it jumps to `out` and returns NULL. Could `rb_num_of_entries` be 0? I know that if the writer wraps around, it cannot be 0. But what if the writer *didn't* wrap around? If the writer writes a single event, and the reader reads it, then `rb_num_of_entries` would be 0.
If it is 0, then the code uses `rb_per_cpu_empty` and exits the routine. The problem is that the bug report has hit `WARN_ON(!reader)`. So it seems that `rb_per_cpu_empty` returned FALSE but `rb_num_of_entries` was 0. Wait, that's impossible. `rb_per_cpu_empty` directly returns `!rb_num_of_entries`.
Okay, what if `rb_num_of_entries` is > 0 when `rb_per_cpu_empty` is called, but then becomes 0 when `__rb_get_reader_page` is called? How can this occur? Writers wrapping won't cause this because of the logic dealing with multiple pages. Writers writing an event will increase `rb_num_of_entries`, so it can't become 0 that way. Readers can't cause that as reads are serialized by `reader_lock`. `overrun` increases cannot cause it.
Wait, what if the overrun caused the other pages to contain 0 events? That can't be, because the event will have a padding value. Unless the padding event isn't counted in the number of entries!
|
| 67/2 |
2026/09/04 05:36 |
tool |
codesearch-definition-source |
0m
Args:
Results: map[SourceCode:4323: static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer)
4324: {
4325: local_inc(&cpu_buffer->entries);
4326: rb_end_commit(cpu_buffer);
4327: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 68/2 |
2026/09/04 05:36 |
llm |
expert |
0m
Model:
Tokens:
input: 61738
output: 23
thoughts: 24
Thoughts: Okay, here's my take on those thoughts, framed as my own:
**Investigating `rb_commit` and Padding Events**
Hmm, interesting question. I'm trying to mentally map out the flow here. Specifically, I'm wondering if `rb_commit` gets triggered for padding events. It's a key function, so understanding its scope is crucial.
Now, my initial recall says that padding events are added within the `rb_move_tail` function. That's good context to hold onto. I need to keep those two pieces of information in mind as I trace this interaction. I need to figure out what happens, during the execution of `rb_move_tail`, and whether that process calls out to `rb_commit`, or if it executes other methods that may call out to `rb_commit`. The details here are critical for overall system performance and correctness, and I need to ensure a holistic understanding before making any conclusions.
|
| 69/2 |
2026/09/04 05:36 |
tool |
codesearch-definition-source |
0m
Args:
Results: map[SourceCode:3887: /*
3888: * This is the slow path, force gcc not to inline it.
3889: */
3890: static noinline struct ring_buffer_event *
3891: rb_move_tail(struct ring_buffer_per_cpu *cpu_buffer,
3892: unsigned long tail, struct rb_event_info *info)
3893: {
3894: struct buffer_page *tail_page = info->tail_page;
3895: struct buffer_page *commit_page = cpu_buffer->commit_page;
3896: struct trace_buffer *buffer = cpu_buffer->buffer;
3897: struct buffer_page *next_page;
3898: int ret;
3899:
3900: next_page = tail_page;
3901:
3902: rb_inc_page(&next_page);
3903:
3904: /*
3905: * If for some reason, we had an interrupt storm that made
3906: * it all the way around the buffer, bail, and warn
3907: * about it.
3908: */
3909: if (unlikely(next_page == commit_page)) {
3910: local_inc(&cpu_buffer->commit_overrun);
3911: goto out_reset;
3912: }
3913:
3914: /*
3915: * This is where the fun begins!
3916: *
3917: * We are fighting against races between a reader that
3918: * could be on another CPU trying to swap its reader
3919: * page with the buffer head.
3920: *
3921: * We are also fighting against interrupts coming in and
3922: * moving the head or tail on us as well.
3923: *
3924: * If the next page is the head page then we have filled
3925: * the buffer, unless the commit page is still on the
3926: * reader page.
3927: */
3928: if (rb_is_head_page(next_page, &tail_page->list)) {
3929:
3930: /*
3931: * If the commit is not on the reader page, then
3932: * move the header page.
3933: */
3934: if (!rb_is_reader_page(cpu_buffer->commit_page)) {
3935: /*
3936: * If we are not in overwrite mode,
3937: * this is easy, just stop here.
3938: */
3939: if (!(buffer->flags & RB_FL_OVERWRITE)) {
3940: local_inc(&cpu_buffer->dropped_events);
3941: goto out_reset;
3942: }
3943:
3944: ret = rb_handle_head_page(cpu_buffer,
3945: tail_page,
3946: next_page);
3947: if (ret < 0)
3948: goto out_reset;
3949: if (ret)
3950: goto out_again;
3951: } else {
3952: /*
3953: * We need to be careful here too. The
3954: * commit page could still be on the reader
3955: * page. We could have a small buffer, and
3956: * have filled up the buffer with events
3957: * from interrupts and such, and wrapped.
3958: *
3959: * Note, if the tail page is also on the
3960: * reader_page, we let it move out.
3961: */
3962: if (unlikely((cpu_buffer->commit_page !=
3963: cpu_buffer->tail_page) &&
3964: (cpu_buffer->commit_page ==
3965: cpu_buffer->reader_page))) {
3966: local_inc(&cpu_buffer->commit_overrun);
3967: goto out_reset;
3968: }
3969: }
3970: }
3971:
3972: rb_tail_page_update(cpu_buffer, tail_page, next_page);
3973:
3974: out_again:
3975:
3976: rb_reset_tail(cpu_buffer, tail, info);
3977:
3978: /* Commit what we have for now. */
3979: rb_end_commit(cpu_buffer);
3980: /* rb_end_commit() decs committing */
3981: local_inc(&cpu_buffer->committing);
3982:
3983: /* fail and let the caller try again */
3984: return ERR_PTR(-EAGAIN);
3985:
3986: out_reset:
3987: /* reset write */
3988: rb_reset_tail(cpu_buffer, tail, info);
3989:
3990: return NULL;
3991: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 70/2 |
2026/09/04 05:36 |
llm |
expert |
0m
Model:
Tokens:
input: 63464
output: 23
thoughts: 32
Thoughts: Okay, here's my take on those thoughts, framed as my own mental process, given my expertise:
**Checking `rb_reset_tail` and Padding**
Alright, so `rb_reset_tail` is adding the padding event – that much is clear from the context. Now, the key question that immediately pops into my head is: *does* this function correctly manage the `entries` counter? That's the critical bit. We need to make sure the accounting is perfect here; a miscount could throw off the whole ring buffer logic down the line. We can't have off-by-one errors creeping in.
Therefore, let's just go ahead and dive directly into the implementation of `rb_reset_tail` to be absolutely certain of the incrementing logic. No point in assuming; we've got to confirm this for ourselves.
|
| 71/2 |
2026/09/04 05:36 |
tool |
codesearch-definition-source |
0m
Args:
Results: map[SourceCode:3807: static inline void
3808: rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer,
3809: unsigned long tail, struct rb_event_info *info)
3810: {
3811: struct buffer_page *tail_page = info->tail_page;
3812: unsigned long bsize = rb_page_capacity(tail_page);
3813: struct ring_buffer_event *event;
3814: unsigned long length = info->length;
3815:
3816: /*
3817: * Only the event that crossed the page boundary
3818: * must fill the old tail_page with padding.
3819: */
3820: if (tail >= bsize) {
3821: /*
3822: * If the page was filled, then we still need
3823: * to update the real_end. Reset it to zero
3824: * and the reader will ignore it.
3825: */
3826: if (tail == bsize)
3827: tail_page->real_end = 0;
3828:
3829: local_sub(length, &tail_page->write);
3830: return;
3831: }
3832:
3833: event = __rb_page_index(tail_page, tail);
3834:
3835: /*
3836: * Save the original length to the meta data.
3837: * This will be used by the reader to add lost event
3838: * counter.
3839: */
3840: tail_page->real_end = tail;
3841:
3842: /*
3843: * If this event is bigger than the minimum size, then
3844: * we need to be careful that we don't subtract the
3845: * write counter enough to allow another writer to slip
3846: * in on this page.
3847: * We put in a discarded commit instead, to make sure
3848: * that this space is not used again, and this space will
3849: * not be accounted into 'entries_bytes'.
3850: *
3851: * If we are less than the minimum size, we don't need to
3852: * worry about it.
3853: */
3854: if (tail > (bsize - RB_EVNT_MIN_SIZE)) {
3855: /* No room for any events */
3856:
3857: /* Mark the rest of the page with padding */
3858: rb_event_set_padding(event);
3859:
3860: /* Make sure the padding is visible before the write update */
3861: smp_wmb();
3862:
3863: /* Set the write back to the previous setting */
3864: local_sub(length, &tail_page->write);
3865: return;
3866: }
3867:
3868: /* Put in a discarded event */
3869: event->array[0] = (bsize - tail) - RB_EVNT_HDR_SIZE;
3870: event->type_len = RINGBUF_TYPE_PADDING;
3871: /* time delta must be non zero */
3872: event->time_delta = 1;
3873:
3874: /* account for padding bytes */
3875: local_add(bsize - tail, &cpu_buffer->entries_bytes);
3876:
3877: /* Make sure the padding is visible before the tail_page->write update */
3878: smp_wmb();
3879:
3880: /* Set write to end of buffer */
3881: length = (tail + length) - bsize;
3882: local_sub(length, &tail_page->write);
3883: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 72/2 |
2026/09/04 05:36 |
llm |
expert |
0m
Model:
Tokens:
input: 64833
output: 27
thoughts: 750
Thoughts: **Analyzing a Ring Buffer's Entry Count**
Okay, so I'm digging into this ring buffer logic, and something's bugging me about `rb_num_of_entries`. Initially, I was looking at `rb_reset_tail` and realizing it *doesn't* increment `entries`. It just adds padding. That means padding events aren't counted in `entries` at all. This is key!
So, if padding isn't counted, `entries` *only* reflects real events written. If a write happens, `entries` is incremented. I got that. If it spans a page, padding is inserted, then the write happens on the next page, and `entries` increases by 1. Therefore, `entries` should exactly represent the number of real events written.
Now, `overrun` is incremented by `rb_page_entries(next_page)` in `rb_handle_head_page`, and this function returns the number of REAL events in that page. So, if I do the math, `entries - overrun - read` should give me the number of real events *currently* in the ring. Since the writer must have filled any other pages, `entries - overrun - read` *has* to be greater than 0, right? This means `rb_num_of_entries` CANNOT be 0.
Wait, hold on. What if the writer *didn't* write real events to those other pages? But how could that happen? Unless the event itself is larger than the page size? Nope, events can't be bigger than the page. Okay, what if the event is *almost* the size of a page? It fits in the first page. Then, I write another event almost the size of a page, doesn't fit on the first page, so padding goes into the first page, and the event goes into a second page. That means the first page has ONE real event, and `entries` increases by 1! So, every other page MUST have at least one real event. In other words, `entries - overrun - read` must be greater or equal to the number of other pages in the ring.
Since the ring has at least two pages, and the writer overwrote the head page, there's at least one other page. Therefore, `entries - overrun - read` MUST be >= 1. Again, `rb_num_of_entries` CANNOT be 0!
Alright, so if `rb_num_of_entries` couldn't have been 0 earlier in `rb_per_cpu_empty` and it wasn't zero, WHY did `rb_get_reader_page` return NULL?
Let's trace back. I'm looking at `ring_buffer_map_get_reader` again. Something isn't adding up.
```c
reader = rb_get_reader_page(cpu_buffer);
if (WARN_ON(!reader))
goto out;
```
Could it be that `rb_get_reader_page` returns NULL even when `rb_num_of_entries` was > 0, but `rb_per_cpu_empty` ALSO returned TRUE, but `ring_buffer_map_get_reader` DID NOT `goto out`?
```c
if (rb_per_cpu_empty(cpu_buffer))
goto out;
```
If `rb_per_cpu_empty` returns TRUE, it goes to `out`. Wait a second! I see it! There's a label called `consume:`
```c
consume:
if (rb_per_cpu_empty(cpu_buffer))
goto out;
```
Aha! There IS a `goto consume;` in the flow! Let me check `ring_buffer_map_get_reader`...
|
| 73/2 |
2026/09/04 05:36 |
tool |
codesearch-definition-source |
0m
Args: map[Name:ring_buffer_map_get_reader]
Results: map[SourceCode:7891: int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
7892: {
7893: struct ring_buffer_per_cpu *cpu_buffer;
7894: struct buffer_page *reader;
7895: unsigned long missed_events;
7896: unsigned long reader_size;
7897: unsigned long flags;
7898:
7899: cpu_buffer = rb_get_mapped_buffer(buffer, cpu);
7900: if (IS_ERR(cpu_buffer))
7901: return (int)PTR_ERR(cpu_buffer);
7902:
7903: raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
7904:
7905: consume:
7906: if (rb_per_cpu_empty(cpu_buffer))
7907: goto out;
7908:
7909: reader_size = rb_page_size(cpu_buffer->reader_page);
7910:
7911: /*
7912: * There are data to be read on the current reader page, we can
7913: * return to the caller. But before that, we assume the latter will read
7914: * everything. Let's update the kernel reader accordingly.
7915: */
7916: if (cpu_buffer->reader_page->read < reader_size) {
7917: while (cpu_buffer->reader_page->read < reader_size)
7918: rb_advance_reader(cpu_buffer);
7919: goto out;
7920: }
7921:
7922: /* Did the reader catch up with the writer? */
7923: if (cpu_buffer->reader_page == cpu_buffer->commit_page)
7924: goto out;
7925:
7926: reader = rb_get_reader_page(cpu_buffer);
7927: if (WARN_ON(!reader))
7928: goto out;
7929:
7930: /* Check if any events were dropped */
7931: missed_events = cpu_buffer->lost_events;
7932:
7933: if (missed_events) {
7934: if (cpu_buffer->reader_page != cpu_buffer->commit_page) {
7935: struct buffer_data_page *dpage = reader->page;
7936: unsigned int commit;
7937: /*
7938: * Use the real_end for the data size,
7939: * This gives us a chance to store the lost events
7940: * on the page.
7941: */
7942: if (reader->real_end)
7943: local_set(&dpage->commit, reader->real_end);
7944: /*
7945: * If there is room at the end of the page to save the
7946: * missed events, then record it there.
7947: */
7948: commit = rb_page_size(reader);
7949: if (rb_page_capacity(reader) - commit >= sizeof(missed_events)) {
7950: memcpy(&dpage->data[commit], &missed_events,
7951: sizeof(missed_events));
7952: local_add(RB_MISSED_STORED, &dpage->commit);
7953: }
7954: local_add(RB_MISSED_EVENTS, &dpage->commit);
7955: } else if (!WARN_ONCE(cpu_buffer->reader_page == cpu_buffer->tail_page,
7956: "Reader on commit with %ld missed events",
7957: missed_events)) {
7958: /*
7959: * There shouldn't be any missed events if the tail_page
7960: * is on the reader page. But if the tail page is not on the
7961: * reader page and the commit_page is, that would mean that
7962: * there's a commit_overrun (an interrupt preempted an
7963: * addition of an event and then filled the buffer
7964: * with new events). In this case it's not an
7965: * error, but it should still be reported.
7966: *
7967: * TODO: Add missed events to the page for user space to know.
7968: */
7969: pr_info("Ring buffer [%d] commit overrun lost %ld events at timestamp:%lld\n",
7970: cpu, missed_events, cpu_buffer->reader_page->page->time_stamp);
7971: }
7972: }
7973:
7974: cpu_buffer->lost_events = 0;
7975:
7976: goto consume;
7977:
7978: out:
7979: /* Some archs do not have data cache coherency between kernel and user-space */
7980: flush_kernel_vmap_range(cpu_buffer->reader_page->page,
7981: rb_subbuf_size(buffer));
7982:
7983: rb_update_meta_page(cpu_buffer);
7984:
7985: raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
7986: rb_put_mapped_buffer(cpu_buffer);
7987:
7988: return 0;
7989: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 74/2 |
2026/09/04 05:36 |
llm |
expert |
0m
Model:
Tokens:
input: 68011
output: 25
thoughts: 2679
Thoughts: **Analyzing a Ring Buffer Issue: A Deep Dive**
Okay, so I'm staring at this `goto consume;` and tracking how the code flows through `ring_buffer_map_get_reader`. The core seems to be getting a reader page, checking if it's empty, and then deciding whether to advance the reader. The initial check is whether `rb_per_cpu_empty` is true. If it is, we exit. If it's *not* empty, we check `cpu_buffer->reader_page->read < reader_size`, which, given that `rb_get_reader_page` initializes `read` to zero, should always be true initially, unless `reader_size` is zero.
The immediate question: can `reader_size` be zero? It's derived from `rb_page_size`. If `rb_page_size` returns zero (which I need to check), then we skip the read check, which is unexpected, but could happen. This then leads to checking if `cpu_buffer->reader_page == cpu_buffer->commit_page`, and if not, we call `rb_get_reader_page` again.
Here's the problem: `rb_per_cpu_empty` returns false, meaning there *should* be data, yet `rb_get_reader_page` might return a page where `rb_page_size` is zero. I have to understand the scenarios where this inconsistency occurs. If the reader page has 0 events, but it is not empty, events must be in the other pages. I realize that the reader page gets swapped with the head page, which the writer is working on. So, what if the writer is on a brand new page, but hasn't written any data? Then we get 0 events.
So this explains `rb_get_reader_page` returning a page of zero size, which causes the read size check to be skipped, and then a call to `rb_get_reader_page` again. However, in that function, the code checks if `cpu_buffer->reader_page->read < rb_page_size(reader)`, if the size is zero, then it bails.
Next, I look at the `rb_num_of_entries` check. If it's zero, we return null. But I know `rb_per_cpu_empty` returned false, so this shouldn't happen. Then `rb_get_reader_page` swaps, and goes to consume again.
When does `rb_get_reader_page` return NULL? It seems to be when `rb_num_of_entries` is zero. If it returns NULL, the reader will not advance! But, again, `rb_per_cpu_empty` said there are events! So that can't be true, right?
Alright, can the situation in which `rb_num_of_entries` is zero arise between the call to `rb_per_cpu_empty` and `rb_get_reader_page`? I prove that writers wrapping around and the reader not reading can't cause that to happen.
Then I look back at `rb_get_reader_page`. I realize that if the reader is on the commit page, it can't return NULL, it should just return the current page.
So, I think, what if `rb_set_head_page` returns NULL? That seems unlikely, because it requires an `RB_WARN_ON`, and the reports don't indicate one. Could `rb_get_reader_page` return NULL because of `USECS_WAIT`? I don't see one.
Now, I look at the loop limit: `RB_WARN_ON(cpu_buffer, ++nr_loops > max_loops)`. Is it possible that `RB_WARN_ON` is triggered but not reported? Then I remember that syzbot often groups warnings. If `RB_WARN_ON` was triggered, the bug title should be about `__rb_get_reader_page` not `ring_buffer_map_get_reader`. So that can't be it either.
Okay, let's revisit when `rb_get_reader_page` returns NULL. I have it. If `rb_num_of_entries` is zero, it returns NULL. And, again, the fact that `rb_per_cpu_empty` returned false, should mean this is impossible. It looks like it is impossible...
Can `rb_num_of_entries` be 0 because the reader increases, and then the reader decreases? No. `read` and `entries` are atomically updated. So that's wrong.
Is it possible that `rb_num_of
|
| 75/2 |
2026/09/04 05:37 |
tool |
codesearch-definition-source |
0m
Args: map[Name:ring_buffer_discard_commit]
Results: map[SourceCode:5093: /**
5094: * ring_buffer_discard_commit - discard an event that has not been committed
5095: * @buffer: the ring buffer
5096: * @event: non committed event to discard
5097: *
5098: * Sometimes an event that is in the ring buffer needs to be ignored.
5099: * This function lets the user discard an event in the ring buffer
5100: * and then that event will not be read later.
5101: *
5102: * This function only works if it is called before the item has been
5103: * committed. It will try to free the event from the ring buffer
5104: * if another event has not been added behind it.
5105: *
5106: * If another event has been added behind it, it will set the event
5107: * up as discarded, and perform the commit.
5108: *
5109: * If this function is called, do not call ring_buffer_unlock_commit on
5110: * the event.
5111: */
5112: void ring_buffer_discard_commit(struct trace_buffer *buffer,
5113: struct ring_buffer_event *event)
5114: {
5115: struct ring_buffer_per_cpu *cpu_buffer;
5116: int cpu;
5117:
5118: /* The event is discarded regardless */
5119: rb_event_discard(event);
5120:
5121: cpu = smp_processor_id();
5122: cpu_buffer = buffer->buffers[cpu];
5123:
5124: /*
5125: * This must only be called if the event has not been
5126: * committed yet. Thus we can assume that preemption
5127: * is still disabled.
5128: */
5129: RB_WARN_ON(buffer, !local_read(&cpu_buffer->committing));
5130:
5131: rb_decrement_entry(cpu_buffer, event);
5132: rb_try_to_discard(cpu_buffer, event);
5133: rb_end_commit(cpu_buffer);
5134:
5135: trace_recursive_unlock(cpu_buffer);
5136:
5137: preempt_enable_notrace();
5138:
5139: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 76/2 |
2026/09/04 05:37 |
llm |
expert |
0m
Model:
Tokens:
input: 72446
output: 27
thoughts: 2761
Thoughts: **Analysis of a Ring Buffer Race Condition**
Okay, here's the breakdown of what I've discovered. It looks like we have a race condition lurking within the kernel's ring buffer implementation. My initial thought process led me to realize that `rb_event_discard` sets the event type to `RINGBUF_TYPE_PADDING`. Then, the crucial detail surfaced: `entries` only gets incremented during `rb_commit`. The kicker? If an event is discarded *instead* of committed (e.g., via `ring_buffer_discard_commit`), then `entries` doesn't get incremented, even though the space is still allocated.
My suspicion grew: if a writer repeatedly reserves an event, discards it, and doesn't increment `entries`, then an entire page could fill with effectively "dead" events. Moreover, if every other page is filled with discarded events, then `entries` remains at zero! This would mean we could wrap around and overwrite valid data. `overrun` increases by the number of real events, leading `entries - overrun - read` to become zero!
Aha! We've found the potential problem: `rb_num_of_entries` can indeed become zero under these circumstances.
Digging deeper, I explored the timing of discard and interrupt scenarios. If an interrupt occurs *between* a reserve and a discard, the event becomes padding because `rb_try_to_discard` can fail. I wondered if a user-space process could trigger this behavior, but then I remembered that `ring_buffer_discard_commit` is used by kernel tracers. This gave me the crucial information: a filter rejecting events and discarding them is what causes this discard.
However, I started thinking about size; what if an event itself is too big? I already know about the padding that's not counted, but what happens with the next page? A real event! If the tracer filters out ALL events, then ALL events are discarded, but that leaves the pages filled with padding! Thus, `entries` is zero.
The consequence is that `rb_num_of_entries` becomes zero. And if `rb_num_of_entries` is zero, then `rb_per_cpu_empty` returns true!
I have it! This is where the race condition emerges: We need a situation where `rb_num_of_entries` is initially greater than zero, then drops to zero. Here's the scenario: 1. The head page has 1 valid event, so `rb_num_of_entries` is initially 1. 2. The code calls `rb_per_cpu_empty`, so it returns `FALSE`. 3. The writer filters out events. 4. Discarded events fill the remaining pages, and the number of `entries` does *not* increase. 5. The writer wraps around and overwrites the head page. 6. `overrun` increases by 1. 7. `rb_num_of_entries` goes to 0. This is the race: we must have both a non-zero number of entries *and* the number of entries equaling zero! 8. Now `ring_buffer_map_get_reader` calls `rb_get_reader_page`. 9. `__rb_get_reader_page` checks `rb_num_of_entries == 0`, and returns NULL. 10. `WARN_ON(!reader)` triggers.
This is a **race condition** between a reader trying to access a mapped ring buffer and a writer overwriting that buffer with discarded events!
Now, the important question is: is this exploitable? Thankfully, it's just a `WARN_ON`, which the code safely handles by jumping to `out`. No memory corruption, and the null pointer returned from `rb_get_reader_page` is checked immediately before dereferencing, so there's no way to dereference a null pointer. The worst outcome is a spurious warning.
The code goes to `out` and returns 0. The user-space reader thinks it succeeds, but the reader page was *not* actually updated. So, the user will either see the old (possibly stale) page or, perhaps, an empty one.
Is there any way for an unprivileged user to trigger this?
Since unprivileged users cannot mount `tracefs` (by default), they cannot cause this to happen, unless they are in the tracing group and the mount permissions change. It's also not accessible from user namespaces.
This bug can only lead to a warning, and is NOT exploitable. It doesn't cause a denial of service, because the `WARN_ON` is handled, and is not accessible from unprivileged processes or user namespaces.
|
| 77/2 |
2026/09/04 05:37 |
tool |
codesearch-find-references |
0m
Args: map[Name:ring_buffer_map_get_reader]
Results: map[References:[map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:tracing_buffers_ioctl SourceFile:kernel/trace/trace.c SourceLine:7392]] TruncatedOutput:false]
|
| 78/2 |
2026/09/04 05:37 |
llm |
expert |
0m
Model:
Tokens:
input: 76322
output: 66
thoughts: 579
Thoughts: **My Analysis of the `tracing_buffers_ioctl` Vulnerability**
Okay, so let's break this down. From what I can see, the code path that uses `tracing_buffers_ioctl` is tightly constrained. It's only called by `tracing_buffers_ioctl`, which in turn is only used in `tracing_buffers_fops`, and that's *exclusively* for `trace_pipe_raw`. That means it's strictly confined to tracefs, which, as we know, means it's not accessible to unprivileged users. So, we can immediately rule out unprivileged exploitation.
Now, let's circle back and look for other angles. Is there any way that the `reader` variable is used before initialization or that it could trigger a crash? The code grabs a reader page:
```c
reader = rb_get_reader_page(cpu_buffer);
if (WARN_ON(!reader))
goto out;
```
A NULL `reader` goes to `out`. Then, there's a flush operation:
```c
out:
/* Some archs do not have data cache coherency between kernel and user-space */
flush_kernel_vmap_range(cpu_buffer->reader_page->page,
rb_subbuf_size(buffer));
```
Wait a second... here we're dereferencing `cpu_buffer->reader_page`! Could *that* be NULL? No, the structure `cpu_buffer->reader_page` is allocated during buffer creation and is *never* NULL. If `rb_get_reader_page` fails, it returns NULL, and the old reader page remains in place. So, `cpu_buffer->reader_page` is still valid, and the flush operation is safe, just flushing the previous reader page. No crash, no memory corruption.
Next, I wondered about `rb_update_meta_page(cpu_buffer)`. However,
```c
static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
{
struct trace_buffer_meta *meta = cpu_buffer->meta_page;
if (!meta)
return;
...
```
This just updates the meta page and is protected with a check that just exits, so it's safe.
Therefore, the `if (WARN_ON(!reader)) goto out;` check effectively handles the case where `rb_get_reader_page` fails. It's a benign race condition that results in a warning. The appropriate fix would involve removing the `WARN_ON` or handling the race condition more robustly, perhaps by retrying the operation or returning `-EAGAIN`.
In conclusion, I'd rate this as:
* Exploitable: False
* DenialOfService: False
* Unprivileged: False
* UserNamespace: False
* VMGuestTrigger: False
* VMHostTrigger: False
* NetworkTrigger: False
* RemoteTrigger: False
* PeripheralTrigger: False
* FilesystemTrigger: False
|
| 79/2 |
2026/09/04 05:37 |
tool |
set-results |
0m
Args: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 80/2 |
2026/09/04 05:37 |
llm |
expert |
0m
Model:
Tokens:
input: 53595
output: 519
thoughts: 1881
Thoughts: **Analysis of the Ring Buffer Reader Issue in Tracefs**
Okay, so I'm looking at this warning in `ring_buffer_map_get_reader` at `kernel/trace/ring_buffer.c:7927`. It's a simple `WARN_ON(!reader)`. It seems to be triggered by the `tracing_buffers_ioctl` function, specifically with the `TRACE_MMAP_IOCTL_GET_READER` command. That ioctl handles the mapping of the trace buffer, typically via files under `/sys/kernel/tracing`, like `trace_pipe_raw`.
First, let's establish the context: tracefs is, by default, accessible only to root, because it requires CAP_SYS_ADMIN. I've checked and there's no way to mount tracefs in a user namespace, so unprivileged users are out of the picture. The code's only reachable by root, or someone with similar privileges. So, no unprivileged access.
The core of the problem is `rb_get_reader_page` returning NULL. Diving into that, `__rb_get_reader_page` has several conditions where it can return NULL: exceeding the loop limit, the reader and commit pages being equal (empty buffer), or the buffer is empty in the first place, or `rb_set_head_page` fails. Importantly, if `rb_num_of_entries` is zero, we'll get a NULL.
Now, looking back at the `ring_buffer_map_get_reader` function, there's a check with `rb_per_cpu_empty` before `rb_get_reader_page` is called, and this checks for `!rb_num_of_entries(cpu_buffer)`. But wait, if after that check and before the call to `rb_get_reader_page` the ring buffer is overwritten due to, say, an overrun, so that the buffer becomes empty (due to `overrun` increasing and decrementing `rb_num_of_entries`), then `rb_get_reader_page` will get a NULL, leading to the warning. It can also happen if `rb_set_head_page` fails for some reason.
Let's consider exploitability. This is *just* a `WARN_ON`. The code gracefully handles the NULL reader, jumping to the `out` label. It doesn't cause a crash unless `panic_on_warn` is set, and even then the code would be very safe. It's just releasing locks and returning 0. There's no memory corruption, no information leak, nothing that could be directly exploited. It's simply a warning that the reader couldn't get a page, likely due to a race condition (writers incrementing `overrun` and, thus, decrementing `rb_num_of_entries`).
Therefore, this is not an exploitable condition. It's a non-issue from a security perspective. It is not denial of service, because it's only a warning. It is not privileged, because of tracefs is restricted to root only. Also, no user namespace access.
I think I've covered everything. I'll write up the reasoning.
|
| 81/1 |
2026/09/04 05:38 |
action |
format-explanation |
0m
Results: map[Explanation:An analysis of the bug report and the corresponding kernel source code reveals
the following:
**Bug Mechanism and Impact:**
The bug is a `WARN_ON(!reader)` in `ring_buffer_map_get_reader()` located in
`kernel/trace/ring_buffer.c`. It is triggered via the
`TRACE_MMAP_IOCTL_GET_READER` ioctl on tracefs ring buffer files (e.g.,
`per_cpu/cpuX/trace_pipe_raw`).
The warning is hit when `rb_get_reader_page(cpu_buffer)` returns `NULL`. This
can happen due to race conditions with concurrent writers. For instance,
`ring_buffer_map_get_reader()` first checks if the buffer is empty using
`rb_per_cpu_empty()`. If a writer subsequently overwrites events (increasing the
overrun count), the number of entries can drop to zero before
`rb_get_reader_page()` is called. `rb_get_reader_page()` will then return `NULL`
because it sees an empty buffer or fails to swap the head page.
When `reader` is `NULL`, the code hits the `WARN_ON(!reader)` and safely bails
out by jumping to the `out` label. At the `out` label, it flushes the cache,
updates the meta page, releases the reader lock, and returns `0`. There is no
invalid memory access, memory corruption, or information leak. The kernel state
remains consistent, and execution continues normally. Therefore, this is not a
memory safety issue and is not exploitable. It also does not cause a Denial of
Service (DoS) in production builds where `panic_on_warn` is disabled.
**Accessibility:**
The ioctl is exposed through tracefs (typically mounted at
`/sys/kernel/tracing`). Tracefs is restricted to root (requires capabilities
like `CAP_SYS_ADMIN` or `CAP_TRACING`) and its files are not accessible to
unprivileged users. Furthermore, tracefs does not have the `FS_USERNS_MOUNT`
flag, meaning it cannot be mounted inside an unprivileged user namespace. Thus,
the bug cannot be triggered by unprivileged users or from within user
namespaces.
**Conclusion:**
The bug is a benign warning caused by a race condition in the tracing subsystem.
It is only reachable by highly privileged users (root) and does not lead to any
memory corruption, information leak, or denial of service.]
|