On 9/21/2026 10:12 AM, Vincent Donnefort wrote:
On Fri, Sep 18, 2026 at 02:34:25PM +0000, syzbot wrote:
From: Krystian Kaniewski <[email protected]>
In ring_buffer_map_get_reader(), an unconditional WARN_ON(!reader) is
triggered when rb_get_reader_page() returns NULL:
WARNING: CPU: 1 PID: 5906 at kernel/trace/ring_buffer.c:7998
ring_buffer_map_get_reader+0x940/0x9d0
CPU: 1 UID: 0 PID: 5906 Comm: task Not tainted
RIP: 0010:ring_buffer_map_get_reader+0x940/0x9d0
kernel/trace/ring_buffer.c:7998
Call Trace:
<TASK>
tracing_buffers_ioctl+0x258/0x300 kernel/trace/trace.c:7381
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
</TASK>
This warning is triggered due to a race between a writer committing events
and a reader mapping the ring buffer via TRACE_MMAP_IOCTL_GET_READER. When
a writer commits an event in rb_set_commit_to_write(), it advances
cpu_buffer->commit_page to cpu_buffer->tail_page in its first loop before
updating the commit counter (commit_page->page->commit) in the second loop.
If a reader invokes ring_buffer_map_get_reader() at this moment, the
initial check cpu_buffer->reader_page == cpu_buffer->commit_page is false,
and it calls rb_get_reader_page(). Inside __rb_get_reader_page(), the
reader swaps reader_page with the head page (which is the new commit_page).
Because the writer has not yet updated the commit count on the new page,
rb_page_size() is zero and reader_page->read < rb_page_size() evaluates to
false. __rb_get_reader_page() then checks if cpu_buffer->commit_page ==
cpu_buffer->reader_page. Since both now point to the swapped page, the
condition evaluates to true and rb_get_reader_page() legitimately returns
NULL to indicate the reader caught up to the writer.
This seems to make the check above reader_page == commit_page redundant. Doesn't
it?
Furthermore, rb_get_reader_page() can legitimately return NULL when there
is no data to read. Re-checking mutable writer state such as
We are checking rb_per_cpu_empty() with the reader_lock held few lines above. I
don't believe we expect NULL here for that reason.
cpu_buffer->reader_page != cpu_buffer->commit_page is insufficient because
a writer on another CPU can advance commit_page before the check is
evaluated without being stopped by reader_lock.
Because WARN_ON must not be used for conditions that can legitimately
happen, and pr_err should be used instead if necessary, remove the
WARN_ON() entirely since returning NULL here is an expected condition.
Fixes: 117c39200d9d ("ring-buffer: Introducing ring-buffer mapping functions")
Assisted-by: Gemini:gemini-3.8-flash syzbot
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=de3d7f9bcc9212f3fae1
Link:
https://syzkaller.appspot.com/ai_job?id=488adc18-a10e-42d2-a205-25327c38837f
Signed-off-by: Krystian Kaniewski <[email protected]>
---
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9c03a555a..1a4da3d47 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7995,7 +7995,7 @@ int ring_buffer_map_get_reader(struct trace_buffer
*buffer, int cpu)
goto out;
reader = rb_get_reader_page(cpu_buffer);
- if (WARN_ON(!reader))
+ if (!reader)
goto out;
/* Check if any events were dropped */
base-commit: df2908090cda368b01ff43709f51890076c56157
--
See https://goo.gle/syzbot-ai-patches for information about AI-generated
patches.
The person who has signed off on the patch is responsible for
addressing comments.
syzbot engineers can be reached at [email protected].
Thanks for taking a look into my patch!
>
> This seems to make the check above reader_page == commit_page
redundant. Doesn't
> it?
>
Yes, it is no longer needed for correctness once a NULL result is
handled without a warning. It remains a fast path when the reader has
already caught up, avoiding a call to rb_get_reader_page(). I can remove
it if you prefer in v2 patch.
>>
>> Furthermore, rb_get_reader_page() can legitimately return NULL when
there
>> is no data to read. Re-checking mutable writer state such as
>
> We are checking rb_per_cpu_empty() with the reader_lock held few
lines above. I
> don't believe we expect NULL here for that reason.
>
Yes, you are right, an empty result exits before the helper. But
reader_lock does not stop the writer and a nonempty entry count does not
guarantee a committed page. After swapping the reader page, the helper
can catch up to commit_page and return NULL. I can clarify the commit
message as part of v2 patch if needed.
