Re: [PATCH] ring-buffer: Fix false warning in ring_buffer_map_get_reader()

[Vincent Donnefort]

On Fri, Sep 18, 2026 at 02:34:25PM +0000, syzbot wrote:
> From: Krystian Kaniewski <krystianmkaniewski@xxxxxxxxx>
> 
> 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: syzbot+de3d7f9bcc9212f3fae1@xxxxxxxxxxxxxxxxxxxxxxxxx
> 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 <krystianmkaniewski@xxxxxxxxx>
> 
> ---
> 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
> -- 
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-- 
Vincent