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

["syzbot" <syzbot@kernel.org>]

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.

Because WARN_ON must not be used for conditions that can legitimately
happen, ring_buffer_map_get_reader() should not unconditionally warn when
rb_get_reader_page() returns NULL. Fix this by only warning if
rb_get_reader_page() returns NULL while cpu_buffer->reader_page !=
cpu_buffer->commit_page.

Fixes: 117c39200d9d ("ring-buffer: Introducing ring-buffer mapping functions")
Assisted-by: Gemini:gemini-3.7-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+de3d7f9bcc9212f3fae1@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=de3d7f9bcc9212f3fae1
Link: https://syzkaller.appspot.com/ai_job?id=f119d7de-b21d-4d27-b61e-5503e31edccd
To: <linux-trace-kernel@vger.kernel.org>
To: "Masami Hiramatsu" <mhiramat@kernel.org>
To: "Steven Rostedt" <rostedt@goodmis.org>
To: "Vincent Donnefort" <vdonnefort@google.com>
Cc: <linux-kernel@vger.kernel.org>
Cc: "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>

---
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9c03a555a..98130182e 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7995,8 +7995,10 @@ 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) {
+		WARN_ON(cpu_buffer->reader_page != cpu_buffer->commit_page);
 		goto out;
+	}
 
 	/* Check if any events were dropped */
 	missed_events = cpu_buffer->lost_events;


base-commit: df2908090cda368b01ff43709f51890076c56157
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