From: Steven Rostedt rostedt@goodmis.org

The function ring_buffer_map_get_reader() is a bit more strict than the other get reader functions, and except for certain situations the rb_get_reader_page() should not return NULL. If it does, it triggers a warning.

This warning was triggering but after looking at why, it was because another acceptable situation was happening and it wasn't checked for.

If the reader catches up to the writer and there's still data to be read on the reader page, then the rb_get_reader_page() will return NULL as there's no new page to get.

In this situation, the reader page should not be updated and no warning should trigger.

Cc: stable@vger.kernel.org Cc: Masami Hiramatsu mhiramat@kernel.org Cc: Mathieu Desnoyers mathieu.desnoyers@efficios.com Cc: Vincent Donnefort vdonnefort@google.com Reported-by: syzbot+92a3745cea5ec6360309@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/690babec.050a0220.baf87.0064.GAE@google.com/ Link: https://lore.kernel.org/20251016132848.1b11bb37@gandalf.local.home Fixes: 117c39200d9d7 ("ring-buffer: Introducing ring-buffer mapping functions") Signed-off-by: Steven Rostedt (Google) rostedt@goodmis.org --- kernel/trace/ring_buffer.c | 4 ++++ 1 file changed, 4 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 1244d2c5c384..afcd3747264d 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -7344,6 +7344,10 @@ int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu) goto out; }

+ /* 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;