ring-buffer: Do not warn in ring_buffer_map_get_reader() when reader catches up

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>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 1244d2c5c384ade76c85bd4c5a6e8cad40cb8e02..afcd3747264d2ee8fa8de819b50041a7f287e409 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7344,6 +7344,10 @@ consume:
                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;