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

commit aa997d2d2a0b2e76f4df0f1f12829f02acb4fb6b upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 95641a46db4cd37b15d56088b344562191c0cd50..a785cc3839338660dc1016eed8897712209cb892 100644 (file)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7223,6 +7223,10 @@ 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;