[PATCH 3.2 049/115] tracing: Fix crash from reading trace_pipe with sendfile

[Ben Hutchings]

3.2.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>

commit a29054d9478d0435ab01b7544da4f674ab13f533 upstream.

If tracing contains data and the trace_pipe file is read with sendfile(),
then it can trigger a NULL pointer dereference and various BUG_ON within the
VM code.

There's a patch to fix this in the splice_to_pipe() code, but it's also a
good idea to not let that happen from trace_pipe either.

Link: http://lkml.kernel.org/r/1457641146-9068-1-git-send-email-rabin@xxxxxx

Reported-by: Rabin Vincent <rabin.vincent@xxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/trace/trace.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3565,7 +3565,10 @@ static ssize_t tracing_splice_read_pipe(
 
 	spd.nr_pages = i;
 
-	ret = splice_to_pipe(pipe, &spd);
+	if (i)
+		ret = splice_to_pipe(pipe, &spd);
+	else
+		ret = 0;
 out:
 	splice_shrink_spd(&spd);
 	return ret;