[PATCH] tracing: Return from tracing_buffers_read() if the file has been closed

[Steven Rostedt <rostedt@goodmis.org>]

From: Steven Rostedt <rostedt@goodmis.org>

When running the following:

 # cd /sys/kernel/tracing/
 # echo 1 > events/sched/sched_waking/enable
 # echo 1 > events/sched/sched_switch/enable
 # echo 0 > tracing_on
 # dd if=per_cpu/cpu0/trace_pipe_raw of=/tmp/raw0.dat

The dd task would get stuck in an infinite loop in the kernel. What would
happen is the following:

When ring_buffer_read_page() returns -1 (no data) then a check is made to
see if the buffer is empty (as happens when the page is not full), it will
call wait_on_pipe() to wait until the ring buffer has data. When it is it
will try again to read data (unless O_NONBLOCK is set).

The issue happens when there's a reader and the file descriptor is closed.
The wait_on_pipe() will return when that is the case. But this loop will
continue to try again and wait_on_pipe() will again return immediately and
the loop will continue and never stop.

Simply check if the file was closed before looping and exit out if it is.

Cc: stable@vger.kernel.org
Fixes: 2aa043a55b9a7 ("tracing/ring-buffer: Fix wait_on_pipe() race")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 10cd38bce2f1..ebe7ce2f5f4a 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7956,7 +7956,7 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
 	trace_access_unlock(iter->cpu_file);
 
 	if (ret < 0) {
-		if (trace_empty(iter)) {
+		if (trace_empty(iter) && !iter->closed) {
 			if ((filp->f_flags & O_NONBLOCK))
 				return -EAGAIN;
 
-- 
2.43.0