From: Vasily Averin <redacted>

[ Upstream commit 6722b23e7a2ace078344064a9735fb73e554e9ef ]

if seq_file .next fuction does not change position index,
read after some lseek can generate unexpected output.

Without patch:
 # dd bs=30 skip=1 if=/sys/kernel/tracing/events/sched/sched_switch/trigger
 dd: /sys/kernel/tracing/events/sched/sched_switch/trigger: cannot skip to specified offset
 n traceoff snapshot stacktrace enable_event disable_event enable_hist disable_hist hist
 # Available triggers:
 # traceon traceoff snapshot stacktrace enable_event disable_event enable_hist disable_hist hist
 6+1 records in
 6+1 records out
 206 bytes copied, 0.00027916 s, 738 kB/s

Notice the printing of "# Available triggers:..." after the line.

With the patch:
 # dd bs=30 skip=1 if=/sys/kernel/tracing/events/sched/sched_switch/trigger
 dd: /sys/kernel/tracing/events/sched/sched_switch/trigger: cannot skip to specified offset
 n traceoff snapshot stacktrace enable_event disable_event enable_hist disable_hist hist
 2+1 records in
 2+1 records out
 88 bytes copied, 0.000526867 s, 167 kB/s

It only prints the end of the file, and does not restart.

Link: http://lkml.kernel.org/r/3c35ee24-dd3a-8119-9c19-552ed253388a@virtuozzo.com

https://bugzilla.kernel.org/show_bug.cgi?id=206283
Signed-off-by: Vasily Averin <redacted>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/trace/trace_events_trigger.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
index 7e6971ba95417..8a88e85c8c615 100644
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c

@@ -126,9 +126,10 @@ static void *trigger_next(struct seq_file *m, void *t, loff_t *pos)
 {
 	struct trace_event_file *event_file = event_file_data(m->private);
 
-	if (t == SHOW_AVAILABLE_TRIGGERS)
+	if (t == SHOW_AVAILABLE_TRIGGERS) {
+		(*pos)++;
 		return NULL;
-
+	}
 	return seq_list_next(t, &event_file->triggers, pos);
 }
 

-- 
2.20.1