[PATCH 4.14 153/237] trigger_next should increase position index

Greg Kroah-Hartman
2020-02-27T13:59:03Z

From: Vasily Averin <vvs@virtuozzo.com>

[ 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 <vvs@virtuozzo.com>
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 e2da180ca172a..31e91efe243e5 100644
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -127,9 +127,10 @@ 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