From: Vasily Averin <vvs@xxxxxxxxxxxxx> [ 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@xxxxxxxxxxxxx https://bugzilla.kernel.org/show_bug.cgi?id=206283 Signed-off-by: Vasily Averin <vvs@xxxxxxxxxxxxx> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- 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 8be66a2b0cacf..6524920c6ebc8 100644 --- a/kernel/trace/trace_events_trigger.c +++ b/kernel/trace/trace_events_trigger.c @@ -121,9 +121,10 @@ 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
