tracing/hist: Fix issue of losting command info in error_log

When input some constructed invalid 'trigger' command, command info
in 'error_log' are lost [1].

The root cause is that there is a path that event_hist_trigger_parse()
is recursely called once and 'last_cmd' which save origin command is
cleared, then later calling of hist_err() will no longer record origin
command info:

  event_hist_trigger_parse() {
    last_cmd_set()  // <1> 'last_cmd' save origin command here at first
    create_actions() {
      onmatch_create() {
        action_create() {
          trace_action_create() {
            trace_action_create_field_var() {
              create_field_var_hist() {
                event_hist_trigger_parse() {  // <2> recursely called once
                  hist_err_clear()  // <3> 'last_cmd' is cleared here
                }
                hist_err()  // <4> No longer find origin command!!!

Since 'glob' is empty string while running into the recurse call, we
can trickly check it and bypass the call of hist_err_clear() to solve it.

[1]
 # cd /sys/kernel/tracing
 # echo "my_synth_event int v1; int v2; int v3;" >> synthetic_events
 # echo 'hist:keys=pid' >> events/sched/sched_waking/trigger
 # echo "hist:keys=next_pid:onmatch(sched.sched_waking).my_synth_event(\
pid,pid1)" >> events/sched/sched_switch/trigger
 # cat error_log
[  8.405018] hist:sched:sched_switch: error: Couldn't find synthetic event
  Command:
hist:keys=next_pid:onmatch(sched.sched_waking).my_synth_event(pid,pid1)
                                                          ^
[  8.816902] hist:sched:sched_switch: error: Couldn't find field
  Command:
hist:keys=next_pid:onmatch(sched.sched_waking).my_synth_event(pid,pid1)
                          ^
[  8.816902] hist:sched:sched_switch: error: Couldn't parse field variable
  Command:
hist:keys=next_pid:onmatch(sched.sched_waking).my_synth_event(pid,pid1)
                          ^
[  8.999880] : error: Couldn't find field
  Command:
           ^
[  8.999880] : error: Couldn't parse field variable
  Command:
           ^
[  8.999880] : error: Couldn't find field
  Command:
           ^
[  8.999880] : error: Couldn't create histogram for field
  Command:
           ^

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: <[email protected]>
Cc: <[email protected]>
Fixes: f404da6 ("tracing: Add 'last error' error facility for hist triggers")
Signed-off-by: Zheng Yejian <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: Zheng Yejian

kernel/trace/trace_events_hist.c

@@ -6576,7 +6576,7 @@ static int event_hist_trigger_parse(struct event_command *cmd_ops,
 	if (se)
 		se->ref++;
  out:
-	if (ret == 0)
+	if (ret == 0 && glob[0])
 		hist_err_clear();
 
 	return ret;