[PATCH 5.15.y] tracing: Do not add length to print format in synthetic events

[Sasha Levin <sashal@kernel.org>]

From: Steven Rostedt <rostedt@goodmis.org>

[ Upstream commit e1a453a57bc76be678bd746f84e3d73f378a9511 ]

The following causes a vsnprintf fault:

  # echo 's:wake_lat char[] wakee; u64 delta;' >> /sys/kernel/tracing/dynamic_events
  # echo 'hist:keys=pid:ts=common_timestamp.usecs if !(common_flags & 0x18)' > /sys/kernel/tracing/events/sched/sched_waking/trigger
  # echo 'hist:keys=next_pid:delta=common_timestamp.usecs-$ts:onmatch(sched.sched_waking).trace(wake_lat,next_comm,$delta)' > /sys/kernel/tracing/events/sched/sched_switch/trigger

Because the synthetic event's "wakee" field is created as a dynamic string
(even though the string copied is not). The print format to print the
dynamic string changed from "%*s" to "%s" because another location
(__set_synth_event_print_fmt()) exported this to user space, and user
space did not need that. But it is still used in print_synth_event(), and
the output looks like:

          <idle>-0       [001] d..5.   193.428167: wake_lat: wakee=(efault)sshd-sessiondelta=155
    sshd-session-879     [001] d..5.   193.811080: wake_lat: wakee=(efault)kworker/u34:5delta=58
          <idle>-0       [002] d..5.   193.811198: wake_lat: wakee=(efault)bashdelta=91
            bash-880     [002] d..5.   193.811371: wake_lat: wakee=(efault)kworker/u35:2delta=21
          <idle>-0       [001] d..5.   193.811516: wake_lat: wakee=(efault)sshd-sessiondelta=129
    sshd-session-879     [001] d..5.   193.967576: wake_lat: wakee=(efault)kworker/u34:5delta=50

The length isn't needed as the string is always nul terminated. Just print
the string and not add the length (which was hard coded to the max string
length anyway).

Cc: stable@vger.kernel.org
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Tom Zanussi <zanussi@kernel.org>
Cc: Douglas Raillard <douglas.raillard@arm.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/20250407154139.69955768@gandalf.local.home
Fixes: 4d38328eb442d ("tracing: Fix synth event printk format for str fields");
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
[ offset calculations instead of union-based data structures ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/trace/trace_events_synth.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
index ab54810bd8d99..62d146254f470 100644
--- a/kernel/trace/trace_events_synth.c
+++ b/kernel/trace/trace_events_synth.c
@@ -364,13 +364,11 @@ static enum print_line_t print_synth_event(struct trace_iterator *iter,
 				str_field = (char *)entry + data_offset;
 
 				trace_seq_printf(s, print_fmt, se->fields[i]->name,
-						 STR_VAR_LEN_MAX,
 						 str_field,
 						 i == se->n_fields - 1 ? "" : " ");
 				n_u64++;
 			} else {
 				trace_seq_printf(s, print_fmt, se->fields[i]->name,
-						 STR_VAR_LEN_MAX,
 						 (char *)&entry->fields[n_u64],
 						 i == se->n_fields - 1 ? "" : " ");
 				n_u64 += STR_VAR_LEN_MAX / sizeof(u64);
-- 
2.51.0