linux-kernel - [PATCH] tracing: Do not add length to print format in synthetic events

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Message-ID: <20250407154139.69955768@gandalf.local.home>
Date: Mon, 7 Apr 2025 15:41:39 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Tom Zanussi <zanussi@...nel.org>, Douglas
 Raillard <douglas.raillard@....com>
Subject: [PATCH] tracing: Do not add length to print format in synthetic
 events

From: Steven Rostedt <rostedt@...dmis.org>

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@...r.kernel.org
Fixes: 4d38328eb442d ("tracing: Fix synth event printk format for str fields");
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/trace_events_synth.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
index 969f48742d72..33cfbd4ed76d 100644
--- a/kernel/trace/trace_events_synth.c
+++ b/kernel/trace/trace_events_synth.c
@@ -370,7 +370,6 @@ static enum print_line_t print_synth_event(struct trace_iterator *iter,
 				union trace_synth_field *data = &entry->fields[n_u64];
 
 				trace_seq_printf(s, print_fmt, se->fields[i]->name,
-						 STR_VAR_LEN_MAX,
 						 (char *)entry + data->as_dynamic.offset,
 						 i == se->n_fields - 1 ? "" : " ");
 				n_u64++;
-- 
2.47.2