tracing: Do not add length to print format in synthetic events

JIRA: https://issues.redhat.com/browse/RHEL-115358

commit e1a453a
Author: Steven Rostedt <rostedt@goodmis.org>
Date:   Mon Apr 7 15:41:39 2025 -0400

    tracing: Do not add length to print format in synthetic events

    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: 4d38328 ("tracing: Fix synth event printk format for str fields");
    Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>

Author: jeromemarchand

kernel/trace/trace_events_synth.c

@@ -377,7 +377,6 @@ 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++;