tracing: Fix NULL dereference when copying keys for a field variable

In event_hist_trigger_parse() where it needs to create actions like
"onmatch", it calls:

  event_hist_trigger_parse() {
    create_actions() {
      action_create() {
        trace_action_create() {
          trace_action_create_field_var() {
            create_field_var_hist()

Where create_field_var_hist() does a loop on the hist_data
representing the keys. The issue is, if the keys uses one of the
pseudo field types (like common_cpu), the hist_data field element
will have NULL for its field member causing a NULL pointer
dereference when accessing the key_field->field->name.

  # echo 'hist:keys=common_cpu:ts0=common_timestamp.usecs' > \
      events/sched/sched_waking/trigger
  # echo 'my_synth u64 lat; int prio' > synthetic_events
  # echo 'hist:keys=common_cpu:wakeup_lat=common_timestamp.usecs-$ts0:\
      onmatch(sched.sched_waking).my_synth($wakeup_lat,prio)' > \
      events/sched/sched_switch/trigger

  KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
  RIP: 0010:create_field_var_hist+0x771/0x1380
  Call Trace:
   trace_action_create_field_var+0x360/0x600
   trace_action_create+0x393/0xe80
   event_hist_trigger_parse+0x3e47/0x69e0
   trigger_process_regex+0x1a6/0x250
   event_trigger_write+0xce/0x160

Instead of accessing it directly, use the proper handler
expr_field_str() to get the name.

The three previous patches are prerequisites: without them a .buckets
key and a common_stacktrace key each render into a command that cannot
be parsed back.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260914053443.981201-5-donggeunyoo.kernel@gmail.com
Fixes: 02205a6752f2 ("tracing: Add support for 'field variables'")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Assisted-by: Claude:claude-fable-5
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

0 files changed, 0 insertions, 0 deletions