data->sample_flags may be modified in perf_prepare_sample(),
in perf_tp_event(), different swevents use the same on-stack
perf_sample_data, the previous swevent may change sample_flags in
perf_prepare_sample(), as a result, some members of perf_sample_data are
not correctly initialized when next swevent_event preparing sample
(for example data->id, the value varies according to swevent).

A simple scenario triggers this problem is as follows:

  # perf record -e sched:sched_switch --switch-output-event sched:sched_switch -a sleep 1
  [ perf record: dump data: Woken up 0 times ]
  [ perf record: Dump perf.data.2023041209014396 ]
  [ perf record: dump data: Woken up 0 times ]
  [ perf record: Dump perf.data.2023041209014662 ]
  [ perf record: dump data: Woken up 0 times ]
  [ perf record: Dump perf.data.2023041209014910 ]
  [ perf record: Woken up 0 times to write data ]
  [ perf record: Dump perf.data.2023041209015164 ]
  [ perf record: Captured and wrote 0.069 MB perf.data.<timestamp> ]
  # ls -l
  total 860
  -rw------- 1 root root  95694 Apr 12 09:01 perf.data.2023041209014396
  -rw------- 1 root root 606430 Apr 12 09:01 perf.data.2023041209014662
  -rw------- 1 root root  82246 Apr 12 09:01 perf.data.2023041209014910
  -rw------- 1 root root  82342 Apr 12 09:01 perf.data.2023041209015164
  # perf script -i perf.data.2023041209014396
  0x11d58 [0x80]: failed to process type: 9 [Bad address]

Solution: Re-initialize perf_sample_data after each event is processed.
Note that data->raw->frag.data may be accessed in perf_tp_event_match().
Therefore, need to init sample_data and then go through swevent hlist to prevent
reference of NULL pointer, reported by [1].

After fix:

  # perf record -e sched:sched_switch --switch-output-event sched:sched_switch -a sleep 1
  [ perf record: dump data: Woken up 0 times ]
  [ perf record: Dump perf.data.2023041209442259 ]
  [ perf record: dump data: Woken up 0 times ]
  [ perf record: Dump perf.data.2023041209442514 ]
  [ perf record: dump data: Woken up 0 times ]
  [ perf record: Dump perf.data.2023041209442760 ]
  [ perf record: Woken up 0 times to write data ]
  [ perf record: Dump perf.data.2023041209443003 ]
  [ perf record: Captured and wrote 0.069 MB perf.data.<timestamp> ]
  # ls -l
  total 864
  -rw------- 1 root root 100166 Apr 12 09:44 perf.data.2023041209442259
  -rw------- 1 root root 606438 Apr 12 09:44 perf.data.2023041209442514
  -rw------- 1 root root  82246 Apr 12 09:44 perf.data.2023041209442760
  -rw------- 1 root root  82342 Apr 12 09:44 perf.data.2023041209443003
  # perf script -i perf.data.2023041209442259 | head -n 5
              perf   232 [000]    66.846217: sched:sched_switch: prev_comm=perf prev_pid=232 prev_prio=120 prev_state=D ==> next_comm=perf next_pid=234 next_prio=120
              perf   234 [000]    66.846449: sched:sched_switch: prev_comm=perf prev_pid=234 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=232 next_prio=120
              perf   232 [000]    66.846546: sched:sched_switch: prev_comm=perf prev_pid=232 prev_prio=120 prev_state=R ==> next_comm=perf next_pid=234 next_prio=120
              perf   234 [000]    66.846606: sched:sched_switch: prev_comm=perf prev_pid=234 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=232 next_prio=120
              perf   232 [000]    66.846646: sched:sched_switch: prev_comm=perf prev_pid=232 prev_prio=120 prev_state=R ==> next_comm=perf next_pid=234 next_prio=120

[1] Link: https://lore.kernel.org/oe-lkp/202304250929.efef2caa-yujie.liu@intel.com

Fixes: bb447c27 ("perf/core: Set data->sample_flags in perf_prepare_sample()")
Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230425103217.130600-1-yangjihong1@huawei.com