Re: [PATCH] perf/core: Fix perf_sample_data not properly initialized for different swevents in perf_tp_event()

[Peter Zijlstra]

On Wed, Apr 12, 2023 at 09:52:40AM +0000, Yang Jihong wrote:
> 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: Add perf_sample_data_flags_{save, restore} helpers to save and
> restore sample_flags when processing different swevents
> 
> 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

This seems a little bit short on analysis; what actual flags are the
problem? Much of the data will in fact be identical between these
invocations and endlessly re-computing that is wasteful.

I'm thinking perhaps those flags that update ->dyn_size are the problem?
At the same time, Should you not also then clear dyn_size?