Re: [PATCH 19/19] perf: Make perf_pmu_unregister() useable

[Ravi Bangoria]

Hi Peter,

>> 4) A race with perf_event_set_bpf_prog(). perf_event_set_bpf_prog() might
>>    be in process of setting event->prog, where as perf_pmu_unregister(),
>>    which internally calls perf_event_free_bpf_prog(), will clear the
>>    event->prog pointer.
>>
>>    perf_pmu_unregister()                perf_event_set_bpf_prog()
>>      ...                                  perf_event_set_bpf_handler()
>>        perf_event_free_bpf_prog()           event->prog = prog;
>>          event->prog = NULL;
>>
>> I've yet to inspect other code paths, so there might be more races.
> 
> Weird, that should be serialized by perf_event_ctx_lock(), both
> __pmu_detach_event() and _perf_ioctl() are called under that.

There are multiple code paths leading to perf_event_set_bpf_prog(). The
one starting from _perf_ioctl() is serialized. However, this is not:

  __sys_bpf()
    link_create()
      bpf_perf_link_attach()
        perf_event_set_bpf_prog()
          perf_event_set_bpf_handler()
            event->prog = prog;

Thanks,
Ravi