linux-kernel - Re: [PATCH 19/19] perf: Make perf_pmu

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Message-ID: <795d9f60-73c8-48da-87a4-6336c170b388@amd.com>
Date: Fri, 17 Jan 2025 10:50:26 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "mingo@...nel.org" <mingo@...nel.org>,
 "lucas.demarchi@...el.com" <lucas.demarchi@...el.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "willy@...radead.org" <willy@...radead.org>,
 "acme@...nel.org" <acme@...nel.org>,
 "namhyung@...nel.org" <namhyung@...nel.org>,
 "mark.rutland@....com" <mark.rutland@....com>,
 "alexander.shishkin@...ux.intel.com" <alexander.shishkin@...ux.intel.com>,
 "jolsa@...nel.org" <jolsa@...nel.org>,
 "irogers@...gle.com" <irogers@...gle.com>,
 "adrian.hunter@...el.com" <adrian.hunter@...el.com>,
 "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>,
 Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH 19/19] perf: Make perf_pmu_unregister() useable

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