linux-kernel - Regarding perfmon

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Date:   Fri, 1 Jul 2022 10:37:37 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Regarding perfmon_capable()

Hello,

In perf event subsystem and related platform drivers registering a PMU,
should perfmon_capable() be used directly ? OR just wondering if instead
perf_allow_[cpu|kernel|tracepoint]() helpers should be used which also
checks for 'sysctl_perf_event_paranoid' ? Should not both capabilities
and 'sysctl_perf_event_paranoid' decide whether kernel/cpu/tracepoint
events will be captured for unprivileged users.

arch/parisc/kernel/perf.c:      if (!perfmon_capable())
arch/powerpc/perf/imc-pmu.c:    if (!perfmon_capable())
arch/powerpc/perf/imc-pmu.c:    if (!perfmon_capable())
drivers/gpu/drm/i915/i915_perf.c:           i915_perf_stream_paranoid && !perfmon_capable()) {
drivers/gpu/drm/i915/i915_perf.c:                       if (oa_freq_hz > i915_oa_max_sample_rate && !perfmon_capable()) {
drivers/gpu/drm/i915/i915_perf.c:       if (i915_perf_stream_paranoid && !perfmon_capable()) {
drivers/gpu/drm/i915/i915_perf.c:       if (i915_perf_stream_paranoid && !perfmon_capable()) {
drivers/media/rc/bpf-lirc.c:            if (perfmon_capable())
drivers/perf/arm_spe_pmu.c:     if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR) && perfmon_capable())
drivers/perf/arm_spe_pmu.c:     if (!perfmon_capable() &&

Although BPF might use perfmon_capabale() alone, because it was never
dependent on 'sysctl_perf_event_paranoid' ?

- Anshuman