linux-kernel - Re: [RFC PATCH 2/2] perf: arm_spe: Enable CONTEXT packets in SPE traces if the profiler runs in CPU mode.

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Date:   Tue, 18 Jan 2022 09:52:58 +0000
From:   Will Deacon <will@...nel.org>
To:     German Gomez <german.gomez@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, james.clark@....com, leo.yan@...aro.org
Subject: Re: [RFC PATCH 2/2] perf: arm_spe: Enable CONTEXT packets in SPE
 traces if the profiler runs in CPU mode.

On Mon, Jan 17, 2022 at 12:44:32PM +0000, German Gomez wrote:
> Enable CONTEXT packets in SPE traces if the profiler runs in CPU mode.
> This is no less permissive than the existing behavior for the following
> reason:
> 
> If perf_event_paranoid <= 0, then non perfmon_capable() users can open
> a per-CPU event. With a per-CPU event, unpriviledged users are allowed
> to profile _all_ processes, even ones owned by root.
> 
> Without this change, users could see kernel addresses, root processes,
> etc, but not gather the PIDs of those processes. The PID is probably the
> least sensitive of all the information.
> 
> It would be more idiomatic to check the perf_event_paranoid level with
> perf_allow_cpu(), but this function is not exported so cannot be used
> from a module. Looking for cpu != -1 is the indirect way of checking
> the same thing as it could never get to arm_spe_pmu_event_init() without
> perf_event_paranoid <= 0.

perf_allow_cpu() is a static inline so there's no need to export it. What's
missing?

Will