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

[German Gomez]

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.

Co-authored-by: James Clark <james.clark@xxxxxxx>
Signed-off-by: German Gomez <german.gomez@xxxxxxx>
---
 drivers/perf/arm_spe_pmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
index 8515bf85c..7d9a7fa4f 100644
--- a/drivers/perf/arm_spe_pmu.c
+++ b/drivers/perf/arm_spe_pmu.c
@@ -711,7 +711,7 @@ static int arm_spe_pmu_event_init(struct perf_event *event)
 	    !(spe_pmu->features & SPE_PMU_FEAT_FILT_LAT))
 		return -EOPNOTSUPP;
 
-	spe_pmu->pmscr_cx = perfmon_capable();
+	spe_pmu->pmscr_cx = perfmon_capable() || (event->cpu != -1);
 	reg = arm_spe_event_to_pmscr(event);
 	if (!perfmon_capable() &&
 	    (reg & (BIT(SYS_PMSCR_EL1_PA_SHIFT) |
-- 
2.25.1