arm_pmu: Validate single/group leader events

In the case where there is only a cycle counter available (i.e.
PMCR_EL0.N is 0) and an event other than CPU cycles is opened, the open
should fail as the event can never possibly be scheduled. However, the
event validation when an event is opened is skipped when the group
leader is opened. Fix this by always validating the group leader events.

Reported-by: Al Grant <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Acked-by: Mark Rutland <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Cc: <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

drivers/perf/arm_pmu.c

···
 	if (!validate_event(event->pmu, &fake_pmu, leader))
 		return -EINVAL;
 
+	if (event == leader)
+		return 0;
+
 	for_each_sibling_event(sibling, leader) {
 		if (!validate_event(event->pmu, &fake_pmu, sibling))
 			return -EINVAL;
···
 		local64_set(&hwc->period_left, hwc->sample_period);
 	}
 
-	if (event->group_leader != event) {
-		if (validate_group(event) != 0)
-			return -EINVAL;
-	}
-
-	return 0;
+	return validate_group(event);
 }
 
 static int armpmu_event_init(struct perf_event *event)