Re: pmu::read() called erroneously in v4.13-rc{3,4}

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Aug 10, 2017 at 06:35:51PM +0100, Mark Rutland wrote:
> Hi,
> 
> While running Vince's perf fuzzer on arm64 v4.13-rc3, I found we call
> pmu::read() for an event whose event::cpu != smp_processor_id(), and
> event::oncpu == -1, violating the usual pmu::read() requirements.

It looks like I have an event that wasn't entirely detached from its
group_leader in perf_group_detach().

The below diff seems to get rid of the problem, though I think this is
masking some futher issues, noted below.

----
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 407dad6..cac84b6 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1724,6 +1724,7 @@ static void perf_group_detach(struct perf_event *event)
        if (event->group_leader != event) {
                list_del_init(&event->group_entry);
                event->group_leader->nr_siblings--;
+               event->group_leader = event;
                goto out;
        }
 
----

If perf_group_detach() iterates over siblings, it re-inits each of their
group_leader entries, but doesn't do this if provided a sibling directly.
Is that deliberate?

It looks like without the above, we could get into
perf_output_read_group(), and follow a stale event->group_leader,
read()ing that without checking its state. We check the state of
siblings, so shouldn't we check the leader, too?

I'm also confused by perf_output_read_group() when event == leader.
AFAICT, in that case we won't read() the event at all, and we'll only
read() the siblings. Is that right?

Thanks,
Mark.