The following command segfaults on my x86 broadwell:

$ ./perf stat  -M frontend_bound,retiring,backend_bound,bad_speculation sleep 1
WARNING: grouped events cpus do not match, disabling group:
  anon group { raw 0x10e }
  anon group { raw 0x10e }
perf: util/evsel.c:1596: get_group_fd: Assertion `!(!leader->core.fd)' failed.
Aborted (core dumped)

The issue shows itself as a use-after-free in evlist__check_cpu_maps(),
whereby the leader of an event selector (evsel) has been deleted (yet we
still attempt to verify for an evsel).

Fundamentally the problem comes from metricgroup__setup_events() ->
find_evsel_group(), and has developed from the previous fix attempt in
commit 9c880c24cb0d ("perf metricgroup: Fix for metrics containing
duration_time").

The problem now is that the logic in checking if an evsel is in the same
group is subtely broken for "cycles" event. For "cycles" event, the
pmu_name is NULL; however the logic in find_evsel_group() may set an event
matched against "cycles" as used, when it should not be.

This leads to a condition where an evsel is set, yet its leader is not.

Fix the check for evsel pmu_name by not matching evsels when either has a
NULL pmu_name.

There is still a pre-existing metric issue whereby the ordering of the
metrics may break the 'stat' function, as discussed at:
https://lore.kernel.org/lkml/49c6fccb-b716-1bf0-18a6-cace1cdb66b9@huawei.com/ (local)

Fixes: 9c880c24cb0d ("perf metricgroup: Fix for metrics containing duration_time")
Signed-off-by: John Garry <redacted>
---
 tools/perf/util/metricgroup.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/metricgroup.c b/tools/perf/util/metricgroup.c
index 8336dd8e8098..c456fdeae06a 100644
--- a/tools/perf/util/metricgroup.c
+++ b/tools/perf/util/metricgroup.c

@@ -162,10 +162,10 @@ static bool contains_event(struct evsel **metric_events, int num_events,
 	return false;
 }
 
-static bool evsel_same_pmu(struct evsel *ev1, struct evsel *ev2)
+static bool evsel_same_pmu_or_none(struct evsel *ev1, struct evsel *ev2)
 {
 	if (!ev1->pmu_name || !ev2->pmu_name)
-		return false;
+		return true;
 
 	return !strcmp(ev1->pmu_name, ev2->pmu_name);
 }

@@ -288,7 +288,7 @@ static struct evsel *find_evsel_group(struct evlist *perf_evlist,
 			 */
 			if (!has_constraint &&
 			    ev->leader != metric_events[i]->leader &&
-			    evsel_same_pmu(ev->leader, metric_events[i]->leader))
+			    evsel_same_pmu_or_none(ev->leader, metric_events[i]->leader))
 				break;
 			if (!strcmp(metric_events[i]->name, ev->name)) {
 				set_bit(ev->idx, evlist_used);

-- 
2.26.2