perf metricgroup: Fix find_evsel_group() event selector

perf metricgroup: Fix find_evsel_group() event selector

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 subtly broken for the "cycles" event. For the "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/

Fixes: 9c880c24cb0d ("perf metricgroup: Fix for metrics containing duration_time")
Signed-off-by: John Garry <john.garry@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com> # On a Thinkpad T450S
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/1623335580-187317-2-git-send-email-john.garry@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>