[PATCH] perf top & record: Fix segfault when default cycles event is not supported

[Yang Jihong]

The perf-record and perf-top call parse_event() to add a cycles event to
an empty evlist. For the system that does not support hardware cycles
event, such as QEMU, the evlist is empty due to the following code process:

  parse_event(evlist, "cycles:P" or ""cycles:Pu")
    parse_events(evlist, "cycles:P")
      __parse_events
        ...
        ret = parse_events__scanner(str, &parse_state);
        // ret = 0
        ...
        ret2 = parse_events__sort_events_and_fix_groups()
        if (ret2 < 0)
          return ret;
        // The cycles event is not supported, here ret2 = -EINVAL,
        // Here return 0.
        ...
        evlist__splice_list_tail(evlist)
        // The code here does not execute to, so the evlist is still empty.

A null pointer occurs when the content in the evlist is accessed later.

Before:

  # perf list hw

  List of pre-defined events (to be used in -e or -M):

  # perf record true
  libperf: Miscounted nr_mmaps 0 vs 1
  WARNING: No sample_id_all support, falling back to unordered processing
  perf: Segmentation fault
  Obtained 1 stack frames.
  [0xc5beff]
  Segmentation fault

Solution:
  If cycles event is not supported, try to fall back to cpu-clock event.

After:
  # perf record true
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.006 MB perf.data ]
  #

Fixes: 7b100989b4f6 ("perf evlist: Remove __evlist__add_default")
Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
---
 tools/perf/builtin-record.c |  4 +---
 tools/perf/builtin-top.c    |  3 +--
 tools/perf/util/evlist.c    | 18 ++++++++++++++++++
 tools/perf/util/evlist.h    |  1 +
 4 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index aec18db7ff23..29ae2b84a63a 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -4161,9 +4161,7 @@ int cmd_record(int argc, const char **argv)
 		record.opts.tail_synthesize = true;
 
 	if (rec->evlist->core.nr_entries == 0) {
-		bool can_profile_kernel = perf_event_paranoid_check(1);
-
-		err = parse_event(rec->evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
+		err = evlist__add_default(rec->evlist);
 		if (err)
 			goto out;
 	}
diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
index c363c04e16df..798cb9252a5f 100644
--- a/tools/perf/builtin-top.c
+++ b/tools/perf/builtin-top.c
@@ -1665,8 +1665,7 @@ int cmd_top(int argc, const char **argv)
 		goto out_delete_evlist;
 
 	if (!top.evlist->core.nr_entries) {
-		bool can_profile_kernel = perf_event_paranoid_check(1);
-		int err = parse_event(top.evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
+		int err = evlist__add_default(top.evlist);
 
 		if (err)
 			goto out_delete_evlist;
diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 7ef43f72098e..60efa762405e 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -287,6 +287,24 @@ struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide)
 	return evsel;
 }
 
+int evlist__add_default(struct evlist *evlist)
+{
+	bool can_profile_kernel;
+	int err;
+
+	can_profile_kernel = perf_event_paranoid_check(1);
+	err = parse_event(evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
+	if (err)
+		return err;
+
+	if (!evlist->core.nr_entries) {
+		pr_debug("The cycles event is not supported, trying to fall back to cpu-clock event\n");
+		return parse_event(evlist, "cpu-clock");
+	}
+
+	return 0;
+}
+
 #ifdef HAVE_LIBTRACEEVENT
 struct evsel *evlist__add_sched_switch(struct evlist *evlist, bool system_wide)
 {
diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
index 664c6bf7b3e0..47eea809ee91 100644
--- a/tools/perf/util/evlist.h
+++ b/tools/perf/util/evlist.h
@@ -116,6 +116,7 @@ int arch_evlist__cmp(const struct evsel *lhs, const struct evsel *rhs);
 
 int evlist__add_dummy(struct evlist *evlist);
 struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide);
+int evlist__add_default(struct evlist *evlist);
 static inline struct evsel *evlist__add_dummy_on_all_cpus(struct evlist *evlist)
 {
 	return evlist__add_aux_dummy(evlist, true);
-- 
2.30.GIT