perf/core: Fix crash due to account/unaccount_sb_event() inconsistency

unaccount_pmu_sb_event() did not check for attributes in event->attr
before calling detach_sb_event(), while account_pmu_event() did.

This caused NULL pointer reference in cgroup events that did not
have any of the attributes checked by account_pmu_event().

To trigger the bug just wait for a cgroup event to terminate, e.g.:

  $ mkdir /dev/cgroup/devices/test
  $ perf stat -e cycles -a -G test sleep 0

... see crash ...

Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
Reviewed-by: Stephane Eranian <eranian@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Zheng <zheng.z.yan@intel.com>
Link: http://lkml.kernel.org/r/1464809585-66072-1-git-send-email-davidcc@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -3682,15 +3682,28 @@ static void detach_sb_event(struct perf_event *event)
 	raw_spin_unlock(&pel->lock);
 }
 
-static void unaccount_pmu_sb_event(struct perf_event *event)
+static bool is_sb_event(struct perf_event *event)
 {
+	struct perf_event_attr *attr = &event->attr;
+
 	if (event->parent)
-		return;
+		return false;
 
 	if (event->attach_state & PERF_ATTACH_TASK)
-		return;
+		return false;
 
-	detach_sb_event(event);
+	if (attr->mmap || attr->mmap_data || attr->mmap2 ||
+	    attr->comm || attr->comm_exec ||
+	    attr->task ||
+	    attr->context_switch)
+		return true;
+	return false;
+}
+
+static void unaccount_pmu_sb_event(struct perf_event *event)
+{
+	if (is_sb_event(event))
+		detach_sb_event(event);
 }
 
 static void unaccount_event_cpu(struct perf_event *event, int cpu)
@@ -8666,18 +8679,7 @@ static void attach_sb_event(struct perf_event *event)
  */
 static void account_pmu_sb_event(struct perf_event *event)
 {
-	struct perf_event_attr *attr = &event->attr;
-
-	if (event->parent)
-		return;
-
-	if (event->attach_state & PERF_ATTACH_TASK)
-		return;
-
-	if (attr->mmap || attr->mmap_data || attr->mmap2 ||
-	    attr->comm || attr->comm_exec ||
-	    attr->task ||
-	    attr->context_switch)
+	if (is_sb_event(event))
 		attach_sb_event(event);
 }