fanotify: use merge argument to determine actual event added to queue

fanotify needs to know the actual event added to queues so it can be
correctly checked for return values from userspace.  To do this we need to
pass that information from the merger code back to the main even handling
routine.  Currently that information is unused, but it will be.
Signed-off-by: Eric Paris <eparis@redhat.com>

fs/notify/fanotify/fanotify.c

@@ -27,6 +27,7 @@ static bool should_merge(struct fsnotify_event *old, struct fsnotify_event *new)
 	return false;
 }
 
+/* Note, if we return an event in *arg that a reference is being held... */
 static int fanotify_merge(struct list_head *list,
 			  struct fsnotify_event *event,
 			  void **arg)
@@ -34,17 +35,22 @@ static int fanotify_merge(struct list_head *list,
 	struct fsnotify_event_holder *test_holder;
 	struct fsnotify_event *test_event;
 	struct fsnotify_event *new_event;
+	struct fsnotify_event **return_event = (struct fsnotify_event **)arg;
 	int ret = 0;
 
 	pr_debug("%s: list=%p event=%p\n", __func__, list, event);
 
+	*return_event = NULL;
+
 	/* and the list better be locked by something too! */
 
 	list_for_each_entry_reverse(test_holder, list, event_list) {
 		test_event = test_holder->event;
 		if (should_merge(test_event, event)) {
-			ret = -EEXIST;
+			fsnotify_get_event(test_event);
+			*return_event = test_event;
 
+			ret = -EEXIST;
 			/* if they are exactly the same we are done */
 			if (test_event->mask == event->mask)
 				goto out;
@@ -66,11 +72,14 @@ static int fanotify_merge(struct list_head *list,
 				goto out;
 			}
 
+			/* we didn't return the test_event, so drop that ref */
+			fsnotify_put_event(test_event);
+			/* the reference we return on new_event is from clone */
+			*return_event = new_event;
+
 			/* build new event and replace it on the list */
 			new_event->mask = (test_event->mask | event->mask);
 			fsnotify_replace_event(test_holder, new_event);
-			/* match ref from fsnotify_clone_event() */
-			fsnotify_put_event(new_event);
 
 			break;
 		}
@@ -82,7 +91,7 @@ static int fanotify_merge(struct list_head *list,
 static int fanotify_handle_event(struct fsnotify_group *group, struct fsnotify_event *event)
 {
 	int ret;
-
+	struct fsnotify_event *used_event;
 
 	BUILD_BUG_ON(FAN_ACCESS != FS_ACCESS);
 	BUILD_BUG_ON(FAN_MODIFY != FS_MODIFY);
@@ -94,10 +103,12 @@ static int fanotify_handle_event(struct fsnotify_group *group, struct fsnotify_e
 
 	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
 
-	ret = fsnotify_add_notify_event(group, event, NULL, fanotify_merge, NULL);
+	ret = fsnotify_add_notify_event(group, event, NULL, fanotify_merge, (void **)&used_event);
 	/* -EEXIST means this event was merged with another, not that it was an error */
 	if (ret == -EEXIST)
 		ret = 0;
+	if (used_event)
+		fsnotify_put_event(used_event);
 	return ret;
 }