fsnotify: Do not return merged event from fsnotify_add_notify_event()

The event returned from fsnotify_add_notify_event() cannot ever be used
safely as the event may be freed by the time the function returns (after
dropping notification_mutex). So change the prototype to just return
whether the event was added or merged into some existing event.

Reported-and-tested-by: Jiri Kosina <[e-mail]>
Reported-and-tested-by: Dave Jones <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/notify/fanotify/fanotify.c

···
 }
 
 /* and the list better be locked by something too! */
-static struct fsnotify_event *fanotify_merge(struct list_head *list,
-					     struct fsnotify_event *event)
+static int fanotify_merge(struct list_head *list, struct fsnotify_event *event)
 {
 	struct fsnotify_event *test_event;
 	bool do_merge = false;
···
 	 * one we should check for permission response.
 	 */
 	if (event->mask & FAN_ALL_PERM_EVENTS)
-		return NULL;
+		return 0;
 #endif
 
 	list_for_each_entry_reverse(test_event, list, list) {
···
 	}
 
 	if (!do_merge)
-		return NULL;
+		return 0;
 
 	test_event->mask |= event->mask;
-	return test_event;
+	return 1;
 }
 
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
···
 	int ret = 0;
 	struct fanotify_event_info *event;
 	struct fsnotify_event *fsn_event;
-	struct fsnotify_event *notify_fsn_event;
 
 	BUILD_BUG_ON(FAN_ACCESS != FS_ACCESS);
 	BUILD_BUG_ON(FAN_MODIFY != FS_MODIFY);
···
 	event->response = 0;
 #endif
 
-	notify_fsn_event = fsnotify_add_notify_event(group, fsn_event,
-						     fanotify_merge);
-	if (notify_fsn_event) {
+	ret = fsnotify_add_notify_event(group, fsn_event, fanotify_merge);
+	if (ret) {
 		/* Our event wasn't used in the end. Free it. */
 		fsnotify_destroy_event(group, fsn_event);
-		if (IS_ERR(notify_fsn_event))
-			return PTR_ERR(notify_fsn_event);
+		ret = 0;
 	}
 
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS

fs/notify/inotify/inotify_fsnotify.c

···
 	return false;
 }
 
-static struct fsnotify_event *inotify_merge(struct list_head *list,
-					    struct fsnotify_event *event)
+static int inotify_merge(struct list_head *list,
+			  struct fsnotify_event *event)
 {
 	struct fsnotify_event *last_event;
 
 	last_event = list_entry(list->prev, struct fsnotify_event, list);
-	if (!event_compare(last_event, event))
-		return NULL;
-	return last_event;
+	return event_compare(last_event, event);
 }
 
 int inotify_handle_event(struct fsnotify_group *group,
···
 {
 	struct inotify_inode_mark *i_mark;
 	struct inotify_event_info *event;
-	struct fsnotify_event *added_event;
 	struct fsnotify_event *fsn_event;
-	int ret = 0;
+	int ret;
 	int len = 0;
 	int alloc_len = sizeof(struct inotify_event_info);
 
···
 	if (len)
 		strcpy(event->name, file_name);
 
-	added_event = fsnotify_add_notify_event(group, fsn_event, inotify_merge);
-	if (added_event) {
+	ret = fsnotify_add_notify_event(group, fsn_event, inotify_merge);
+	if (ret) {
 		/* Our event wasn't used in the end. Free it. */
 		fsnotify_destroy_event(group, fsn_event);
-		if (IS_ERR(added_event))
-			ret = PTR_ERR(added_event);
 	}
 
 	if (inode_mark->mask & IN_ONESHOT)
 		fsnotify_destroy_mark(inode_mark, group);
 
-	return ret;
+	return 0;
 }
 
 static void inotify_freeing_mark(struct fsnotify_mark *fsn_mark, struct fsnotify_group *group)

fs/notify/notification.c

···
 
 /*
  * Add an event to the group notification queue.  The group can later pull this
- * event off the queue to deal with.  If the event is successfully added to the
- * group's notification queue, a reference is taken on event.
+ * event off the queue to deal with.  The function returns 0 if the event was
+ * added to the queue, 1 if the event was merged with some other queued event.
  */
-struct fsnotify_event *fsnotify_add_notify_event(struct fsnotify_group *group,
-						 struct fsnotify_event *event,
-						 struct fsnotify_event *(*merge)(struct list_head *,
-										 struct fsnotify_event *))
+int fsnotify_add_notify_event(struct fsnotify_group *group,
+			      struct fsnotify_event *event,
+			      int (*merge)(struct list_head *,
+					   struct fsnotify_event *))
 {
-	struct fsnotify_event *return_event = NULL;
+	int ret = 0;
 	struct list_head *list = &group->notification_list;
 
 	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
···
 		/* Queue overflow event only if it isn't already queued */
 		if (list_empty(&group->overflow_event.list))
 			event = &group->overflow_event;
-		return_event = event;
+		ret = 1;
 	}
 
 	if (!list_empty(list) && merge) {
-		return_event = merge(list, event);
-		if (return_event) {
+		ret = merge(list, event);
+		if (ret) {
 			mutex_unlock(&group->notification_mutex);
-			return return_event;
+			return ret;
 		}
 	}
 
···
 
 	wake_up(&group->notification_waitq);
 	kill_fasync(&group->fsn_fa, SIGIO, POLL_IN);
-	return return_event;
+	return ret;
 }
 
 /*

include/linux/fsnotify_backend.h

···
 extern void fsnotify_destroy_event(struct fsnotify_group *group,
 				   struct fsnotify_event *event);
 /* attach the event to the group notification queue */
-extern struct fsnotify_event *fsnotify_add_notify_event(struct fsnotify_group *group,
-							struct fsnotify_event *event,
-							struct fsnotify_event *(*merge)(struct list_head *,
-											struct fsnotify_event *));
+extern int fsnotify_add_notify_event(struct fsnotify_group *group,
+				     struct fsnotify_event *event,
+				     int (*merge)(struct list_head *,
+						  struct fsnotify_event *));
 /* true if the group notification queue is empty */
 extern bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group);
 /* return, but do not dequeue the first event on the notification queue */