[PATCH 2/2] fsnotify: Let userspace know about lost events due to ENOMEM

[Jan Kara <jack@suse.cz>]

Currently if notification event is lost due to event allocation failing
we ENOMEM, we just silently continue (except for fanotify permission
events where we deny the access). This is undesirable as userspace has
no way of knowing whether the notifications it got are complete or not.
Treat lost events due to ENOMEM the same way as lost events due to queue
overflow so that userspace knows something bad happened and it likely
needs to rescan the filesystem.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/notify/fanotify/fanotify.c        | 9 ++++++++-
 fs/notify/inotify/inotify_fsnotify.c | 8 +++++++-
 fs/notify/notification.c             | 3 ++-
 include/linux/fsnotify_backend.h     | 6 ++++++
 4 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 928f2a5eedb7..d51e1bb781cf 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -221,8 +221,15 @@ static int fanotify_handle_event(struct fsnotify_group *group,
 
 	event = fanotify_alloc_event(group, inode, mask, data);
 	ret = -ENOMEM;
-	if (unlikely(!event))
+	if (unlikely(!event)) {
+		/*
+		 * We don't queue overflow events for permission events as
+		 * there the access is denied and so no event is in fact lost.
+		 */
+		if (!fanotify_is_perm_event(mask))
+			fsnotify_queue_overflow(group);
 		goto finish;
+	}
 
 	fsn_event = &event->fse;
 	ret = fsnotify_add_event(group, fsn_event, fanotify_merge);
diff --git a/fs/notify/inotify/inotify_fsnotify.c b/fs/notify/inotify/inotify_fsnotify.c
index 8b73332735ba..6440df8fb29a 100644
--- a/fs/notify/inotify/inotify_fsnotify.c
+++ b/fs/notify/inotify/inotify_fsnotify.c
@@ -99,8 +99,14 @@ int inotify_handle_event(struct fsnotify_group *group,
 			      fsn_mark);
 
 	event = kmalloc(alloc_len, GFP_KERNEL);
-	if (unlikely(!event))
+	if (unlikely(!event)) {
+		/*
+		 * Treat lost event due to ENOMEM the same way as queue
+ 		 * overflow to let userspace know event was lost.
+		 */
+		fsnotify_queue_overflow(group);
 		return -ENOMEM;
+	}
 
 	fsn_event = &event->fse;
 	fsnotify_init_event(fsn_event, inode, mask);
diff --git a/fs/notify/notification.c b/fs/notify/notification.c
index 66f85c651c52..3c3e36745f59 100644
--- a/fs/notify/notification.c
+++ b/fs/notify/notification.c
@@ -111,7 +111,8 @@ int fsnotify_add_event(struct fsnotify_group *group,
 		return 2;
 	}
 
-	if (group->q_len >= group->max_events) {
+	if (event == group->overflow_event ||
+	    group->q_len >= group->max_events) {
 		ret = 2;
 		/* Queue overflow event only if it isn't already queued */
 		if (!list_empty(&group->overflow_event->list)) {
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 067d52e95f02..9f1edb92c97e 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -331,6 +331,12 @@ extern int fsnotify_add_event(struct fsnotify_group *group,
 			      struct fsnotify_event *event,
 			      int (*merge)(struct list_head *,
 					   struct fsnotify_event *));
+/* Queue overflow event to a notification group */
+static inline void fsnotify_queue_overflow(struct fsnotify_group *group)
+{
+	fsnotify_add_event(group, group->overflow_event, NULL);
+}
+
 /* 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 */
-- 
2.13.6