fanotify: dont merge permission events

Boyd Yang reported a problem for the case that multiple threads of the same thread group are waiting for a reponse for a permission event.
In this case it is possible that some of the threads are never woken up, even if the response for the event has been received
(see http://marc.info/?l=linux-kernel&m=131822913806350&w=2).

The reason is that we are currently merging permission events if they belong to the same thread group. But we are not prepared to wake up more than one waiter for each event. We do

wait_event(group->fanotify_data.access_waitq, event->response ||

			atomic_read(&group->fanotify_data.bypass_perm));

and after that
event->response = 0;

which is the reason that even if we woke up all waiters for the same event some of them may see event->response being already set 0 again, then go back to sleep and block forever.

With this patch we avoid that more than one thread is waiting for a response by not merging permission events for the same thread group any more.

---
 fs/notify/fanotify/fanotify.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

index f35794b..aeb5b5a 100644
@@ -18,6 +18,12 @@ static bool should_merge(struct fsnotify_event *old, struct fsnotify_event *new)
 	    old->tgid == new->tgid) {
 		switch (old->data_type) {
 		case (FSNOTIFY_EVENT_PATH):
+#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
+			/* dont merge two permission events */
+			if ((old->mask & FAN_ALL_PERM_EVENTS) &&
+			    (new->mask & FAN_ALL_PERM_EVENTS))
+				return false;
+#endif
 			if ((old->path.mnt == new->path.mnt) &&
 			    (old->path.dentry == new->path.dentry))
 				return true;

1.5.6.5