fanotify: Track permission event state

Track whether permission event got already reported to userspace and
whether userspace already answered to the permission event. Protect
stores to this field together with updates to ->response field by
group->notification_lock. This will allow aborting wait for reply to
permission event from userspace.

Reviewed-by: Amir Goldstein <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/notify/fanotify/fanotify.c

···
 
 	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
 
-	wait_event(group->fanotify_data.access_waitq, event->response);
+	wait_event(group->fanotify_data.access_waitq,
+		   event->state == FAN_EVENT_ANSWERED);
 
 	/* userspace responded, convert to something usable */
 	switch (event->response & ~FAN_AUDIT) {
···
 	/* Check if the response should be audited */
 	if (event->response & FAN_AUDIT)
 		audit_fanotify(event->response & ~FAN_AUDIT);
-
-	event->response = 0;
 
 	pr_debug("%s: group=%p event=%p about to return ret=%d\n", __func__,
 		 group, event, ret);
···
 			goto out;
 		event = &pevent->fae;
 		pevent->response = 0;
+		pevent->state = FAN_EVENT_INIT;
 		goto init;
 	}
 	event = kmem_cache_alloc(fanotify_event_cachep, gfp);

fs/notify/fanotify/fanotify.h

···
 extern struct kmem_cache *fanotify_event_cachep;
 extern struct kmem_cache *fanotify_perm_event_cachep;
 
+/* Possible states of the permission event */
+enum {
+	FAN_EVENT_INIT,
+	FAN_EVENT_REPORTED,
+	FAN_EVENT_ANSWERED
+};
+
 /*
  * 3 dwords are sufficient for most local fs (64bit ino, 32bit generation).
  * For 32bit arch, fid increases the size of fanotify_event by 12 bytes and
···
  */
 struct fanotify_perm_event {
 	struct fanotify_event fae;
-	int response;	/* userspace answer to question */
+	unsigned short response;	/* userspace answer to the event */
+	unsigned short state;		/* state of the event */
 	int fd;		/* fd we passed to userspace for this event */
 };
 

fs/notify/fanotify/fanotify_user.c

···
 /*
  * Get an fsnotify notification event if one exists and is small
  * enough to fit in "count". Return an error pointer if the count
- * is not large enough.
+ * is not large enough. When permission event is dequeued, its state is
+ * updated accordingly.
  */
 static struct fsnotify_event *get_one_event(struct fsnotify_group *group,
 					    size_t count)
···
 		goto out;
 	}
 	fsn_event = fsnotify_remove_first_event(group);
+	if (fanotify_is_perm_event(FANOTIFY_E(fsn_event)->mask))
+		FANOTIFY_PE(fsn_event)->state = FAN_EVENT_REPORTED;
 out:
 	spin_unlock(&group->notification_lock);
 	return fsn_event;
···
 	return client_fd;
 }
 
+/*
+ * Finish processing of permission event by setting it to ANSWERED state and
+ * drop group->notification_lock.
+ */
+static void finish_permission_event(struct fsnotify_group *group,
+				    struct fanotify_perm_event *event,
+				    unsigned int response)
+				    __releases(&group->notification_lock)
+{
+	assert_spin_locked(&group->notification_lock);
+	event->response = response;
+	event->state = FAN_EVENT_ANSWERED;
+	spin_unlock(&group->notification_lock);
+}
+
 static int process_access_response(struct fsnotify_group *group,
 				   struct fanotify_response *response_struct)
 {
···
 			continue;
 
 		list_del_init(&event->fae.fse.list);
-		event->response = response;
-		spin_unlock(&group->notification_lock);
+		finish_permission_event(group, event, response);
 		wake_up(&group->fanotify_data.access_waitq);
 		return 0;
 	}
···
 			fsnotify_destroy_event(group, kevent);
 		} else {
 			if (ret <= 0) {
-				FANOTIFY_PE(kevent)->response = FAN_DENY;
+				spin_lock(&group->notification_lock);
+				finish_permission_event(group,
+					FANOTIFY_PE(kevent), FAN_DENY);
 				wake_up(&group->fanotify_data.access_waitq);
 			} else {
 				spin_lock(&group->notification_lock);
···
 		event = list_first_entry(&group->fanotify_data.access_list,
 				struct fanotify_perm_event, fae.fse.list);
 		list_del_init(&event->fae.fse.list);
-		event->response = FAN_ALLOW;
+		finish_permission_event(group, event, FAN_ALLOW);
+		spin_lock(&group->notification_lock);
 	}
 
 	/*
···
 		if (!(FANOTIFY_E(fsn_event)->mask & FANOTIFY_PERM_EVENTS)) {
 			spin_unlock(&group->notification_lock);
 			fsnotify_destroy_event(group, fsn_event);
-			spin_lock(&group->notification_lock);
 		} else {
-			FANOTIFY_PE(fsn_event)->response = FAN_ALLOW;
+			finish_permission_event(group, FANOTIFY_PE(fsn_event),
+						FAN_ALLOW);
 		}
+		spin_lock(&group->notification_lock);
 	}
 	spin_unlock(&group->notification_lock);