fsnotify: implement ordering between notifiers

fanotify needs to be able to specify that some groups get events before
others.  They use this idea to make sure that a hierarchical storage
manager gets access to files before programs which actually use them.  This
is purely infrastructure.  Everything will have a priority of 0, but the
infrastructure will exist for it to be non-zero.

Signed-off-by: Eric Paris <eparis@redhat.com>

fs/notify/inode_mark.c

@@ -177,7 +177,8 @@ void fsnotify_set_inode_mark_mask_locked(struct fsnotify_mark *mark,
  * Attach an initialized mark to a given inode.
  * These marks may be used for the fsnotify backend to determine which
  * event types should be delivered to which group and for which inodes.  These
- * marks are ordered according to the group's location in memory.
+ * marks are ordered according to priority, highest number first, and then by
+ * the group's location in memory.
  */
 int fsnotify_add_inode_mark(struct fsnotify_mark *mark,
 			    struct fsnotify_group *group, struct inode *inode,
@@ -211,7 +212,11 @@ int fsnotify_add_inode_mark(struct fsnotify_mark *mark,
 			goto out;
 		}
 
-		if (mark->group < lmark->group)
+		if (mark->group->priority < lmark->group->priority)
+			continue;
+
+		if ((mark->group->priority == lmark->group->priority) &&
+		    (mark->group < lmark->group))
 			continue;
 
 		hlist_add_before_rcu(&mark->i.i_list, &lmark->i.i_list);

fs/notify/vfsmount_mark.c

@@ -169,7 +169,11 @@ int fsnotify_add_vfsmount_mark(struct fsnotify_mark *mark,
 			goto out;
 		}
 
-		if (mark->group < lmark->group)
+		if (mark->group->priority < lmark->group->priority)
+			continue;
+
+		if ((mark->group->priority == lmark->group->priority) &&
+		    (mark->group < lmark->group))
 			continue;
 
 		hlist_add_before_rcu(&mark->m.m_list, &lmark->m.m_list);

include/linux/fsnotify_backend.h

@@ -129,6 +129,14 @@ struct fsnotify_group {
 	wait_queue_head_t notification_waitq;	/* read() on the notification file blocks on this waitq */
 	unsigned int q_len;			/* events on the queue */
 	unsigned int max_events;		/* maximum events allowed on the list */
+	/*
+	 * Valid fsnotify group priorities.  Events are send in order from highest
+	 * priority to lowest priority.  We default to the lowest priority.
+	 */
+	#define FS_PRIO_0	0 /* normal notifiers, no permissions */
+	#define FS_PRIO_1	1 /* fanotify content based access control */
+	#define FS_PRIO_2	2 /* fanotify pre-content access */
+	unsigned int priority;
 
 	/* stores all fastpath marks assoc with this group so they can be cleaned on unregister */
 	spinlock_t mark_lock;		/* protect marks_list */