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>

3 files changed, 20 insertions, 3 deletions

diff --git a/fs/notify/inode_mark.c b/fs/notify/inode_mark.c
index 21ed10660b80..4c29fcf557d1 100644
--- a/fs/notify/inode_mark.c
+++ b/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);
diff --git a/fs/notify/vfsmount_mark.c b/fs/notify/vfsmount_mark.c
index 56772b578fbd..85eebff6d0d7 100644
--- a/fs/notify/vfsmount_mark.c
+++ b/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);
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index e40190d16878..825329534162 100644
--- a/include/linux/fsnotify_backend.h
+++ b/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 */