inotify: fix race

There is a race between setting an inode's children's "parent watched" flag
when placing the first watch on a parent, and instantiating new children of
that parent: a child could miss having its flags set by
set_dentry_child_flags, but then inotify_d_instantiate might still see
!inotify_inode_watched.

The solution is to set_dentry_child_flags after adding the watch.  Locking is
taken care of, because both set_dentry_child_flags and inotify_d_instantiate
hold dcache_lock and child->d_locks.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Robert Love <rlove@google.com>
Cc: John McCutchan <ttb@tentacle.dhs.org>
Cc: Jan Kara <jack@ucw.cz>
Cc: Yan Zheng <yanzheng@21cn.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/inotify.c b/fs/inotify.c
index 2c5b921528760b9da6d8abcc3f90668a8e9381d7..b2b109bf29d6ab9d39575a3e4f76ea4277f8202a 100644 (file)
--- a/fs/inotify.c
+++ b/fs/inotify.c
@@ -627,6 +627,7 @@ s32 inotify_add_watch(struct inotify_handle *ih, struct inotify_watch *watch,
                      struct inode *inode, u32 mask)
 {
        int ret = 0;
+       int newly_watched;
 
        /* don't allow invalid bits: we don't want flags set */
        mask &= IN_ALL_EVENTS | IN_ONESHOT;
@@ -653,12 +654,18 @@ s32 inotify_add_watch(struct inotify_handle *ih, struct inotify_watch *watch,
         */
        watch->inode = igrab(inode);
 
-       if (!inotify_inode_watched(inode))
-               set_dentry_child_flags(inode, 1);
-
        /* Add the watch to the handle's and the inode's list */
+       newly_watched = !inotify_inode_watched(inode);
        list_add(&watch->h_list, &ih->watches);
        list_add(&watch->i_list, &inode->inotify_watches);
+       /*
+        * Set child flags _after_ adding the watch, so there is no race
+        * windows where newly instantiated children could miss their parent's
+        * watched flag.
+        */
+       if (newly_watched)
+               set_dentry_child_flags(inode, 1);
+
 out:
        mutex_unlock(&ih->mutex);
        mutex_unlock(&inode->inotify_mutex);