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>

1 files changed, 10 insertions, 3 deletions

diff --git a/fs/inotify.c b/fs/inotify.c
index 2c5b92152876..b2b109bf29d6 100644
--- 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);