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Old 02-06-2008, 08:37 AM
Nick Piggin
 
Default inotify: fix race Bug: #104837

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: Tim Gardner <tim.gardner@canonical.com>
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>
---
fs/inotify.c | 13 ++++++++++---
1 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/inotify.c b/fs/inotify.c
index 2c5b921..b2b109b 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);
--
1.5.4.3
 

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