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fsnotify: do not set group for a mark before it is on the i_list 

fsnotify_add_mark is supposed to add a mark to the g_list and i_list and to
set the group and inode for the mark.  fsnotify_destroy_mark_by_entry uses
the fact that ->group != NULL to know if this group should be destroyed or
if it's already been done.

But fsnotify_add_mark sets the group and inode before it actually adds the
mark to the i_list and g_list.  This can result in a race in inotify, it
requires 3 threads.

sys_inotify_add_watch("file")	sys_inotify_add_watch("file")	sys_inotify_rm_watch([a])
inotify_update_watch()
inotify_new_watch()
inotify_add_to_idr()
   ^--- returns wd = [a]
				inotfiy_update_watch()
				inotify_new_watch()
				inotify_add_to_idr()
				fsnotify_add_mark()
				   ^--- returns wd = [b]
				returns to userspace;
								inotify_idr_find([a])
								   ^--- gives us the pointer from task 1
fsnotify_add_mark()
   ^--- this is going to set the mark->group and mark->inode fields, but will
return -EEXIST because of the race with [b].
								fsnotify_destroy_mark()
								   ^--- since ->group != NULL we call back
									into inotify_freeing_mark() which calls
								inotify_remove_from_idr([a])

since fsnotify_add_mark() failed we call:
inotify_remove_from_idr([a])     <------WHOOPS it's not in the idr, this could
					have been any entry added later!

The fix is to make sure we don't set mark->group until we are sure the mark is
on the inode and fsnotify_add_mark will return success.

Signed-off-by: Eric Paris <eparis@redhat.com>
This commit is contained in:
Eric Paris 2009-09-11 13:03:19 -04:00
parent 2fdc246aaf
commit 9f0d793b52
1 changed files with 3 additions and 3 deletions
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6
fs/notify/inode_mark.c
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@ -324,11 +324,11 @@ int fsnotify_add_mark(struct fsnotify_mark_entry *entry,
spin_lock(&group->mark_lock);
spin_lock(&inode->i_lock);
entry->group = group;
entry->inode = inode;
lentry = fsnotify_find_mark_entry(group, inode);
if (!lentry) {
entry->group = group;
entry->inode = inode;
hlist_add_head(&entry->i_list, &inode->i_fsnotify_mark_entries);
list_add(&entry->g_list, &group->mark_entries);