xfs: prevent bogus assert when trying to remove non-existent attribute

If the attribute fork on an inode is in btree format and has
multiple levels (i.e node format rather than leaf format), then a
lookup failure will trigger an assert failure in xfs_da_path_shift
if the flag XFS_DA_OP_OKNOENT is not set. This flag is used to
indicate to the directory btree code that not finding an entry is
not a fatal error. In the case of doing a lookup for a directory
name removal, this is valid as a user cannot insert an arbitrary
name to remove from the directory btree.

However, in the case of the attribute tree, a user has direct
control over the attribute name and can ask for any random name to
be removed without any validation. In this case, fsstress is asking
for a non-existent user.selinux attribute to be removed, and that is
causing xfs_da_path_shift() to fall off the bottom of the tree where
it asserts that a lookup failure is allowed. Because the flag is not
set, we die a horrible death on a debug enable kernel.

Prevent this assert from firing on attribute removes by adding the
op_flag XFS_DA_OP_OKNOENT to atribute removal operations.

Discovered when testing on a SELinux enabled system by fsstress in
test 070 by trying to remove a non-existent user.selinux attribute.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/xfs_attr.c

@@ -489,6 +489,13 @@ xfs_attr_remove_int(xfs_inode_t *dp, struct xfs_name *name, int flags)
 	args.total = 0;
 	args.whichfork = XFS_ATTR_FORK;
 
+	/*
+	 * we have no control over the attribute names that userspace passes us
+	 * to remove, so we have to allow the name lookup prior to attribute
+	 * removal to fail.
+	 */
+	args.op_flags = XFS_DA_OP_OKNOENT;
+
 	/*
 	 * Attach the dquots to the inode.
 	 */