ceph: give up on paths longer than PATH_MAX

If the full path to be built by ceph_mdsc_build_path() happens to be
longer than PATH_MAX, then this function will enter an endless (retry)
loop, effectively blocking the whole task.  Most of the machine
becomes unusable, making this a very simple and effective DoS
vulnerability.

I cannot imagine why this retry was ever implemented, but it seems
rather useless and harmful to me.  Let's remove it and fail with
ENAMETOOLONG instead.

Cc: stable@vger.kernel.org
Reported-by: Dario Weißer <dario@cure53.de>
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Alex Markuze <amarkuze@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

Author: MaxKellermann

fs/ceph/mds_client.c

@@ -2800,12 +2800,11 @@ char *ceph_mdsc_build_path(struct ceph_mds_client *mdsc, struct dentry *dentry,
 
 	if (pos < 0) {
 		/*
-		 * A rename didn't occur, but somehow we didn't end up where
-		 * we thought we would. Throw a warning and try again.
+		 * The path is longer than PATH_MAX and this function
+		 * cannot ever succeed.  Creating paths that long is
+		 * possible with Ceph, but Linux cannot use them.
 		 */
-		pr_warn_client(cl, "did not end path lookup where expected (pos = %d)\n",
-			       pos);
-		goto retry;
+		return ERR_PTR(-ENAMETOOLONG);
 	}
 
 	*pbase = base;