namei: only return -ECHILD from follow_dotdot_rcu()

It's over-zealous to return hard errors under RCU-walk here, given that
a REF-walk will be triggered for all other cases handling ".." under
RCU.

The original purpose of this check was to ensure that if a rename occurs
such that a directory is moved outside of the bind-mount which the
resolution started in, it would be detected and blocked to avoid being
able to mess with paths outside of the bind-mount. However, triggering a
new REF-walk is just as effective a solution.

Cc: "Eric W. Biederman" <[email protected]>
Fixes: 397d425 ("vfs: Test for and handle paths that are unreachable from their mnt_root")
Suggested-by: Al Viro <[email protected]>
Signed-off-by: Aleksa Sarai <[email protected]>
Signed-off-by: Al Viro <[email protected]>

Author: cyphar

fs/namei.c

@@ -1365,7 +1365,7 @@ static int follow_dotdot_rcu(struct nameidata *nd)
 			nd->path.dentry = parent;
 			nd->seq = seq;
 			if (unlikely(!path_connected(&nd->path)))
-				return -ENOENT;
+				return -ECHILD;
 			break;
 		} else {
 			struct mount *mnt = real_mount(nd->path.mnt);