ovl: use is_subdir() for testing if one thing is a subdir of another

Rather than using lock_rename(), use the more obvious is_subdir() for
ensuring that neither upper nor workdir contain the other.
Also be explicit in the comment that the two directories cannot be the
same.

As this is a point-it-time sanity check and does not provide any
on-going guarantees, the removal of locking does not introduce any
interesting races.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260224222542.3458677-14-neilb@ownmail.net
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: neilbrown

fs/overlayfs/super.c

@@ -451,18 +451,13 @@ static int ovl_lower_dir(const char *name, const struct path *path,
 	return 0;
 }
 
-/* Workdir should not be subdir of upperdir and vice versa */
+/*
+ * Workdir should not be subdir of upperdir and vice versa, and
+ * they should not be the same.
+ */
 static bool ovl_workdir_ok(struct dentry *workdir, struct dentry *upperdir)
 {
-	bool ok = false;
-
-	if (workdir != upperdir) {
-		struct dentry *trap = lock_rename(workdir, upperdir);
-		if (!IS_ERR(trap))
-			unlock_rename(workdir, upperdir);
-		ok = (trap == NULL);
-	}
-	return ok;
+	return !is_subdir(workdir, upperdir) && !is_subdir(upperdir, workdir);
 }
 
 static int ovl_setup_trap(struct super_block *sb, struct dentry *dir,