ovl: check for emptiness of redirect dir

Before introducing redirect_dir feature, the condition
!ovl_lower_positive(dentry) for a directory, implied that it is a pure
upper directory, which may be removed if empty.

Now that directory can be redirect, it is possible that upper does not
cover any lower (i.e. !ovl_lower_positive(dentry)), but the directory is a
merge (with redirected path) and maybe non empty.

Check for this case in ovl_remove_upper().

This change fixes the following test case from rename-pop-dir.py
of unionmount-testsuite:

    """Remove dir and rename old name"""
    d = ctx.non_empty_dir()
    d2 = ctx.no_dir()

    ctx.rmdir(d, err=ENOTEMPTY)
    ctx.rename(d, d2)
    ctx.rmdir(d, err=ENOENT)
    ctx.rmdir(d2, err=ENOTEMPTY)

./run --ov rename-pop-dir
/mnt/a/no_dir103: Expected error (Directory not empty) was not produced
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/dir.c

@@ -674,8 +674,17 @@ static int ovl_remove_upper(struct dentry *dentry, bool is_dir)
 	struct dentry *upperdir = ovl_dentry_upper(dentry->d_parent);
 	struct inode *dir = upperdir->d_inode;
 	struct dentry *upper;
+	struct dentry *opaquedir = NULL;
 	int err;
 
+	/* Redirect dir can be !ovl_lower_positive && OVL_TYPE_MERGE */
+	if (is_dir && ovl_dentry_get_redirect(dentry)) {
+		opaquedir = ovl_check_empty_and_clear(dentry);
+		err = PTR_ERR(opaquedir);
+		if (IS_ERR(opaquedir))
+			goto out;
+	}
+
 	inode_lock_nested(dir, I_MUTEX_PARENT);
 	upper = lookup_one_len(dentry->d_name.name, upperdir,
 			       dentry->d_name.len);
@@ -684,14 +693,15 @@ static int ovl_remove_upper(struct dentry *dentry, bool is_dir)
 		goto out_unlock;
 
 	err = -ESTALE;
-	if (upper == ovl_dentry_upper(dentry)) {
+	if ((opaquedir && upper != opaquedir) ||
+	    (!opaquedir && upper != ovl_dentry_upper(dentry)))
+		goto out_dput_upper;
+
 	if (is_dir)
 		err = vfs_rmdir(dir, upper);
 	else
 		err = vfs_unlink(dir, upper, NULL);
 	ovl_dentry_version_inc(dentry->d_parent);
-	}
-	dput(upper);
 
 	/*
 	 * Keeping this dentry hashed would mean having to release
@@ -701,9 +711,12 @@ static int ovl_remove_upper(struct dentry *dentry, bool is_dir)
 	 */
 	if (!err)
 		d_drop(dentry);
+out_dput_upper:
+	dput(upper);
 out_unlock:
 	inode_unlock(dir);
-
+	dput(opaquedir);
+out:
 	return err;
 }