ext2: Avoid reading renamed directory if parent does not change

The VFS will not be locking moved directory if its parent does not
change. Change ext2 rename code to avoid reading renamed directory if
its parent does not change. Although it is currently harmless it is a
bad practice to read directory contents without inode->i_rwsem.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ext2/namei.c

@@ -325,6 +325,7 @@ static int ext2_rename (struct mnt_idmap * idmap,
 	struct ext2_dir_entry_2 * dir_de = NULL;
 	struct folio * old_folio;
 	struct ext2_dir_entry_2 * old_de;
+	bool old_is_dir = S_ISDIR(old_inode->i_mode);
 	int err;
 
 	if (flags & ~RENAME_NOREPLACE)
@@ -342,7 +343,7 @@ static int ext2_rename (struct mnt_idmap * idmap,
 	if (IS_ERR(old_de))
 		return PTR_ERR(old_de);
 
-	if (S_ISDIR(old_inode->i_mode)) {
+	if (old_is_dir && old_dir != new_dir) {
 		err = -EIO;
 		dir_de = ext2_dotdot(old_inode, &dir_folio);
 		if (!dir_de)
@@ -354,7 +355,7 @@ static int ext2_rename (struct mnt_idmap * idmap,
 		struct ext2_dir_entry_2 *new_de;
 
 		err = -ENOTEMPTY;
-		if (dir_de && !ext2_empty_dir (new_inode))
+		if (old_is_dir && !ext2_empty_dir(new_inode))
 			goto out_dir;
 
 		new_de = ext2_find_entry(new_dir, &new_dentry->d_name,
@@ -368,14 +369,14 @@ static int ext2_rename (struct mnt_idmap * idmap,
 		if (err)
 			goto out_dir;
 		inode_set_ctime_current(new_inode);
-		if (dir_de)
+		if (old_is_dir)
 			drop_nlink(new_inode);
 		inode_dec_link_count(new_inode);
 	} else {
 		err = ext2_add_link(new_dentry, old_inode);
 		if (err)
 			goto out_dir;
-		if (dir_de)
+		if (old_is_dir)
 			inode_inc_link_count(new_dir);
 	}
 
@@ -387,7 +388,7 @@ static int ext2_rename (struct mnt_idmap * idmap,
 	mark_inode_dirty(old_inode);
 
 	err = ext2_delete_entry(old_de, old_folio);
-	if (!err && dir_de) {
+	if (!err && old_is_dir) {
 		if (old_dir != new_dir)
 			err = ext2_set_link(old_inode, dir_de, dir_folio,
 					    new_dir, false);