ubifs: dirty_cow_znode: Fix memleak in error handling path

Following process will cause a memleak for copied up znode:

dirty_cow_znode
  zn = copy_znode(c, znode);
  err = insert_old_idx(c, zbr->lnum, zbr->offs);
  if (unlikely(err))
     return ERR_PTR(err);   // No one refers to zn.

Fix it by adding copied znode back to tnc, then it will be freed
by ubifs_destroy_tnc_subtree() while closing tnc.

Fetch a reproducer in [Link].

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216705
Fixes: 1e51764a3c ("UBIFS: add new flash file system")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/tnc.c

@@ -267,11 +267,18 @@ static struct ubifs_znode *dirty_cow_znode(struct ubifs_info *c,
 	if (zbr->len) {
 		err = insert_old_idx(c, zbr->lnum, zbr->offs);
 		if (unlikely(err))
-			return ERR_PTR(err);
+			/*
+			 * Obsolete znodes will be freed by tnc_destroy_cnext()
+			 * or free_obsolete_znodes(), copied up znodes should
+			 * be added back to tnc and freed by
+			 * ubifs_destroy_tnc_subtree().
+			 */
+			goto out;
 		err = add_idx_dirt(c, zbr->lnum, zbr->len);
 	} else
 		err = 0;
 
+out:
 	zbr->znode = zn;
 	zbr->lnum = 0;
 	zbr->offs = 0;