ubifs: Handle re-linking of inodes correctly while recovery

UBIFS's recovery code strictly assumes that a deleted inode will never
come back, therefore it removes all data which belongs to that inode
as soon it faces an inode with link count 0 in the replay list.
Before O_TMPFILE this assumption was perfectly fine. With O_TMPFILE
it can lead to data loss upon a power-cut.

Consider a journal with entries like:
0: inode X (nlink = 0) /* O_TMPFILE was created */
1: data for inode X /* Someone writes to the temp file */
2: inode X (nlink = 0) /* inode was changed, xattr, chmod, … */
3: inode X (nlink = 1) /* inode was re-linked via linkat() */

Upon replay of entry #2 UBIFS will drop all data that belongs to inode X,
this will lead to an empty file after mounting.

As solution for this problem, scan the replay list for a re-link entry
before dropping data.

Fixes: 474b9370 ("ubifs: Implement O_TMPFILE")
Cc: stable@vger.kernel.org
Cc: Russell Senior <russell@personaltelco.net>
Cc: Rafał Miłecki <zajec5@gmail.com>
Reported-by: Russell Senior <russell@personaltelco.net>
Reported-by: Rafał Miłecki <zajec5@gmail.com>
Tested-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/replay.c

@@ -212,6 +212,38 @@ static int trun_remove_range(struct ubifs_info *c, struct replay_entry *r)
 	return ubifs_tnc_remove_range(c, &min_key, &max_key);
 }
 
+/**
+ * inode_still_linked - check whether inode in question will be re-linked.
+ * @c: UBIFS file-system description object
+ * @rino: replay entry to test
+ *
+ * O_TMPFILE files can be re-linked, this means link count goes from 0 to 1.
+ * This case needs special care, otherwise all references to the inode will
+ * be removed upon the first replay entry of an inode with link count 0
+ * is found.
+ */
+static bool inode_still_linked(struct ubifs_info *c, struct replay_entry *rino)
+{
+	struct replay_entry *r;
+
+	ubifs_assert(c, rino->deletion);
+	ubifs_assert(c, key_type(c, &rino->key) == UBIFS_INO_KEY);
+
+	/*
+	 * Find the most recent entry for the inode behind @rino and check
+	 * whether it is a deletion.
+	 */
+	list_for_each_entry_reverse(r, &c->replay_list, list) {
+		ubifs_assert(c, r->sqnum >= rino->sqnum);
+		if (key_inum(c, &r->key) == key_inum(c, &rino->key))
+			return r->deletion == 0;
+
+	}
+
+	ubifs_assert(c, 0);
+	return false;
+}
+
 /**
  * apply_replay_entry - apply a replay entry to the TNC.
  * @c: UBIFS file-system description object
@@ -239,6 +271,11 @@ static int apply_replay_entry(struct ubifs_info *c, struct replay_entry *r)
 			{
 				ino_t inum = key_inum(c, &r->key);
 
+				if (inode_still_linked(c, r)) {
+					err = 0;
+					break;
+				}
+
 				err = ubifs_tnc_remove_ino(c, inum);
 				break;
 			}