kernel: fix ubifs loosing O_TMPFILE data after power cut

There was a bug in ubifs related to the O_TMPFILE. When reapplying
changes after power cut data could be lost. This problem was exposed by
overlayfs and the upstream commit 3a1e819b4e80 ("ovl: store file handle
of lower inode on copy up").

This fixes a regression introduced when switching from 4.9 to 4.14.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/generic/backport-4.14/500-ubifs-Handle-re-linking-of-inodes-correctly-while-re.patch

@@ -0,0 +1,89 @@
+From: Richard Weinberger <richard@nod.at>
+Date: Wed, 7 Nov 2018 23:04:43 +0100
+Subject: [PATCH] ubifs: Handle re-linking of inodes correctly while recovery
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+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: 474b93704f32 ("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>
+Signed-off-by: Richard Weinberger <richard@nod.at>
+---
+ fs/ubifs/replay.c | 37 +++++++++++++++++++++++++++++++++++++
+ 1 file changed, 37 insertions(+)
+
+--- a/fs/ubifs/replay.c
++++ b/fs/ubifs/replay.c
+@@ -210,6 +210,38 @@ static int trun_remove_range(struct ubif
+ }
+ 
+ /**
++ * 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(rino->deletion);
++	ubifs_assert(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(r->sqnum >= rino->sqnum);
++		if (key_inum(c, &r->key) == key_inum(c, &rino->key))
++			return r->deletion == 0;
++
++	}
++
++	ubifs_assert(0);
++	return false;
++}
++
++/**
+  * apply_replay_entry - apply a replay entry to the TNC.
+  * @c: UBIFS file-system description object
+  * @r: replay entry to apply
+@@ -239,6 +271,11 @@ static int apply_replay_entry(struct ubi
+ 			{
+ 				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;
+ 			}