[PATCH 4/4] xfs_repair: check plausiblitiy of root dir pointer

["Darrick J. Wong" <darrick.wong@oracle.com>]

From: Darrick J. Wong <darrick.wong@oracle.com>

If sb_rootino doesn't point to where we think mkfs was supposed to have
preallocated an inode chunk, check to see if the alleged root directory
actually looks like a root directory.  If so, we'll let it go because
someone could have changed sunit since formatting time.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 repair/xfs_repair.c |   50 +++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 49 insertions(+), 1 deletion(-)


diff --git a/repair/xfs_repair.c b/repair/xfs_repair.c
index 6798b88c..f6134cca 100644
--- a/repair/xfs_repair.c
+++ b/repair/xfs_repair.c
@@ -395,12 +395,60 @@ do_log(char const *msg, ...)
 	va_end(args);
 }
 
+/*
+ * If sb_rootino points to a different inode than we were expecting, try
+ * loading the alleged root inode to see if it's a plausibly a root directory.
+ * If so, we'll readjust the computations.
+ */
+static void
+check_misaligned_root(
+	struct xfs_mount	*mp)
+{
+	struct xfs_inode	*ip;
+	xfs_ino_t		ino;
+	int			error;
+
+	error = -libxfs_iget(mp, NULL, mp->m_sb.sb_rootino, 0, &ip,
+			&xfs_default_ifork_ops);
+	if (error)
+		return;
+	if (!S_ISDIR(VFS_I(ip)->i_mode))
+		goto out_rele;
+
+	error = -libxfs_dir_lookup(NULL, ip, &xfs_name_dotdot, &ino, NULL);
+	if (error)
+		goto out_rele;
+
+	if (ino == mp->m_sb.sb_rootino) {
+		do_warn(
+_("sb root inode value %" PRIu64 " inconsistent with calculated value %u but looks like a root directory\n"),
+			mp->m_sb.sb_rootino, first_prealloc_ino);
+		last_prealloc_ino += (int)ino - first_prealloc_ino;
+		first_prealloc_ino = ino;
+	}
+
+out_rele:
+	libxfs_irele(ip);
+}
+
 static void
-calc_mkfs(xfs_mount_t *mp)
+calc_mkfs(
+	struct xfs_mount	*mp)
 {
 	libxfs_ialloc_find_prealloc(mp, &first_prealloc_ino,
 			&last_prealloc_ino);
 
+	/*
+	 * If the root inode isn't where we think it is, check its plausibility
+	 * as a root directory.  It's possible that somebody changed sunit since
+	 * the filesystem was created, which can change the value of the above
+	 * computation.  Try to avoid blowing up the filesystem if this is the
+	 * case.
+	 */
+	if (mp->m_sb.sb_rootino != NULLFSINO &&
+	    mp->m_sb.sb_rootino != first_prealloc_ino)
+		check_misaligned_root(mp);
+
 	/*
 	 * now the first 3 inodes in the system
 	 */