[PATCH 5/6] xfs_repair: check plausibility of root dir pointer before trashing it

["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 should have allocated
the root directory, check to see if the alleged root directory actually
looks like a root directory.  If so, we'll let it live because someone
could have changed sunit since formatting time, and that changes the
root directory inode estimate.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 repair/xfs_repair.c |   45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)


diff --git a/repair/xfs_repair.c b/repair/xfs_repair.c
index 53b04dae..372616c4 100644
--- a/repair/xfs_repair.c
+++ b/repair/xfs_repair.c
@@ -426,6 +426,37 @@ _("would reset superblock %s inode pointer to %"PRIu64"\n"),
 	*ino = expected_ino;
 }
 
+/* Does the root directory inode look like a plausible root directory? */
+static bool
+has_plausible_rootdir(
+	struct xfs_mount	*mp)
+{
+	struct xfs_inode	*ip;
+	xfs_ino_t		ino;
+	int			error;
+	bool			ret = false;
+
+	error = -libxfs_iget(mp, NULL, mp->m_sb.sb_rootino, 0, &ip,
+			&xfs_default_ifork_ops);
+	if (error)
+		goto out;
+	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;
+
+	/* The root directory '..' entry points to the directory. */
+	if (ino == mp->m_sb.sb_rootino)
+		ret = true;
+
+out_rele:
+	libxfs_irele(ip);
+out:
+	return ret;
+}
+
 /*
  * Make sure that the first 3 inodes in the filesystem are the root directory,
  * the realtime bitmap, and the realtime summary, in that order.
@@ -438,6 +469,20 @@ calc_mkfs(
 
 	rootino = libxfs_ialloc_calc_rootino(mp, mp->m_sb.sb_unit);
 
+	/*
+	 * 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.  Don't blow up the root directory if this is the
+	 * case.
+	 */
+	if (mp->m_sb.sb_rootino != rootino && has_plausible_rootdir(mp)) {
+		do_warn(
+_("sb root inode value %" PRIu64 " inconsistent with alignment (expected %"PRIu64")\n"),
+			mp->m_sb.sb_rootino, rootino);
+		rootino = mp->m_sb.sb_rootino;
+	}
+
 	ensure_fixed_ino(&mp->m_sb.sb_rootino, rootino,
 			_("root"));
 	ensure_fixed_ino(&mp->m_sb.sb_rbmino, rootino + 1,