xfs: tighten up validation of root block in inode forks

Check that root blocks that sit in the inode fork and thus have a NULL
bp don't have siblings.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/libxfs/xfs_btree.c

@@ -107,7 +107,7 @@ __xfs_btree_check_lblock(
 {
 	struct xfs_mount	*mp = cur->bc_mp;
 	xfs_failaddr_t		fa;
-	xfs_fsblock_t		fsb = NULLFSBLOCK;
+	xfs_fsblock_t		fsb;
 
 	if (xfs_has_crc(mp)) {
 		if (!uuid_equal(&block->bb_u.l.bb_uuid, &mp->m_sb.sb_meta_uuid))
@@ -127,9 +127,19 @@ __xfs_btree_check_lblock(
 	    cur->bc_ops->get_maxrecs(cur, level))
 		return __this_address;
 
-	if (bp)
-		fsb = XFS_DADDR_TO_FSB(mp, xfs_buf_daddr(bp));
+	/*
+	 * For inode-rooted btrees, the root block sits in the inode fork.  In
+	 * that case bp is NULL, and the block must not have any siblings.
+	 */
+	if (!bp) {
+		if (block->bb_u.l.bb_leftsib != cpu_to_be64(NULLFSBLOCK))
+			return __this_address;
+		if (block->bb_u.l.bb_rightsib != cpu_to_be64(NULLFSBLOCK))
+			return __this_address;
+		return NULL;
+	}
 
+	fsb = XFS_DADDR_TO_FSB(mp, xfs_buf_daddr(bp));
 	fa = xfs_btree_check_lblock_siblings(mp, fsb, block->bb_u.l.bb_leftsib);
 	if (!fa)
 		fa = xfs_btree_check_lblock_siblings(mp, fsb,