xfs: fix memcpy fortify errors in BUI log format copying

Starting in 6.1, CONFIG_FORTIFY_SOURCE checks the length parameter of
memcpy.  Unfortunately, it doesn't handle flex arrays correctly:

------------[ cut here ]------------
memcpy: detected field-spanning write (size 48) of single field "dst_bui_fmt" at fs/xfs/xfs_bmap_item.c:628 (size 16)

Fix this by refactoring the xfs_bui_copy_format function to handle the
copying of the head and the flex array members separately.  While we're
at it, fix a minor validation deficiency in the recovery function.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/xfs_bmap_item.c

@@ -608,28 +608,18 @@ static const struct xfs_item_ops xfs_bui_item_ops = {
 	.iop_relog	= xfs_bui_item_relog,
 };
 
-/*
- * Copy an BUI format buffer from the given buf, and into the destination
- * BUI format structure.  The BUI/BUD items were designed not to need any
- * special alignment handling.
- */
-static int
+static inline void
 xfs_bui_copy_format(
-	struct xfs_log_iovec		*buf,
-	struct xfs_bui_log_format	*dst_bui_fmt)
+	struct xfs_bui_log_format	*dst,
+	const struct xfs_bui_log_format	*src)
 {
-	struct xfs_bui_log_format	*src_bui_fmt;
-	uint				len;
+	unsigned int			i;
 
-	src_bui_fmt = buf->i_addr;
-	len = xfs_bui_log_format_sizeof(src_bui_fmt->bui_nextents);
+	memcpy(dst, src, offsetof(struct xfs_bui_log_format, bui_extents));
 
-	if (buf->i_len == len) {
-		memcpy(dst_bui_fmt, src_bui_fmt, len);
-		return 0;
-	}
-	XFS_ERROR_REPORT(__func__, XFS_ERRLEVEL_LOW, NULL);
-	return -EFSCORRUPTED;
+	for (i = 0; i < src->bui_nextents; i++)
+		memcpy(&dst->bui_extents[i], &src->bui_extents[i],
+				sizeof(struct xfs_map_extent));
 }
 
 /*
@@ -646,23 +636,31 @@ xlog_recover_bui_commit_pass2(
 	struct xlog_recover_item	*item,
 	xfs_lsn_t			lsn)
 {
-	int				error;
 	struct xfs_mount		*mp = log->l_mp;
 	struct xfs_bui_log_item		*buip;
 	struct xfs_bui_log_format	*bui_formatp;
+	size_t				len;
 
 	bui_formatp = item->ri_buf[0].i_addr;
 
+	if (item->ri_buf[0].i_len < xfs_bui_log_format_sizeof(0)) {
+		XFS_ERROR_REPORT(__func__, XFS_ERRLEVEL_LOW, log->l_mp);
+		return -EFSCORRUPTED;
+	}
+
 	if (bui_formatp->bui_nextents != XFS_BUI_MAX_FAST_EXTENTS) {
 		XFS_ERROR_REPORT(__func__, XFS_ERRLEVEL_LOW, log->l_mp);
 		return -EFSCORRUPTED;
 	}
-	buip = xfs_bui_init(mp);
-	error = xfs_bui_copy_format(&item->ri_buf[0], &buip->bui_format);
-	if (error) {
-		xfs_bui_item_free(buip);
-		return error;
+
+	len = xfs_bui_log_format_sizeof(bui_formatp->bui_nextents);
+	if (item->ri_buf[0].i_len != len) {
+		XFS_ERROR_REPORT(__func__, XFS_ERRLEVEL_LOW, log->l_mp);
+		return -EFSCORRUPTED;
 	}
+
+	buip = xfs_bui_init(mp);
+	xfs_bui_copy_format(&buip->bui_format, bui_formatp);
 	atomic_set(&buip->bui_next_extent, bui_formatp->bui_nextents);
 	/*
 	 * Insert the intent into the AIL directly and drop one reference so

fs/xfs/xfs_ondisk.h

@@ -134,6 +134,11 @@ xfs_check_ondisk_structs(void)
 	XFS_CHECK_STRUCT_SIZE(struct xfs_trans_header,		16);
 	XFS_CHECK_STRUCT_SIZE(struct xfs_attri_log_format,	40);
 	XFS_CHECK_STRUCT_SIZE(struct xfs_attrd_log_format,	16);
+	XFS_CHECK_STRUCT_SIZE(struct xfs_bui_log_format,	16);
+	XFS_CHECK_STRUCT_SIZE(struct xfs_bud_log_format,	16);
+	XFS_CHECK_STRUCT_SIZE(struct xfs_map_extent,		32);
+
+	XFS_CHECK_OFFSET(struct xfs_bui_log_format, bui_extents,	16);
 
 	/*
 	 * The v5 superblock format extended several v4 header structures with