xfs: don't use the incore struct xfs_sb for offsets into struct xfs_dsb

Currently, the XFS_SB_CRC_OFF macro uses the incore superblock struct
(xfs_sb) to compute the address of sb_crc within the ondisk superblock
struct (xfs_dsb).  This is a landmine if we ever change the layout of
the incore superblock (as we're about to do), so redefine the macro
to use xfs_dsb to compute the layout of xfs_dsb.

Signed-off-by: Darrick J. Wong <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>

Author: Darrick J. Wong

fs/xfs/libxfs/xfs_format.h

@@ -90,8 +90,7 @@ struct xfs_ifork;
 #define XFSLABEL_MAX			12
 
 /*
- * Superblock - in core version.  Must match the ondisk version below.
- * Must be padded to 64 bit alignment.
+ * Superblock - in core version.  Must be padded to 64 bit alignment.
  */
 typedef struct xfs_sb {
 	uint32_t	sb_magicnum;	/* magic number == XFS_SB_MAGIC */
@@ -178,10 +177,8 @@ typedef struct xfs_sb {
 	/* must be padded to 64 bit alignment */
 } xfs_sb_t;
 
-#define XFS_SB_CRC_OFF		offsetof(struct xfs_sb, sb_crc)
-
 /*
- * Superblock - on disk version.  Must match the in core version above.
+ * Superblock - on disk version.
  * Must be padded to 64 bit alignment.
  */
 struct xfs_dsb {
@@ -265,6 +262,8 @@ struct xfs_dsb {
 	/* must be padded to 64 bit alignment */
 };
 
+#define XFS_SB_CRC_OFF		offsetof(struct xfs_dsb, sb_crc)
+
 /*
  * Misc. Flags - warning - these will be cleared by xfs_repair unless
  * a feature bit is set when the flag is used.

fs/xfs/libxfs/xfs_ondisk.h

@@ -85,6 +85,7 @@ xfs_check_ondisk_structs(void)
 	XFS_CHECK_STRUCT_SIZE(xfs_attr_leaf_name_remote_t,	12);
 	 */
 
+	XFS_CHECK_OFFSET(struct xfs_dsb, sb_crc,		224);
 	XFS_CHECK_OFFSET(xfs_attr_leaf_name_local_t, valuelen,	0);
 	XFS_CHECK_OFFSET(xfs_attr_leaf_name_local_t, namelen,	2);
 	XFS_CHECK_OFFSET(xfs_attr_leaf_name_local_t, nameval,	3);