xfs: add CRC checks to the AGFL

Add CRC checks, location information and a magic number to the AGFL.
Previously the AGFL was just a block containing nothing but the
free block pointers.  The new AGFL has a real header with the usual
boilerplate instead, so that we can verify it's not corrupted and
written into the right place.

[dchinner@redhat.com] Added LSN field, reworked significantly to fit
into new verifier structure and growfs structure, enabled full
verifier functionality now there is a header to verify and we can
guarantee an initialised AGFL.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_ag.h

@@ -30,6 +30,7 @@ struct xfs_trans;
 
 #define	XFS_AGF_MAGIC	0x58414746	/* 'XAGF' */
 #define	XFS_AGI_MAGIC	0x58414749	/* 'XAGI' */
+#define	XFS_AGFL_MAGIC	0x5841464c	/* 'XAFL' */
 #define	XFS_AGF_VERSION	1
 #define	XFS_AGI_VERSION	1
 
@@ -190,11 +191,31 @@ extern const struct xfs_buf_ops xfs_agi_buf_ops;
  */
 #define XFS_AGFL_DADDR(mp)	((xfs_daddr_t)(3 << (mp)->m_sectbb_log))
 #define	XFS_AGFL_BLOCK(mp)	XFS_HDR_BLOCK(mp, XFS_AGFL_DADDR(mp))
-#define XFS_AGFL_SIZE(mp)	((mp)->m_sb.sb_sectsize / sizeof(xfs_agblock_t))
 #define	XFS_BUF_TO_AGFL(bp)	((xfs_agfl_t *)((bp)->b_addr))
 
+#define XFS_BUF_TO_AGFL_BNO(mp, bp) \
+	(xfs_sb_version_hascrc(&((mp)->m_sb)) ? \
+		&(XFS_BUF_TO_AGFL(bp)->agfl_bno[0]) : \
+		(__be32 *)(bp)->b_addr)
+
+/*
+ * Size of the AGFL.  For CRC-enabled filesystes we steal a couple of
+ * slots in the beginning of the block for a proper header with the
+ * location information and CRC.
+ */
+#define XFS_AGFL_SIZE(mp) \
+	(((mp)->m_sb.sb_sectsize - \
+	 (xfs_sb_version_hascrc(&((mp)->m_sb)) ? \
+		sizeof(struct xfs_agfl) : 0)) / \
+	  sizeof(xfs_agblock_t))
+
 typedef struct xfs_agfl {
-	__be32		agfl_bno[1];	/* actually XFS_AGFL_SIZE(mp) */
+	__be32		agfl_magicnum;
+	__be32		agfl_seqno;
+	uuid_t		agfl_uuid;
+	__be64		agfl_lsn;
+	__be32		agfl_crc;
+	__be32		agfl_bno[];	/* actually XFS_AGFL_SIZE(mp) */
 } xfs_agfl_t;
 
 /*