xfs: add a xfs_valid_startblock helper

Add a helper that validates the startblock is valid.  This checks for a
non-zero block on the main device, but skips that check for blocks on
the realtime device.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/libxfs/xfs_bmap.c

@@ -4515,7 +4515,7 @@ xfs_bmapi_convert_delalloc(
 	if (WARN_ON_ONCE(bma.blkno == NULLFSBLOCK))
 		goto out_finish;
 	error = -EFSCORRUPTED;
-	if (WARN_ON_ONCE(!bma.got.br_startblock && !XFS_IS_REALTIME_INODE(ip)))
+	if (WARN_ON_ONCE(!xfs_valid_startblock(ip, bma.got.br_startblock)))
 		goto out_finish;
 
 	XFS_STATS_ADD(mp, xs_xstrat_bytes, XFS_FSB_TO_B(mp, bma.length));

fs/xfs/libxfs/xfs_bmap.h

@@ -171,6 +171,13 @@ static inline bool xfs_bmap_is_real_extent(struct xfs_bmbt_irec *irec)
 		!isnullstartblock(irec->br_startblock);
 }
 
+/*
+ * Check the mapping for obviously garbage allocations that could trash the
+ * filesystem immediately.
+ */
+#define xfs_valid_startblock(ip, startblock) \
+	((startblock) != 0 || XFS_IS_REALTIME_INODE(ip))
+
 void	xfs_trim_extent(struct xfs_bmbt_irec *irec, xfs_fileoff_t bno,
 		xfs_filblks_t len);
 int	xfs_bmap_add_attrfork(struct xfs_inode *ip, int size, int rsvd);

fs/xfs/xfs_iomap.c

@@ -58,7 +58,7 @@ xfs_bmbt_to_iomap(
 {
 	struct xfs_mount	*mp = ip->i_mount;
 
-	if (unlikely(!imap->br_startblock && !XFS_IS_REALTIME_INODE(ip)))
+	if (unlikely(!xfs_valid_startblock(ip, imap->br_startblock)))
 		return xfs_alert_fsblock_zero(ip, imap);
 
 	if (imap->br_startblock == HOLESTARTBLOCK) {
@@ -297,7 +297,7 @@ xfs_iomap_write_direct(
 		goto out_unlock;
 	}
 
-	if (!(imap->br_startblock || XFS_IS_REALTIME_INODE(ip)))
+	if (unlikely(!xfs_valid_startblock(ip, imap->br_startblock)))
 		error = xfs_alert_fsblock_zero(ip, imap);
 
 out_unlock:
@@ -814,7 +814,7 @@ xfs_iomap_write_unwritten(
 		if (error)
 			return error;
 
-		if (!(imap.br_startblock || XFS_IS_REALTIME_INODE(ip)))
+		if (unlikely(!xfs_valid_startblock(ip, imap.br_startblock)))
 			return xfs_alert_fsblock_zero(ip, &imap);
 
 		if ((numblks_fsb = imap.br_blockcount) == 0) {