xfs: check dabtree node hash values when loading child blocks

When xchk_da_btree_block is loading a non-root dabtree block, we know
that the parent block had to have a (hashval, address) pointer to the
block that we just loaded.  Check that the hashval in the parent matches
the block we just loaded.

This was found by fuzzing nbtree[3].hashval = ones in xfs/394.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/scrub/dabtree.c

@@ -441,6 +441,20 @@ xchk_da_btree_block(
 		goto out_freebp;
 	}
 
+	/*
+	 * If we've been handed a block that is below the dabtree root, does
+	 * its hashval match what the parent block expected to see?
+	 */
+	if (level > 0) {
+		struct xfs_da_node_entry	*key;
+
+		key = xchk_da_btree_node_entry(ds, level - 1);
+		if (be32_to_cpu(key->hashval) != blk->hashval) {
+			xchk_da_set_corrupt(ds, level);
+			goto out_freebp;
+		}
+	}
+
 out:
 	return error;
 out_freebp: