Index: /trunk/mmap.c
===================================================================
--- /trunk/mmap.c	(revision 57)
+++ /trunk/mmap.c	(revision 59)
@@ -81,5 +81,5 @@
 	if (shm->diff) {
 		assert(xc_shm_is_readonly(p));
-		p = PTR_SUB(p, -shm->diff);
+		p = PTR_SUB(p, shm->diff);
 	}
 	assert(xc_shm_is_readwrite(p));