Because you'll fault in the guard zone on one of those memory accesses, and then your app will get killed. EDIT: No, you don't need to check after a stack memo

Because you'll fault in the guard zone on one of those memory accesses, and then your app will get killed. EDIT: No, you don't need to check after a stack memory access either. The farthest you can get out of the sandbox is an access-then-adjust of 4095 bytes, followed by an adjust-then-access of 4095 bytes, which means you access the very tail end (offset 8190) of the guard zone, fault, and die.