Arbitrary computation is just a subset of arbitrary code execution and one that is much less harmful. Arbitrary computation means that you can let the computer

Arbitrary computation is just a subset of arbitrary code execution and one that is much less harmful. Arbitrary computation means that you can let the computer do any pure calculation you want it to. The worst you can do is to put it in an endless loop. Which you can break with timeouts. As Turing machines are idealized machines that only do computation, that’s all that can happen to them and why they are completely useless models for security. Real machines, processes interact with the outside world. Arbitrary code execution is dangerous exactly because it can access those interaction possibilities. It doesn’t matter that the injected code could calculate PI to trillions of digits, but it matters that it could do anything the process could do, namely manipulating files and accessing the network.