> how do you expect a cop to run a hash sum on the vehicle software

Any flash chip that holds vehicle software gets a tiny hardware component that once a minute hashes the entire memory and broadcasts the checksum over the car's message bus. Somewhere in the car there's a read-only hardware interface to the message bus that cops and mechanics can use.

If you don't trust the message bus you need a bit more silicon to provide a signed challenge-response protocol, but then we're still talking about cents per flash chip.