Looks like that device might not be covered by the tiviozation close since it might not be an user product as defined in the GPL

> A “User Product” is either (1) a “consumer product”, which means any tangible personal property which is normally used for personal, family, or household purposes, or (2) anything designed or sold for incorporation into a dwelling.

If you sell to business, that should be alright.

Also, it is fine to make it impossible to modify, if you also don't have this possibility:

> this requirement does not apply if neither you nor any third party retains the ability to install modified object code on the User Product

I guess you're working on credit card terminals?

Anyway: why not have a secure hardware element / TPM that has a GPIO with a pull-up resistor that can be queried by the device? Then, have the bootloader check as part of the boot if the TPM attests with a digital signature that the GPIO is still high, and the application also regularly checking the TPM? Or an e-fuse similar to Samsung's Knox Guard?

That way, a user can remove the pullup resistor (and thus, as it's a hardware modification, has to break the seal of the device) to "unlock" custom firmware loading to fulfill the GPL requirement, but at the same time your application can be reasonably certain at run-time the device hasn't been tampered with.

EDIT: So, I don't think this is actually a technical question. It's a legal question that boils down to the fact that a premise and its inverse can not both be true at the same time.

how do you ensure beyond doubt that the vehicle was not "tampered with" by an "unauthorized unqualified third party"?

would I want to own a car where the oem can turn around burden of proof and can easily claim I have modified the vehicle software, and thus broke vehicle behaviour?

oem: you've patched coreutils, that's what killed your wife, your fault!

me: no. that's technically impossible. the drm disables third party modifications.

now if you provide means to do it anyhow, you'd need to make forensics crystal clear.

so you as a customer want the physical modification to be dead obvious even on a burnt vehicle. especially on a burnt vehicle. so you as a customer can show that you did in fact not unlock software modification.

and state regulations for that reason require drm from car makers, to make it impossible to evade responsibility with flakey claims of third party modifications.