[pabs3]

In the EU it sounds like the modifications allowed are limited to "within its intended purpose", which could block some modifications, although it would be hard to discover modifications you made locally without sharing them.

https://en.wikipedia.org/wiki/Computer_Programs_Directive

Wikipedia says that software ToS can override the legality of reverse engineering in the USA:

https://en.wikipedia.org/wiki/Reverse_engineering#Legality

It also sounds like in the US the modifications allowed are only for circumventing DRM, and only for interoperability, not for adding features or fixing bugs.