If the attacker is assumed to have access to every byte of your software's code and data, how would it be possible to stop that attacker from breaking off part

If the attacker is assumed to have access to every byte of your software's code and data, how would it be possible to stop that attacker from breaking off part of that software's functionality and turning it into a library? It seems like that is the goal for white box cryptography, and I am very skeptical about its feasibility.