This article explores the implementation of user-defined intrinsics within Crystal Palace, a feature that allows developers to replace specific function calls with custom machine code at link-time. The author demonstrates how to declare a function, such as __my_intrinsic, and use a spec file to substitute the instruction with arbitrary bytecode, effectively providing a powerful mechanism for binary manipulation and obfuscation.
The author focuses on a practical use case for C2 agents: dynamic task dispatching. By creating a proof-of-concept __lookup_func intrinsic, the author shows how to resolve function pointers based on command IDs when capabilities are merged at link-time via COFF merging. This approach allows for efficient execution of modular code without knowing the function addresses at compile time, illustrated through assembly examples and successful execution results.
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