Rcmp Save

rcmp - C++17, multi-architecture cross-platform hooking library with clean API.

Features

• Intuitive, modern, compiler/platform-independent API
• x86/x86-64 support (more soon)
• Windows/Linux support
• Calling convention support (cdecl, stdcall, thiscall, fastcall, native-x64)

Building

With CMake (as a subproject)

Clone repository to subfolder and link rcmp to your project:

add_subdirectory(path/to/rcmp)
target_link_libraries(your-project-name PRIVATE rcmp)

Examples

• The most common case: hook function to modify its argument and/or result

int foo(float arg) { /* body */ }

rcmp::hook_function<&foo>([](auto original_foo, float arg) {
    return original_foo(arg * 2) + 1;
});

• However, in most cases you probably want to hook function knowing only its address and signature (in fact, that's everything you need to make hook)

rcmp::hook_function<0xDEADBEEF, int(float)>([](auto original_foo, float arg) {
    return original_foo(arg * 2) + 1;
});

• Trace function calls

void do_something(int id, const char* action) { /* body */ }

rcmp::hook_function<&do_something>([](auto original_function, int id, const char* action) {
    std::cout << "do_something(" << id << ", " << action << ") called..\n";
    original_function(id, action);
});

• Replace return value

bool check_license() { /* body */ }

rcmp::hook_function<&check_license>([](auto /* original_function */) {
    return true;
});

• Accept arguments as a variadic pack

template <class... Args> void print(const Args& ...) { /* implementation */ }

rcmp::hook_function<0xDEADBEEF, unsigned int(int, float, bool, double, void*, long)>([](auto original, auto... args) {
    print("args are: ", args...);
    return original(args...);
});

• Function address can be known at runtime or compile-time

// compile-time address
rcmp::hook_function<0xDEADBEEF, int(int)>([](auto original, int arg) { ... });

// runtime address (i.e. from GetProcAddress/dlopen)
rcmp::hook_function<int(int)>(0xDEADBEEF, [](auto original, int arg) { ... });

• Calling convention support

/// x86, the following calls are synonyms

// good:
rcmp::hook_function<void(int)>(...);                // default convention
rcmp::hook_function<void(*)(int)>(...);             // default convention, but 3 more symbols
rcmp::hook_function<rcmp::cdecl_t<void(int)>>(...); // explicit convention (rcmp::cdecl_t<S> is an alias for rcmp::generic_signature_t<S, rcmp::cconv::cdecl_>)

// bad, compiler-specific
rcmp::hook_function<void(__cdecl*)(int)>(...);               // MSVC
rcmp::hook_function<void(__attributes((cdecl))*)(int)>(...); // gcc/clang

// x86 supported conventions
rcmp::cdecl_t   <void(int)> // same as rcmp::generic_signature_t<void(int), rcmp::cconv::cdecl_>
rcmp::stdcall_t <void(int)> // same as rcmp::generic_signature_t<void(int), rcmp::cconv::stdcall_>
rcmp::thiscall_t<void(int)> // same as rcmp::generic_signature_t<void(int), rcmp::cconv::thiscall_>
rcmp::fastcall_t<void(int)> // same as rcmp::generic_signature_t<void(int), rcmp::cconv::fastcall_>

// x64
rcmp::hook_function<void(int)>(...);                                                     // default convention
rcmp::hook_function<void(*)(int)>(...);                                                  // default convention, but more letters
rcmp::hook_function<rcmp::generic_signature_t<void(int), rcmp::cconv::native_x64>>(...); // explicit convention

• VTable hooking (hook_indirect_function)

// Let's assume:
// 5             - index of function in vtable
// int A::f(int) - function signature
using signature_t = rcmp::thiscall_t<int(A*, int)>; // x86, MSVC
using signature_t = rcmp::cdecl_t<int(A*, int)>;    // x86, gcc/clang
using signature_t = int(A*, int);                   // x64

// vtable address can be known at compile-time (0xDEADBEEF)
rcmp::hook_indirect_function<0xDEADBEEF + 5 * sizeof(void*), signature_t>([](auto original, A* self, int arg) { ... });

// ..or at runtime
rcmp::hook_indirect_function<signature_t>(get_vtable_address() + 5 * sizeof(void*), [](auto original, A* self, int arg) { ... });

Motivation

Why yet another hooking library?

There are too many libraries with similar or even more powerful features. Most of them have been perfectly designed; however, they don't provide all the features I need.

Mods, cheats, plugins etc

I like to develop unofficial mods (plugins) and cheats for various games (both for client and server-side). This is a very specific area of development that requires continuous experimentation with function hooking.

Suppose you want to hook a function. All you need to know about this function - its address and signature, that's it. Just write an interceptor (replacement function) and call something like cool_lib::hook_function. Very simple, isn't it? Of course it's not. Most libraries require:

• Create global variable, that holds pointer to replaced original function.
• Write global function, that contains interceptor logic.
• Write same function signature/name multiple times (DRY).
• Use C-style casts or MACRO to call original function from the interceptor.
• Manually create and destroy auxiliary context (i.e. disassembler backend) required for hooking.

That's really annoying. I want to express my intentions in a single expression without boilerplate, code repetitions and ugly C-style code.

So I ended up with developing my own library - rcmp.

Cross-platform and compiler support

At work I need both windows (.dll) and linux (.so) support, but most libraries aren't cross-platform (some of them also use compiler-specific extensions, that's not portable). Moreover, there are moddable games for Oculus Quest VR that works on ARM64 architecture. rcmp was designed to be easily extendable, so I was able to use it even for Android apps (ARM64 support comes soon!).

Modern C++ and canonical code

Most libraries use not-so-modern C++ standards (C++11 and below), so they have limited capabilities. Modern C++ features allow developer to write compact and type-safe code without boilerplate and repetitions (especially in case of hooking). Due to C++17, rcmp has convenient API as well as minimalistic and readable implementation.

Dependencies

rcmp has single-header bundled lightweight dependency - nmd by Nomade040 (only as a length disassembler for x86/x86-64). Most of hooking libraries depend on big, verbose or even deprecated frameworks.

Build & Install

rcmp can be easily added to any cmake-based project. No external requirements or dependencies, no installation or manual non-trivial actions to build - just add two lines in CMakeLists.txt.

Missing features

• No documentation (yet)
• No way to disable hook
• No ellipsis (...) support

References

• catch2 for unit-testing
• nmd library for x86/x86-64 length disassembly
• x86 and x86-64 opcode and instruction reference

License

• MIT