visual-c++,assembly,macros , Macro to push arguments onto stack


Macro to push arguments onto stack

Question:

Tag: visual-c++,assembly,macros

I've been working on a fun little project to mess around with how functions are called and I need a macro to push arguments as it'll be quite time consuming to push the arguments manually for every instance of this obfuscated call.

This is my code so far:

#define pushargs(...)                      \
    // idk what to do here...

#define ObfuscatedPrefix(FunctionPtr)      \
    __asm cmp ebx, FunctionPtr             \
    __asm _emit 0x74 __asm _emit 0x02      \
    __asm _emit 0xEB __asm _emit 0x03      \
    __asm mov [ebp + 4], eax

#define DecryptOffset(FunctionPtr, V)      \
    ((FunctionPtr) - (V))

#define ObfuscatedCall(FunctionPtr, V, ...)\
    __asm push ebx                         \
    __asm push eax                         \
    __asm mov ebx, FunctionPtr             \
    __asm mov eax, V                       \
    __asm sub ebx, eax                     \
    pushargs(__VA_ARGS__)                  \
    __asm _emit 0xEB __asm _emit 0x02      \
    __asm _emit 0xEB __asm _emit 0x05      \
    __asm _emit 0xE8 __asm _emit 0xF9      \
    __asm _emit 0xFF __asm _emit 0xFF      \
    __asm _emit 0xFF __asm pop eax         \
    __asm add eax, 0x07                    \
    __asm jmp ebx                          \
    __asm pop ebx

I feel like this isn't possible, please correct me if I'm wrong...


Answer:

If you are still interested in tricking the Visual C++ compiler you can try this code

#define pushargs(...) for (unsigned int _esp; ;) _esp = varcount(), _esp =(_esp-varcount(__VA_ARGS__))>>2, pushargs_c(_esp, __VA_ARGS__ ); 



unsigned int __declspec(naked) __forceinline varcount(...)
{
    __asm mov eax, esp;
    __asm ret;
}

unsigned int __declspec(naked) __forceinline pushargs_c(unsigned int count, ...)
{
    __asm mov eax, DWORD PTR [esp+04h]
    __asm dec eax
    __asm shl eax, 2
    __asm sub esp, eax
    __asm push DWORD PTR [esp+eax]
    __asm ret;
}

My version of Visual Studio (2013 express) doesn't inline the calls even with __forceinline.
This code is not portable and is implementation specific, I found it working with my VC++ compiler, even in Release mode but may breaks under heavy optimizations or in new Visual Studio versions!

Give it a try, you can use the pushargs macro as you expect. pushargs_c is more reliable but require the first argument to be the number of the remaining arguments (i.e. pushargs_c(5, 1,2,3,4,5)).


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