How to hook __usercall, __userpurge (__spoils) functions?

Question

Know anybody something about hooking __usercall type of functions? I hooking successfully __thiscall, __stdcall and __cdecl calls but this is enough for me.

Know anybody hooking library for __usercall's or how to hook this type of functions using translation to __stdcall or __cdecl?

Function what i must hook at first is:

int __usercall func<eax>(int a<eax>, int b<ecx>, int c, unsigned int d, signed int e);

Answer 1

Use a wrapper which will convert it to __stdcall.

int __stdcall func_hook_payload(int a, int b, int c, unsigned int d, signed int e);

// Wrapper for
// int __usercall func<eax>(int a<eax>, int b<ecx>, int c, unsigned int d, signed int e);
__declspec(naked) void func_hook()
{__asm{
    push ebp
    mov ebp, esp
    push dword ptr[ebp + 0x0C] // or just push e
    push dword ptr[ebp + 0x08] // d
    push dword ptr[ebp + 0x04] // c
    push ecx // b
    push eax // a
    call func_hook_payload
    leave
    ret // note: __usercall is cdecl-like
}}

Answer 2

When all else fails.. walk through it with a debugger.

In particular take note of these like the ESP when you enter the call, and then again just before the function returns..