format PE GUI 4.0

include 'c:\fasm\include\win32ax.inc'

.data
 testvar db "test starin", 0
 oldMessageBox  dd 0
 calc           dd 0
 addrs          dd 0
 tjmp           db 233, 0, 0 , 0, 0, 195


.code

proc HookGeneralFunction, Dll, FuncName, Function, backup

        invoke  GetModuleHandle, [Dll]
        invoke  GetProcAddress, eax, addr FuncName
       ; BYTE tjmp[6] = { 0xe9,                   //jmp
        ;        0x00, 0x00, 0x00, 0x00,         //address
         ;       0xc3 };                         //retn

        invoke  GetCurrentProcess
        invoke  ReadProcessMemory, eax, addr addrs, [backup], 6, 0


        mov     [calc], 0
        mov     eax, [Function]
        mov     [calc], eax
        mov     eax, [addrs]
        sub     [calc], eax
        sub     [calc], 5         ; //((to)-(from)-5)

      ;  memcpy(&jmp[1], &calc, 4); //build the jmp
        invoke  GetCurrentProcess
        invoke  WriteProcessMemory, eax, addr addrs, tjmp, 6, 0

        mov eax, addrs
endp

proc newMessageBox, hWnd, lpText, lpCaption, uType


endp


start:

  stdcall       HookGeneralFunction, "user32.dll", "MessageBoxA", newMessageBox,oldMessageBox
.end start    

//this will hook a process and all of it`s modules (loaded DLLs)
//by writting to kernel`s area
DWORD HookGeneralFunction(const char *Dll, const char *FuncName, void *Function, unsigned char *backup)
{
 DWORD addr = (DWORD)GetProcAddress(GetModuleHandle(Dll), FuncName);
 BYTE jmp[6] = { 0xe9,                      //jmp
               0x00, 0x00, 0x00, 0x00,         //address
           0xc3 };                                            //retn

  ReadProcessMemory(GetCurrentProcess(), (void*)addr, backup, 6, 0);
    DWORD calc = ((DWORD)Function - addr - 5); //((to)-(from)-5)
      memcpy(&jmp[1], &calc, 4); //build the jmp
  WriteProcessMemory(GetCurrentProcess(), (void*)addr, jmp, 6, 0);

        return addr;
}