Possible to hook an address and directly write to it?

Question

I am reversing a game for fun and currently use this function to manipulate the Y coordinate:

void __declspec(naked) hookYCoordASM() {
    __asm {
        mov eax, valueY
        mov [esi + 0x7C], eax
        jmp[returnYCoord]
    }
}

Is it possible to safe the address of esi+0x7C in a variable of mine (for example valueY) and write to that saved address after wards?

Answer 1

Yes, you can. Like this:

uint32_t some_value;
uint32_t ptr_to_some_value; // eax is always 32-bit
void __declspec(naked) hookYCoordASM() {
    __asm {
        // #1: to get the value
        mov eax, [esi + 0x7C];
        mov some_value, eax;

        // #2: to get a pointer to the value
        lea eax, [esi + 0x7C];
        mov ptr_to_some_value, eax;

        mov eax, valueY
        mov [esi + 0x7C], eax
        jmp[returnYCoord]
    }
}

With that being said, I have to clarify this: you're arbitrarily overriding the value of eax. If the hooked function does not use it, then you're good, but whenever you hook a function which does use it, and you override it, everything will probably break. To avoid this, preserve eax, like this:

void __declspec(naked) hookYCoordASM() {
    __asm {
        // save eax for later
        push eax;

        // ...
        // do stuff
        // ...

        // restore eax
        pop eax;

        // ...
        jmp[returnYCoord]
    }
}