Access Violation Exception (5) in function of injected DLL

Question

I am extending the functionality of a Windows component using code injection. I overwrite a method's instructions, calling my own method that does the job of the original one. Let's say we have:

void Target(HDC magic123)
{ ... }

Below are the first few instructions of the method:

push    rbx //
push    rbp // stores registers to recover later
...
sub     rsp, 0x260 // for all 7 pushes
...
mov     r12, [rsp+0x28] // stores a pointer to 'magic123'
...
...a lot more instructions

---

Immediately after mov r12, [rsp+0x28], I overwrite the next instructions with:

mov rcx, r12                // 1st parameter to pass to a called function goes in RCX
add rsp, 0x260              // restore the stack
push 0                      // create shadow space |EDIT: MISALIGNED STACK. WRONG.
mov rax, &DetouredFunction  // function in my injected DLL
call rax                    // call it with the HDC as parameter

My function in the DLL:

void DetouredFunction(uintptr_t hdcPointer)
{
uintptr_t hdcAddress = *(uintptr_t*)(hdcPointer); // convert pointer to address
HDC hdc = (HDC)hdcAddress; // create a HDC from the address


HBITMAP hBitmapWallpaper = (HBITMAP)LoadImage(NULL, L"C:\\Users\\<user>\\Desktop\\image.bmp", IMAGE_BITMAP, 0, 0, LR_LOADFROMFILE);
SelectObject(hdc, hBitmapWallpaper);
}

Everything works until I reach a call to 'LoadImage' in my injected DLL's function. It throws an 'Access Violation Exception (5)' trying to read a non-existing address 0xFFFFFFFFFFFFFFFF.

What is the problem? Please correct any of my comments above if they contain mistakes. Thank you!

Answer 1

My problem was misaligning the stack. It did not have any evident effect until another call to LoadImage was made.