How to build shellcode on Windows, for Windows?

Beginners
1

Hi,
I’ve downloaded the latest LLVM 18 release from GitHub, tried to make a simple cmd script to make a .bin file, but that didn’t work:

code.c:

typedef void *(__stdcall *GetProcAddress_t)(void *hModule, const char *symbol);
typedef int (__stdcall *MessageBoxA_t)(void *hWnd, const char *text, const char *cap, unsigned type);

// ideally should be the first thing in the file (offs=0)
void _start(GetProcAddress_t gpa) {
    char user32[] = "user32.dll"; // will be preloaded before this is called
    char msgboxa[] = "MessageBoxA";
    char text[] = "hi";
    char cap[] = "lolwut";
    MessageBoxA_t MessageBoxA = gpa(user32, msgboxa);
    MessageBoxA(0, text, cap, 0);
}

make.cmd:

@echo off
set LLVM_PATH=D:\SDK\LLVM1812\bin

set CC=%LLVM_PATH%\clang
set LD=%LLVM_PATH%\ld.lld

%CC% code.c -v -nostdlib -target x86_64-windows-gnu -fPIC -fpie -fuse-ld=lld -o code.bin
:: %LD% code.o --oformat binary -o code.bin

clang complains about “mainCRTStartup”, which is the entrypoint of MSVC applications, ok, I can always rename _start to that. Even though I don’t really care about any MSVC libraries here, nor do I need a CRT in this payload.
But even after renaming, lld complains that “oformat” is apparently an unknown argument, even though it’s listed in --help. When I try to call lld separately of clang (through -c and then lld) it complains that the input format of an object isn’t known.
Is there a way to force clang into generating flat raw binary code files for Win64?

2

%CC% code.c -v -nostdlib -target x86_64-windows-gnu -fPIC -fpie -fuse-ld=lld -e _start -o code.bin