glue between nasm and exec

stub.rs

use std::io::Write;
use std::process::Command;

extern "C" {
    fn mprotect(addr: *const u8, len: usize, prot: u32) -> u32;
}

fn main() {
    let mut args = std::env::args();
    let _ = args.next();
    let fname = if let Some(n) = args.next() {
        n
    } else {
        return;
    };
    let res = Command::new("/usr/bin/nasm")
        .args(["-o", "/dev/stdout", "-f", "bin", &fname])
        .output()
        .expect("process runs");

    if res.status.success() {
        unsafe {
            let code = res.stdout.as_ptr();
            let len = res.stdout.len();
            // round down because we might be offset in the first page
            let prefix = (code as isize) & 0x0fff;

            let pages_start = code.offset(-prefix);
            let code_end = len + prefix as usize;

            // rwx
            if mprotect(pages_start, code_end, 7) != 0 {
                return;
            }

            let f: fn() -> ! = std::mem::transmute::<*const u8, fn() -> !>(code);
            f();
        }
    } else {
        let mut out = std::io::stdout();
        out.write_all(&res.stdout).unwrap();
        out.flush().unwrap();
        let mut err = std::io::stderr();
        err.write_all(&res.stderr).unwrap();
        err.flush().unwrap();
    }
}