weird.rs

// libuser syscalls:
/// Creates a thread in the current process.
pub fn create_thread(ip: fn() -> !, arg: usize, sp: *const u8, _priority: u32, _processor_id: u32) -> Result<Thread, KernelError> {
    unsafe {
        let (out_handle, ..) = syscall(nr::CreateThread, ip as usize, arg, sp as _, _priority as _, _processor_id as _, 0)?;
        Ok(Thread(Handle::new(out_handle as _)))
    }
}

// Function I'm writing:
fn test_threads() -> Terminal {
    fn thread_b(c: usize) -> ! {
        for _ in 0..10 {
                println!("{}", c);
                libuser::syscalls::sleep_thread(0);
            }
        }
        libuser::syscalls::exit_thread()
    }

    #[naked]
    fn function_wrapper() {
        unsafe {
            asm!("
            push eax
            call $0
            " :: "i"(thread_b) :: "intel");
        }
    }

    const THREAD_STACK_SIZE: usize = 0x2000;

    let terminal = Arc::new(Mutex::new(terminal));
    let stack = Box::new([0u8; THREAD_STACK_SIZE]);
    let sp = (Box::into_raw(stack) as *const u8).wrapping_offset(THREAD_STACK_SIZE as isize);
    let ip : fn() -> ! = unsafe {
        // Safety: This is changing the return type from () to !. It's safe. It
        // sucks though. This is, yet again, an instance of "naked functions are
        // fucking horrible".
        core::mem::transmute(function_wrapper) // THE BUG IS HERE
    };
    let thread_handle = libuser::syscalls::create_thread(ip, Arc::into_raw(terminal.clone()) as usize, sp, 0, 0)
        .expect("svcCreateThread returned an error");
    thread_handle.start()
        .expect("svcStartThread returned an error");

    // Wait for thread_b to terminate.
    loop {
        if let Ok(terminal) = Arc::try_unwrap(terminal) {
            break terminal.into_inner()
        }
        libuser::syscalls::sleep_thread(0);
    }
}