Function clone 

pub unsafe fn clone(
    flags: usize,
    stack: usize,
    parent_tid: usize,
    child_tid: usize,
    tls: usize,
) -> isize

clone with its arguments in the kernel’s order

Safety

See syscall6; with a new stack or CLONE_VM the child must not return from here (a thread’s start is assembly: linux_thread.rs).