Function do_link 

fn do_link(
    r: &Req<'_>,
    olddirfd: i32,
    old: &[u8],
    newdirfd: i32,
    new: &[u8],
    flags: i64,
) -> Result<Outcome, i32>

link/linkat. An empty old path with AT_EMPTY_PATH links the tracee’s descriptor via its /proc entry. The walk followed the old side when asked to, so the kernel is never asked to follow it again.