Shellcode on a setuid program (2/?)

Resuming learning process of previous episode.

A fellow hacker suggested that dash also supports 'privmode', a
feature that resets euid if different from uid, disabled
with `-p`.
https://sources.debian.org/patches/dash/0.5.12-2/9002-Add-privmode-Part-2.diff/

Good to know...
Still this is not the problem, running id(1) via
execve("/bin/id") does not list the user who owns the setuid
binary.  And there's no shell interpreter invoked here.