permanently drop root privileges on modern Linux
The "canonical" way to do this was implemented by D.J.Bernstein in his 'setuidgid' code, which was originally used into his QMail program, nowadays included in 'daemontools'.
The actual code used in GNU coreutils is based on DJB's description of the procedure, its code is visible here https://github.com/wertarbyte/coreutils/blob/master/src/setuidgid.c