For what it is worth, BSD/OS's "compat setreuid" code fakes the
   whole thing out:
   
           /*
            * If requested ruid == euid, this is effectively a call to setuid.
            */
   
   and:
   
           /*
            * Partially emulate old setreuid usage of exchanging ruid and euid.
            * We assume that the intent of setting ruid is to be able to get
            * back setuid privilege. So we make sure that we will be able to
            * do so, but do not actually set the ruid.
            */
   
   Other setreuid() syscalls are rejected; this last one is turned into
   a seteuid() as needed.


this is the scheme that mycroft replaced with the real syscalls again:

revision 1.35
date: 1996/06/23 11:04:11;  author: mycroft;  state: Exp;  lines: +89 -1
Implement setre[ug]id() compatibly with 4.3BSD, SunOS, and Linux.