setuidgid w. support for supplementary groups

setusergroups

#! /usr/bin/perl

use POSIX qw(setuid setgid);
use Unix::Groups qw(setgroups);

die "usage: setusergroups username child\n"
    unless @ARGV >= 2;
my $username = shift @ARGV;

# get user entry
my @userent = getpwnam($username)
    or die "unknown user: $username\n";

# build list of supp. groups
my @supp_groups;
while (my @e = getgrent) {
    if (grep { $_ eq $username } split /\s+/, $e[3]) {
        push @supp_groups, $e[2];
    }
}

# setgid
setgid($userent[3])
    or die "setgid failed:$!";

# setgroups!
setgroups(@supp_groups)
    or die "setgroups failed:$!";

# setuid
setuid($userent[2])
    or die "setuid failed:$!";

# exec
exec @ARGV
    or die "failed to exec: $ARGV[0]:$!";