#! /usr/bin/perl -w -T

use strict;
use Cwd;

$ENV{PATH} = "/usr/local/bin:/usr/bin:/bin";
delete($ENV{BASH_ENV});

#500 is my non-root uid that I want to test with;
my $newuid = 500;

unless ($< == 0 and $> == 0) {
        die "must be root";
}

system('whoami');
print getcwd."\n";
$> = $< = $newuid ;
system('whoami');
print getcwd."\n";
[download]

Oh grief, I misinterpreted the error to be the result of the "$<" operation. The actual problem is the fact that my now lesser-privileged user is not authorised to perform the next "$< = $> = $newid" iteration.

Hmm, my question should actually read: "How do I perform seteid()" or "How do I temporarily drop my privileges before calling a function? ". The obvious answer is $>. *hides in shame*

-0xbeef

If anyone's interested, here's a snippet with the basic working logic. Although the "id -G" portion is not the safest way of doing this, I just don't know how:

my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,$atime,$mtime,$ctime,
+$blksize,$blocks) = stat($targetdir);
$newuser =  getpwuid($uid);
$groups =  `/usr/bin/id -G $newuser`;
$) = "$groups";
$> = $uid;
if ( not chdir("$targetdir")) {
  print "$targetdir is inaccessible to its owner\n";
  next;
} else {
  $currentdir = Cwd::getcwd();
  my $lasterr = $!;
  if (not defined $currentdir) {
    # incorrect underlying mount-point permissions
    print "getcwd() failed for $targetdir:$lasterr\n";
  } else {
    print "getcwd() for $over is OK.");
  }
}
[download]