I wrote a script which forks an unpriviledged child as it shown in perlipc example and found out that after a successul socketpair() call there is an error recorded in $!:

#!/usr/bin/perl -w

use strict;
use IO::Socket;

print "Before socketpair() call: $!\n";
my @s = IO::Socket->socketpair(AF_UNIX, SOCK_STREAM, PF_UNIX)
    or die "socketpair: $!\n";
print "After socketpair() call: $!\n";
[download]

It prints out:

Before socketpair() call: 
After socketpair() call: Illegal seek
[download]

Despite socketpair() is successful (sockets work as expected and the script does not die) an "Illegal seek" is recorded in $! so I had to explicitly say $!=undef after the call to get rid of it or use a block with "local $!;".

Usually it is not a problem as most errors are reported in some other way (e.g., func() or die "$!") but when I need to change process uid/gid the only way to check for error is to see if $! is set, and if $! stays littered with an error from socketpair() the check fails:

    $) = $run_gid;
    die "Setting egid: $!" if $!;
[download]

I tried to see if I'm doing anything wrong so I tried strace'ing my script, to get the following:

socketpair(PF_LOCAL, SOCK_STREAM, 1, [3, 4]) = 0
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGET
+S, 0x7fff3ff30d00) = -1 ENOTTY (Inappropriate ioctl for device)
lseek(3, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGET
+S, 0x7fff3ff30d00) = -1 ENOTTY (Inappropriate ioctl for device)
lseek(3, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
ioctl(4, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGET
+S, 0x7fff3ff30d00) = -1 ENOTTY (Inappropriate ioctl for device)
lseek(4, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
ioctl(4, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGET
+S, 0x7fff3ff30d00) = -1 ENOTTY (Inappropriate ioctl for device)
lseek(4, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
fcntl(4, F_SETFD, FD_CLOEXEC)           = 0
[download]

As we see, the system socketpair() call is followed by two lseek() calls somewhere inside Perl interpreter, which fail with ESPIPE, which is recorded in $!, but socketpair() still returns success.

Any insights on how to do this (socketpair/fork/drop privileges) properly?

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