mmdebstrap/mmdebstrap

#!/usr/bin/perl
#
# Copyright: 2018 Johannes Schauer <josch@mister-muffin.de>
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to
# deal in the Software without restriction, including without limitation the
# rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
# sell copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.

use strict;
use warnings;

use English;
use Getopt::Long;
use Pod::Usage;
use File::Copy;
use File::Path qw(make_path remove_tree);
use File::Temp qw(tempfile tempdir);
use Cwd qw(abs_path);
require "syscall.ph";
use Fcntl qw(S_IFCHR S_IFBLK FD_CLOEXEC F_GETFD F_SETFD);
use List::Util qw(any none);

# from sched.h
use constant {
    CLONE_NEWNS   => 0x20000,
    CLONE_NEWUTS  => 0x4000000,
    CLONE_NEWIPC  => 0x8000000,
    CLONE_NEWUSER => 0x10000000,
    CLONE_NEWPID  => 0x20000000,
    CLONE_NEWNET  => 0x40000000,
};

# type codes:
#   0 -> normal file
#   1 -> hardlink
#   2 -> symlink
#   3 -> character special
#   4 -> block special
#   5 -> directory
my @devfiles = (
    # filename         mode  type link target        major  minor
    [ "./dev/",        0755, 5,   '',                undef, undef ],
    [ "./dev/console", 0666, 3,   '',                5,     1 ],
    [ "./dev/fd",      0777, 2,   '/proc/self/fd',   undef, undef ],
    [ "./dev/full",    0666, 3,   '',                1,     7 ],
    [ "./dev/null",    0666, 3,   '',                1,     3 ],
    [ "./dev/ptmx",    0666, 3,   '',                5,     2 ],
    [ "./dev/pts/",    0755, 5,   '',                undef, undef ],
    [ "./dev/random",  0666, 3,   '',                1,     8 ],
    [ "./dev/shm/",    0755, 5,   '',                undef, undef ],
    [ "./dev/stderr",  0777, 2,   '/proc/self/fd/2', undef, undef ],
    [ "./dev/stdin",   0777, 2,   '/proc/self/fd/0', undef, undef ],
    [ "./dev/stdout",  0777, 2,   '/proc/self/fd/1', undef, undef ],
    [ "./dev/tty",     0666, 3,   '',                5,     0 ],
    [ "./dev/urandom", 0666, 3,   '',                1,     9 ],
    [ "./dev/zero",    0666, 3,   '',                1,     5 ],
);

# tar cannot figure out the decompression program when receiving data on
# standard input, thus we do it ourselves. This is copied from tar's
# src/suffix.c
sub get_tar_compress_options($) {
    my $filename = shift;
    if ($filename =~ /\.(gz|tgz|taz)$/) {
	return ('--gzip');
    } elsif ($filename =~ /\.(Z|taZ)$/) {
	return ('--compress');
    } elsif ($filename =~ /\.(bz2|tbz|tbz2|tz2)$/) {
	return ('--bzip2');
    } elsif ($filename =~ /\.lz$/) {
	return ('--lzip');
    } elsif ($filename =~ /\.(lzma|tlz)$/) {
	return ('--lzma');
    } elsif ($filename =~ /\.lzo$/) {
	return ('--lzop');
    } elsif ($filename =~ /\.lz4$/) {
	return ('--use-compress-program', 'lz4');
    } elsif ($filename =~ /\.(xz|txz)$/) {
	return ('--xz');
    }
    return ();
}

sub test_unshare($) {
    my $verbose = shift;
    if ($EFFECTIVE_USER_ID == 0) {
	if ($verbose) {
	    print STDERR "E: cannot use unshare mode when executing as root\n";
	}
	return 0;
    }
    # arguments to syscalls have to be stored in their own variable or
    # otherwise we will get "Modification of a read-only value attempted"
    my $unshare_flags = CLONE_NEWUSER;
    # we spawn a new per process because if unshare succeeds, we would
    # otherwise have unshared the mmdebstrap process itself which we don't want
    my $pid = fork() // die "fork() failed: $!";
    if ($pid == 0) {
	my $ret = syscall &SYS_unshare, $unshare_flags;
	if ($ret == 0) {
	    exit 0;
	} else {
	    if ($verbose) {
		print STDERR "E: unshare syscall failed: $!\n";
	    }
	    exit 1;
	}
    }
    waitpid($pid, 0);
    if (($? >> 8) != 0) {
	return 0;
    }
    # if newuidmap and newgidmap exist, the exit status will be 1 when
    # executed without parameters
    system "newuidmap 2>/dev/null";
    if (($? >> 8) != 1) {
	if ($verbose) {
	    if (($? >> 8) == 127) {
		print STDERR "E: cannot find newuidmap\n";
	    } else {
		print STDERR "E: newuidmap returned unknown exit status\n";
	    }
	}
	return 0;
    }
    system "newgidmap 2>/dev/null";
    if (($? >> 8) != 1) {
	if ($verbose) {
	    if (($? >> 8) == 127) {
		print STDERR "E: cannot find newgidmap\n";
	    } else {
		print STDERR "E: newgidmap returned unknown exit status\n";
	    }
	}
	return 0;
    }
    return 1;
}

sub read_subuid_subgid() {
    my $username = getpwuid $<;
    my ($subid, $num_subid, $fh, $n);
    my @result = ();

    if (! -e "/etc/subuid") {
	printf STDERR "/etc/subuid doesn't exist\n";
	return;
    }
    if (! -r "/etc/subuid") {
	printf STDERR "/etc/subuid is not readable\n";
	return;
    }

    open $fh, "<", "/etc/subuid" or die "cannot open /etc/subuid for reading: $!";
    while (my $line = <$fh>) {
	($n, $subid, $num_subid) = split(/:/, $line, 3);
	last if ($n eq $username);
    }
    close $fh;
    push @result, ["u", 0, $subid, $num_subid];

    if (scalar(@result) < 1) {
	printf STDERR "/etc/subuid does not contain an entry for $username\n";
	return;
    }
    if (scalar(@result) > 1) {
	printf STDERR "/etc/subuid contains multiple entries for $username\n";
	return;
    }

    open $fh, "<", "/etc/subgid" or die "cannot open /etc/subgid for reading: $!";
    while (my $line = <$fh>) {
	($n, $subid, $num_subid) = split(/:/, $line, 3);
	last if ($n eq $username);
    }
    close $fh;
    push @result, ["g", 0, $subid, $num_subid];

    if (scalar(@result) < 2) {
	printf STDERR "/etc/subgid does not contain an entry for $username\n";
	return;
    }
    if (scalar(@result) > 2) {
	printf STDERR "/etc/subgid contains multiple entries for $username\n";
	return;
    }

    return @result;
}

# This function spawns two child processes forming the following process tree
#
#   A
#   |
#  fork()
#   |  \
#   B   C
#   |   |
#   |  fork()
#   |   |  \
#   |   D   E
#   |   |   |
#   |unshare()
#   | close()
#   |   |   |
#   |   | read()
#   |   | newuidmap(D)
#   |   | newgidmap(D)
#   |   |  /
#   | waitpid()
#   |   |
#   |  fork()
#   |   |  \
#   |   F   G
#   |   |   |
#   |   |   exec()
#   |   |  /
#   | waitpid()
#   |  /
# waitpid()
#
# To better refer to each individual part, we give each process a new
# identifier after calling fork(). Process A is the main process. After
# executing fork() we call the parent and child B and C, respectively. This
# first fork() is done because we do not want to modify A. B then remains
# waiting for its child C to finish. C calls fork() again, splitting into
# the parent D and its child E. In the parent D we call unshare() and close a
# pipe shared by D and E to signal to E that D is done with calling unshare().
# E notices this by using read() and follows up with executing the tools
# new[ug]idmap on D. E finishes and D continues with doing another fork().
# This is because when unsharing the PID namespace, we need a PID 1 to be kept
# alive or otherwise any child processes cannot fork() anymore themselves. So
# we keep F as PID 1 and finally call exec() in G.
sub get_unshare_cmd(&$) {
    my $cmd = shift;
    my $idmap = shift;

    my $unshare_flags = CLONE_NEWUSER | CLONE_NEWNS | CLONE_NEWPID | CLONE_NEWUTS | CLONE_NEWIPC;

    if (0) {
	$unshare_flags |= CLONE_NEWNET;
    }

    # fork a new process and let the child get unshare()ed
    # we don't want to unshare the parent process
    my $gcpid = fork() // die "fork() failed: $!";
    if ($gcpid == 0) {
	# Create a pipe for the parent process to signal the child process that it is
	# done with calling unshare() so that the child can go ahead setting up
	# uid_map and gid_map.
	pipe my $rfh, my $wfh;

	# We have to do this dance with forking a process and then modifying the
	# parent from the child because:
	#  - new[ug]idmap can only be called on a process id after that process has
	#    unshared the user namespace
	#  - a process looses its capabilities if it performs an execve() with nonzero
	#    user ids see the capabilities(7) man page for details.
	#  - a process that unshared the user namespace by default does not have the
	#    privileges to call new[ug]idmap on itself
	#
	# this also works the other way around (the child setting up a user namespace
	# and being modified from the parent) but that way, the parent would have to
	# stay around until the child exited (so a pid would be wasted). Additionally,
	# that variant would require an additional pipe to let the parent signal the
	# child that it is done with calling new[ug]idmap. The way it is done here,
	# this signaling can instead be done by wait()-ing for the exit of the child.
	my $ppid = $$;
	my $cpid = fork() // die "fork() failed: $!";
	if ($cpid == 0) {
	    # child

	    # Close the writing descriptor at our end of the pipe so that we
	    # see EOF when parent closes its descriptor.
	    close $wfh;

	    # Wait for the parent process to finish its unshare() call by
	    # waiting for an EOF.
	    0 == sysread $rfh, my $c, 1 or die "read() did not receive EOF";

	    # The program's new[ug]idmap have to be used because they are
	    # setuid root.  These privileges are needed to map the ids from
	    # /etc/sub[ug]id to the user namespace set up by the parent.
	    # Without these privileges, only the id of the user itself can be
	    # mapped into the new namespace.
	    #
	    # Since new[ug]idmap is setuid root we also don't need to write
	    # "deny" to /proc/$$/setgroups beforehand (this is otherwise
	    # required for unprivileged processes trying to write to
	    # /proc/$$/gid_map since kernel version 3.19 for security reasons)
	    # and therefore the parent process keeps its ability to change its
	    # own group here.
	    #
	    # Since /proc/$ppid/[ug]id_map can only be written to once,
	    # respectively, instead of making multiple calls to new[ug]idmap,
	    # we assemble a command line that makes one call each.
	    my $uidmapcmd = "";
	    my $gidmapcmd = "";
	    foreach (@{$idmap}) {
		my ($t, $hostid, $nsid, $range) = @{$_};
		if ($t ne "u" and $t ne "g" and $t ne "b") {
		    die "invalid idmap type: $t";
		}
		if ($t eq "u" or $t eq "b") {
		    $uidmapcmd .= " $hostid $nsid $range";
		}
		if ($t eq "g" or $t eq "b") {
		    $gidmapcmd .= " $hostid $nsid $range";
		}
	    }
	    my $idmapcmd = '';
	    if ($uidmapcmd ne "") {
		0 == system "newuidmap $ppid $uidmapcmd" or die "newuidmap $ppid $uidmapcmd failed: $!";
	    }
	    if ($gidmapcmd ne "") {
		0 == system "newgidmap $ppid $gidmapcmd" or die "newgidmap $ppid $gidmapcmd failed: $!";
	    }
	    exit 0;
	}

	# parent

	# After fork()-ing, the parent immediately calls unshare...
	0 == syscall &SYS_unshare, $unshare_flags or die "unshare() failed: $!";

	# .. and then signals the child process that we are done with the
	# unshare() call by sending an EOF.
	close $wfh;

	# Wait for the child process to finish its setup by waiting for its
	# exit.
	$cpid == waitpid $cpid, 0 or die "waitpid() failed: $!";
	my $exit = $? >> 8;
	if ($exit != 0) {
	    die "child had a non-zero exit status: $exit";
	}

	# Currently we are nobody (uid and gid are 65534). So we become root
	# user and group instead.
	#
	# We are using direct syscalls instead of setting $(, $), $< and $>
	# because then perl would do additional stuff which we don't need or
	# want here, like checking /proc/sys/kernel/ngroups_max (which might
	# not exist). It would also also call setgroups() in a way that makes
	# the root user be part of the group unknown.
	0 == syscall &SYS_setgid, 0 or die "setgid failed: $!";
	0 == syscall &SYS_setuid, 0 or die "setuid failed: $!";
	0 == syscall &SYS_setgroups, 0, 0 or die "setgroups failed: $!";

	if (1) {
	    # When the pid namespace is also unshared, then processes expect a
	    # master pid to always be alive within the namespace. To achieve
	    # this, we fork() here instead of exec() to always have one dummy
	    # process running as pid 1 inside the namespace. This is also what
	    # the unshare tool does when used with the --fork option.
	    #
	    # Otherwise, without a pid 1, new processes cannot be forked
	    # anymore after pid 1 finished.
	    my $cpid = fork() // die "fork() failed: $!";
	    if ($cpid != 0) {
		# The parent process will stay alive as pid 1 in this
		# namespace until the child finishes executing. This is
		# important because pid 1 must never die or otherwise nothing
		# new can be forked.
		$cpid == waitpid $cpid, 0 or die "waitpid() failed: $!";
		exit ($? >> 8);
	    }
	}

	&{$cmd}();

	exit 0;
    }

    # parent
    return $gcpid;
}

sub havemknod($) {
    my $root = shift;
    my $havemknod = 0;
    if (-e "$root/test-dev-null") {
	die "/test-dev-null already exists";
    }
    TEST: {
	# we fork so that we can read STDERR
	my $pid = open my $fh, '-|' // die "failed to fork(): $!";
	if ($pid == 0) {
	    open(STDERR, '>&', STDOUT);
	    # we use mknod(1) instead of the system call because creating the
	    # right dev_t argument requires makedev(3)
	    exec 'mknod', "$root/test-dev-null", 'c', '1', '3';
	}
	chomp (my $content = do { local $/; <$fh> });
	close $fh;
	{
	    last TEST unless $? == 0 and $content eq '';
	    last TEST unless -c "$root/test-dev-null";
	    last TEST unless open my $fh, '>', "$root/test-dev-null";
	    last TEST unless print $fh 'test';
	}
	$havemknod = 1;
    }
    if (-e "$root/test-dev-null") {
	unlink "$root/test-dev-null" or die "cannot unlink /test-dev-null";
    }
    return $havemknod;
}

sub print_progress {
    my $perc = shift;
    if (!-t STDERR) {
	return;
    }
    if ($perc eq "done") {
	# \e[2K clears everything on the current line (i.e. the progress bar)
	print STDERR "\e[2Kdone\n";
	return;
    }
    if ($perc >= 100) {
	$perc = 100;
    }
    my $width = 50;
    my $num_x = int($perc*$width/100);
    my $bar = '=' x $num_x;
    if ($num_x != $width) {
	$bar .= '>';
	$bar .= ' ' x ($width - $num_x - 1);
    }
    printf STDERR "%6.2f [%s]\r", $perc, $bar;
}

sub run_dpkg_progress {
    my $options = shift;
    my @args = @{$options->{ARGV}};
    my @debs = @{$options->{PKGS}};
    my $verbose = $options->{VERBOSE} // 0;
    pipe my $rfh, my $wfh;
    my $dpkgpid = open (my $pipe_dpkg, '-|') // die "failed to fork(): $!";
    if ($dpkgpid == 0) {
	close $rfh;
	# Unset the close-on-exec flag, so that the file descriptor does not
	# get closed when we exec dpkg
	my $flags = fcntl( $wfh, F_GETFD, 0 ) or die "fcntl F_GETFD: $!";
	fcntl($wfh, F_SETFD, $flags & ~FD_CLOEXEC ) or die "fcntl F_SETFD: $!";
	my $fd = fileno $wfh;
	# redirect dpkg's stderr to stdout so that we can capture it
	open(STDERR, '>&', STDOUT);
	my @execargs = (@args, "--status-fd=$fd", @debs);
	exec { $args[0] } @execargs;
	die 'cannot exec() ' . (join ' ', @execargs);
    }
    close $wfh;

    # spawn two processes:
    #   parent will parse stdout
    #   child will parse $rfh for the progress meter
    my $pid = fork() // die "failed to fork(): $!";
    if ($pid == 0) {
	# child
	print_progress 0.0 if not $verbose;
	my $num = 0;
	# each package has one install and one configure step, thus the total
	# number is twice the number of packages
	my $total = (scalar @debs) * 2;
	while (my $line = <$rfh>) {
	    if ($line =~ /^processing: (install|configure): /) {
		$num += 1;
	    }
	    my $perc = $num/$total*100;
	    print_progress $perc if not $verbose;
	}
	print_progress "done" if not $verbose;

	exit 0;
    }

    # parent
    my $dpkg_output = '';
    while (my $line = <$pipe_dpkg>) {
	if ($verbose) {
	    print STDERR $line;
	} else {
	    # forward captured apt output
	    $dpkg_output .= $line;
	}
    }
    close $pipe_dpkg;
    my $fail = 0;
    if ($? != 0) {
	$fail = 1;
    }

    waitpid $pid, 0;
    $? == 0 or die "progress parsing failed";

    # only print apt failure after progress output finished or otherwise it
    # might interfere
    if ($fail) {
	print STDERR $dpkg_output;
	die ((join ' ', @args, @debs) . ' failed');
    }
}

sub run_apt_progress {
    my $options = shift;
    my @args = @{$options->{ARGV}};
    my @debs = @{$options->{PKGS} // []};
    my $verbose = $options->{VERBOSE} // 0;
    pipe my $rfh, my $wfh;
    my $aptpid = open(my $pipe_apt, '-|') // die "failed to fork(): $!";
    if ($aptpid == 0) {
	close $rfh;
	# Unset the close-on-exec flag, so that the file descriptor does not
	# get closed when we exec apt-get
	my $flags = fcntl( $wfh, F_GETFD, 0 ) or die "fcntl F_GETFD: $!";
	fcntl($wfh, F_SETFD, $flags & ~FD_CLOEXEC ) or die "fcntl F_SETFD: $!";
	my $fd = fileno $wfh;
	# redirect apt's stderr to stdout so that we can capture it
	open(STDERR, '>&', STDOUT);
	my @execargs = (@args, "-oAPT::Status-Fd=$fd", @debs);
	exec { $args[0] } @execargs;
	die 'cannot exec() ' . (join ' ', @execargs);
    }
    close $wfh;

    # spawn two processes:
    #   parent will parse stdout to look for errors
    #   child will parse $rfh for the progress meter
    my $pid = fork() // die "failed to fork(): $!";
    if ($pid == 0) {
	# child
	print_progress 0.0 if not $verbose;
	while (my $line = <$rfh>) {
	    if ($line =~ /(pmstatus|dlstatus):[^:]+:(\d+\.\d+):.*/) {
		print_progress $2 if not $verbose;
	    }
	    #print STDERR "apt: $line";
	}
	print_progress "done" if not $verbose;

	exit 0;
    }

    # parent
    my $has_error = 0;
    # apt-get doesn't report a non-zero exit if the update failed. Thus, we
    # have to parse its output. See #778357, #776152, #696335 and #745735
    my $apt_output = '';
    while (my $line = <$pipe_apt>) {
	if ($line =~ '^W: ') {
	    $has_error = 1;
	} elsif ($line =~ '^Err:') {
	    $has_error = 1;
	}
	if ($verbose) {
	    print STDERR $line;
	} else {
	    # forward captured apt output
	    $apt_output .= $line;
	}
    }
    close($pipe_apt);
    my $fail = 0;
    if ($? != 0 or $has_error) {
	$fail = 1;
    }

    waitpid $pid, 0;
    $? == 0 or die "progress parsing failed";

    # only print apt failure after progress output finished or otherwise it
    # might interfere
    if ($fail) {
	print STDERR $apt_output;
	die ((join ' ', @args, @debs) . ' failed');
    }
}

sub setup {
    my $options = shift;

    if (0) {
	foreach my $key (sort keys %{$options}) {
	    my $value = $options->{$key};
	    if (!defined $value) {
		next;
	    }
	    if (ref $value eq '') {
		print STDERR "I: $key: $options->{$key}\n";
	    } elsif (ref $value eq 'ARRAY') {
		print STDERR "I: $key: [" . (join ', ', @{$value}) . "]\n";
	    } else {
		die "unknown type";
	    }
	}
    }

    my ($conf, $tmpfile) = tempfile(UNLINK => 1) or die "cannot open apt.conf: $!";
    print $conf "Apt::Architecture \"$options->{nativearch}\";\n";
    # the host system might have configured additional architectures
    # force only the native architecture
    if (scalar @{$options->{foreignarchs}} > 0) {
	print $conf "Apt::Architectures { \"$options->{nativearch}\"; ";
	foreach my $arch (@{$options->{foreignarchs}}) {
	    print $conf "\"$arch\"; ";
	}
	print $conf "};\n";
    } else {
	print $conf "Apt::Architectures \"$options->{nativearch}\";\n";
    }
    print $conf "Dir::Etc \"$options->{root}/etc/apt\";\n";
    print $conf "Dir::State \"$options->{root}/var/lib/apt\";\n";
    print $conf "Dir::Cache \"$options->{root}/var/cache/apt\";\n";
    print $conf "Dir::Log \"$options->{root}/var/log/apt\";\n";
    # for authentication, use the keyrings from the host
    print $conf "Dir::Etc::Trusted \"/etc/apt/trusted.gpg\";\n";
    print $conf "Dir::Etc::TrustedParts \"/etc/apt/trusted.gpg.d\";\n";
    close $conf;

    {
	my @directories = ('/etc/apt/apt.conf.d', '/etc/apt/sources.list.d',
	    '/etc/apt/preferences.d', '/var/cache/apt',
	    '/var/lib/apt/lists/partial', '/var/lib/dpkg',
	    '/etc/dpkg/dpkg.cfg.d/');
	# if dpkg and apt operate from the outside we need some more
	# directories because dpkg and apt might not even be installed inside
	# the chroot
	if ($options->{mode} eq 'chrootless') {
	    push @directories, ('/var/log/apt', '/var/lib/dpkg/triggers',
		'/var/lib/dpkg/info', '/var/lib/dpkg/alternatives',
		'/var/lib/dpkg/updates');
	}
	foreach my $dir (@directories) {
	    make_path("$options->{root}/$dir") or die "failed to create $dir: $!";
	}
    }

    # We put certain configuration items in their own configuration file
    # because they have to be valid for apt invocation from outside as well as
    # from inside the chroot.
    # The config filename is chosen such that any settings in it will be
    # overridden by what the user specified with --aptopt.
    {
	open my $fh, '>', "$options->{root}/etc/apt/apt.conf.d/00mmdebstrap" or die "cannot open /etc/apt/apt.conf.d/00mmdebstrap: $!";
	print $fh "Apt::Install-Recommends false;\n";
	print $fh "Acquire::Languages \"none\";\n";
	close $fh;
    }

    {
	open my $fh, '>', "$options->{root}/var/lib/dpkg/status" or die "failed to open(): $!";
	close $fh;
    }

    # /var/lib/dpkg/available is required to exist or otherwise package
    # removals will fail
    {
	open my $fh, '>', "$options->{root}/var/lib/dpkg/available" or die "failed to open(): $!";
	close $fh;
    }

    if (scalar @{$options->{foreignarchs}} > 0) {
	open my $fh, '>', "$options->{root}/var/lib/dpkg/arch" or die "cannot open /var/lib/dpkg/arch: $!";
	print $fh "$options->{nativearch}\n";
	foreach my $arch (@{$options->{foreignarchs}}) {
	    print $fh "$arch\n";
	}
	close $fh;
    }

    if (scalar @{$options->{aptopts}} > 0) {
	open my $fh, '>', "$options->{root}/etc/apt/apt.conf.d/99mmdebstrap" or die "cannot open /etc/apt/apt.conf.d/99mmdebstrap: $!";
	foreach my $opt (@{$options->{aptopts}}) {
	    if (-r $opt) {
		copy $opt, $fh or die "cannot copy $opt: $!";
	    } else {
		print $fh $opt;
		if ($opt !~ /;$/) {
		    print $fh ';';
		}
		if ($opt !~ /\n$/) {
		    print $fh "\n";
		}
	    }
	}
	close $fh;
    }

    if (scalar @{$options->{dpkgopts}} > 0) {
	# FIXME: in chrootless mode, dpkg will only read the configuration
	# from the host
	open my $fh, '>', "$options->{root}/etc/dpkg/dpkg.cfg.d/99mmdebstrap" or die "cannot open /etc/dpkg/dpkg.cfg.d/99mmdebstrap: $!";
	foreach my $opt (@{$options->{dpkgopts}}) {
	    if (-r $opt) {
		copy $opt, $fh or die "cannot copy $opt: $!";
	    } else {
		print $fh $opt;
		if ($opt !~ /\n$/) {
		    print $fh "\n";
		}
	    }
	}
	close $fh;
    }

    ## setup merged usr
    #my @amd64_dirs = ('lib32', 'lib64', 'libx32'); # only amd64 for now
    #foreach my $dir ("bin", "sbin", "lib", @amd64_dirs) {
    #    symlink "usr/$dir", "$options->{root}/$dir" or die "cannot create symlink: $!";
    #    make_path("$options->{root}/usr/$dir") or die "cannot create /usr/$dir: $!";
    #}

    {
	open my $fh, '>', "$options->{root}/etc/fstab" or die "cannot open fstab: $!";
	print $fh "# UNCONFIGURED FSTAB FOR BASE SYSTEM\n";
	close $fh;
	chmod 0644, "$options->{root}/etc/fstab" or die "cannot chmod fstab: $!";
    }

    # write /etc/apt/sources.list
    {
	open my $fh, '>', "$options->{root}/etc/apt/sources.list" or die "cannot open /etc/apt/sources.list: $!";
	print $fh $options->{sourceslist};
	close $fh;
    }

    # allow network access from within
    copy("/etc/resolv.conf", "$options->{root}/etc/resolv.conf") or die "cannot copy /etc/resolv.conf: $!";
    copy("/etc/hostname", "$options->{root}/etc/hostname") or die "cannot copy /etc/hostname: $!";

    if ($options->{havemknod}) {
	foreach my $file (@devfiles) {
	    my ($fname, $mode, $type, $linkname, $devmajor, $devminor) = @{$file};
	    if ($type == 0) { # normal file
		die "type 0 not implemented";
	    } elsif ($type == 1) { # hardlink
		die "type 1 not implemented";
	    } elsif ($type == 2) { # symlink
		if ($options->{mode} eq 'fakechroot' and $linkname =~ /^\/proc/) {
		    # there is no /proc in fakechroot mode
		    next;
		}
		symlink $linkname, "$options->{root}/$fname" or die "cannot create symlink $fname";
		next; # chmod cannot work on symlinks
	    } elsif ($type == 3) { # character special
		0 == system('mknod', "$options->{root}/$fname", 'c', $devmajor, $devminor) or die "mknod failed: $?";
	    } elsif ($type == 4) { # block special
		0 == system('mknod', "$options->{root}/$fname", 'b', $devmajor, $devminor) or die "mknod failed: $?";
	    } elsif ($type == 5) { # directory
		make_path "$options->{root}/$fname", { error => \my $err };
		if (@$err) {
		    die "cannot create $fname";
		}
	    } else {
		die "unsupported type: $type";
	    }
	    chmod $mode, "$options->{root}/$fname" or die "cannot chmod $fname: $!";
	}
    }

    # we tell apt about the configuration via a config file passed via the
    # APT_CONFIG environment variable instead of using the --option command
    # line arguments because configuration settings like Dir::Etc have already
    # been evaluated at the time that apt takes its command line arguments
    # into account.
    $ENV{"APT_CONFIG"} = "$tmpfile";

    print STDERR "I: running apt-get update...\n";
    run_apt_progress({
	    ARGV => ['apt-get', 'update'],
	    VERBOSE => $options->{verbose}
	});

    # check if anything was downloaded at all
    {
	open my $fh, '-|', 'apt-get', 'indextargets' // die "failed to fork(): $!";
	chomp (my $indextargets = do { local $/; <$fh> });
	close $fh;
	if ($indextargets eq '') {
	    print STDERR "content of /etc/apt/sources.list:\n";
	    copy("$options->{root}/etc/apt/sources.list", *STDERR);
	    die "apt-get update didn't download anything";
	}
    }

    # setting PATH for chroot, ldconfig, start-stop-daemon...
    if (defined $ENV{PATH} && $ENV{PATH} ne "") {
	$ENV{PATH} = "$ENV{PATH}:/usr/sbin:/usr/bin:/sbin:/bin";
    } else {
	$ENV{PATH} = "/usr/sbin:/usr/bin:/sbin:/bin";
    }

    my %pkgs_to_install;
    if (defined $options->{include}) {
	for my $pkg (split /,/, $options->{include}) {
	    $pkgs_to_install{$pkg} = ();
	}
    }
    if ($options->{variant} eq 'buildd') {
	$pkgs_to_install{'build-essential'} = ();
    }
    # To figure out the right package set for the apt variant we can use:
    #     $ apt-get dist-upgrade -o dir::state::status=/dev/null
    # This is because that variants only contain essential packages and
    # apt and libapt treats apt as essential. If we want to install less
    # (essential variant) then we have to compute the package set ourselves.
    # Same if we want to install priority based variants.
    if (any { $_ eq $options->{variant} } ('extract', 'custom')) {
	print STDERR "I: downloading packages with apt...\n";
	run_apt_progress({
		ARGV => ['apt-get', '--yes',
		    '-oApt::Get::Download-Only=true',
		    'install'],
		PKGS => [keys %pkgs_to_install],
		VERBOSE => $options->{verbose}
	    });
    } elsif ($options->{variant} eq 'apt') {
	# if we just want to install Essential:yes packages, apt and their
	# dependencies then we can make use of libapt treating apt as
	# implicitly essential. An upgrade with the (currently) empty status
	# file will trigger an installation of the essential packages plus apt.
	#
	# 2018-09-02, #debian-dpkg on OFTC, times in UTC+2
	# 23:39 < josch> I'll just put it in my script and if it starts
	#                breaking some time I just say it's apt's fault. :P
	# 23:42 < DonKult> that is how it usually works, so yes, do that :P (<-
	#                  and please add that line next to it so you can
	#                  remind me in 5+ years that I said that after I wrote
	#                  in the bugreport: "Are you crazy?!? Nobody in his
	#                  right mind would even suggest depending on it!")
	print STDERR "I: downloading packages with apt...\n";
	run_apt_progress({
		ARGV => ['apt-get', '--yes',
		    '-oApt::Get::Download-Only=true',
		    'dist-upgrade'],
		VERBOSE => $options->{verbose}
	    });
    } elsif (any { $_ eq $options->{variant} } ('essential', 'standard', 'important', 'required', 'buildd', 'minbase')) {
	my %ess_pkgs;
	open(my $pipe_apt, '-|', 'apt-get', 'indextargets', '--format', '$(FILENAME)', 'Created-By: Packages') or die "cannot start apt-get indextargets: $!";
	while (my $fname = <$pipe_apt>) {
	    chomp $fname;
	    open (my $pipe_cat, '-|', '/usr/lib/apt/apt-helper', 'cat-file', $fname) or die "cannot start apt-helper cat-file: $!";

	    my $pkgname;
	    my $ess = '';
	    my $prio = 'optional';
	    my $arch = '';
	    while (my $line = <$pipe_cat>) {
		chomp $line;
		# Dpkg::Index takes 10 seconds to parse a typical Packages
		# file. Thus we instead use a simple parser that just retrieve
		# the information we need.
		if ($line ne "") {
		    if ($line =~ /^Package: (.*)/) {
			$pkgname = $1;
		    } elsif ($line =~ /^Essential: yes$/) {
			$ess = 'yes'
		    } elsif ($line =~ /^Priority: (.*)/) {
			$prio = $1;
		    } elsif ($line =~ /^Architecture: (.*)/) {
			$arch = $1;
		    }
		    next;
		}
		# we are only interested of packages of native architecture or
		# Architecture:all
		if ($arch eq $options->{nativearch} or $arch eq 'all') {
		    # the line is empty, thus a package stanza just finished
		    # processing and we can handle it now
		    if ($ess eq 'yes') {
			$ess_pkgs{$pkgname} = ();
		    } elsif ($options->{variant} eq 'essential') {
			# for this variant we are only interested in the
			# essential packages
		    } elsif (any { $_ eq $options->{variant} } ('standard', 'important', 'required', 'buildd', 'minbase')) {
			if ($prio eq 'optional' or $prio eq 'extra') {
			    # always ignore packages of priority optional and extra
			} elsif ($prio eq 'standard') {
			    if (none { $_ eq $options->{variant} } ('important', 'required', 'buildd', 'minbase')) {
				$pkgs_to_install{$pkgname} = ();
			    }
			} elsif ($prio eq 'important') {
			    if (none { $_ eq $options->{variant} } ('required', 'buildd', 'minbase')) {
				$pkgs_to_install{$pkgname} = ();
			    }
			} elsif ($prio eq 'required') {
			    # required packages are part of all sets except
			    # essential and apt
			    $pkgs_to_install{$pkgname} = ();
			} else {
			    die "unknown priority: $prio";
			}
		    } else {
			die "unknown variant: $options->{variant}";
		    }
		}
		# reset values
		undef $pkgname;
		$ess = '';
		$prio = 'optional';
		$arch = '';
	    }

	    close $pipe_cat;
	    $? == 0 or die "apt-helper cat-file failed: $?";
	}
	close $pipe_apt;
	$? == 0 or die "apt-get indextargets failed: $?";

	print STDERR "I: downloading packages with apt...\n";
	run_apt_progress({
		ARGV => ['apt-get', '--yes',
		    '-oApt::Get::Download-Only=true',
		    'install'],
		PKGS => [keys %ess_pkgs],
		VERBOSE => $options->{verbose}
	    });
    } else {
	die "unknown variant: $options->{variant}";
    }

    # extract the downloaded packages
    my @essential_pkgs;
    {
	my $apt_archives = "/var/cache/apt/archives/";
	opendir my $dh, "$options->{root}/$apt_archives" or die "cannot read $apt_archives";
	while (my $deb = readdir $dh) {
	    if ($deb !~ /\.deb$/) {
		next;
	    }
	    $deb = "$apt_archives/$deb";
	    if (!-f "$options->{root}/$deb") {
		next;
	    }
	    push @essential_pkgs, $deb;
	}
	close $dh;
    }

    if (scalar @essential_pkgs == 0) {
	# check if a file:// URI was used
	open(my $pipe_apt, '-|', 'apt-get', 'indextargets', '--format', '$(URI)', 'Created-By: Packages') or die "cannot start apt-get indextargets: $!";
	while (my $uri = <$pipe_apt>) {
	    if ($uri =~ /^file:\/\//) {
		die "nothing got downloaded -- use copy:// instead of file://";
	    }
	}
	die "nothing got downloaded";
    }

    # We have to extract the packages from @essential_pkgs either if we run in
    # chrootless mode and extract variant or in any other mode.
    # In other words, the only scenario in which the @essential_pkgs are not
    # extracted are in chrootless mode in any other than the extract variant.
    if ($options->{mode} eq 'chrootless' and $options->{variant} ne 'extract') {
	# nothing to do
    } else {
	print STDERR "I: extracting archives...\n";
	print_progress 0.0;
	my $counter = 0;
	my $total = scalar @essential_pkgs;
	foreach my $deb (@essential_pkgs) {
	    $counter += 1;
	    # not using dpkg-deb --extract as that would replace the
	    # merged-usr symlinks with plain directories
	    pipe my $rfh, my $wfh;
	    my $pid1 = fork() // die "fork() failed: $!";
	    if ($pid1 == 0) {
		open(STDOUT, '>&', $wfh);
		exec 'dpkg-deb', '--fsys-tarfile', "$options->{root}/$deb";
	    }
	    my $pid2 = fork() // die "fork() failed: $!";
	    if ($pid2 == 0) {
		open(STDIN, '<&', $rfh);
		exec 'tar', '-C', $options->{root}, '--keep-directory-symlink', '--extract', '--file', '-';
	    }
	    waitpid($pid1, 0);
	    $? == 0 or die "dpkg-deb --fsys-tarfile failed: $?";
	    waitpid($pid2, 0);
	    $? == 0 or die "tar --extract failed: $?";
	    print_progress ($counter/$total*100);
	}
	print_progress "done";
    }

    if ($options->{mode} eq 'chrootless') {
	print STDERR "I: installing packages...\n";
	# FIXME: the dpkg config from the host is parsed before the command
	# line arguments are parsed and might break this mode
	# Example: if the host has --path-exclude set, then this will also
	# affect the chroot.
	my @chrootless_opts = (
	    '-oDPkg::Options::=--force-not-root',
	    '-oDPkg::Options::=--force-script-chrootless',
	    '-oDPkg::Options::=--root=' . $options->{root},
	    '-oDPkg::Options::=--log=' . "$options->{root}/var/log/dpkg.log");
	if ($options->{variant} eq 'extract') {
	    # nothing to do
	} else {
	    run_apt_progress({
		    ARGV => ['apt-get', '--yes',
			@chrootless_opts,
			'install'],
		    PKGS => [map { "$options->{root}/$_" } @essential_pkgs],
		    VERBOSE => $options->{verbose}
		});
	}
	if (any { $_ eq $options->{variant} } ('extract', 'custom')) {
	    # nothing to do
	} elsif (any { $_ eq $options->{variant} } ('essential', 'apt', 'standard', 'important', 'required', 'buildd', 'minbase')) {
	    if (%pkgs_to_install) {
		run_apt_progress({
			ARGV => ['apt-get', '--yes',
			    @chrootless_opts,
			    'install'],
			PKGS => [keys %pkgs_to_install],
			VERBOSE => $options->{verbose}
		    });
	    }
	} else {
	    die "unknown variant: $options->{variant}";
	}
    } elsif (any { $_ eq $options->{mode} } ('root', 'unshare', 'fakechroot', 'proot')) {

	if ($options->{variant} eq 'extract') {
	    # nothing to do
	} elsif (any { $_ eq $options->{variant} } ('custom', 'essential', 'apt', 'standard', 'important', 'required', 'buildd', 'minbase')) {
	    if ($options->{mode} eq 'fakechroot') {
		# this borrows from and extends
		# /etc/fakechroot/debootstrap.env and /etc/fakechroot/chroot.env
		{
		    my @fakechrootsubst = ();
		    foreach my $dir ('/usr/sbin', '/usr/bin', '/sbin', '/bin') {
			push @fakechrootsubst, "$dir/chroot=/usr/sbin/chroot.fakechroot";
			push @fakechrootsubst, "$dir/mkfifo=/bin/true";
			push @fakechrootsubst, "$dir/ldconfig=/bin/true";
			push @fakechrootsubst, "$dir/ldd=/usr/bin/ldd.fakechroot";
			push @fakechrootsubst, "$dir/ischroot=/bin/true";
		    }
		    if (defined $ENV{FAKECHROOT_CMD_SUBST}
			&& $ENV{FAKECHROOT_CMD_SUBST} ne "") {
			push @fakechrootsubst, split /:/, $ENV{FAKECHROOT_CMD_SUBST};
		    }
		    $ENV{FAKECHROOT_CMD_SUBST} = join ':', @fakechrootsubst;
		}
		if (defined $ENV{FAKECHROOT_EXCLUDE_PATH}
		    && $ENV{FAKECHROOT_EXCLUDE_PATH} ne "") {
		    $ENV{FAKECHROOT_EXCLUDE_PATH} = "$ENV{FAKECHROOT_EXCLUDE_PATH}:/dev:/proc:/sys";
		} else {
		    $ENV{FAKECHROOT_EXCLUDE_PATH} = '/dev:/proc:/sys';
		}
		# workaround for long unix socket path if FAKECHROOT_BASE
		# exceeds the limit of 108 bytes
		$ENV{FAKECHROOT_AF_UNIX_PATH} = "/tmp";
		{
		    my @ldsoconf = ('/etc/ld.so.conf');
		    opendir(my $dh, '/etc/ld.so.conf.d') or die "Can't opendir(/etc/ld.so.conf.d): $!";
		    while (my $entry = readdir $dh) {
			# skip the "." and ".." entries
			next if $entry eq ".";
			next if $entry eq "..";
			next if $entry !~ /\.conf$/;
			push @ldsoconf, "/etc/ld.so.conf.d/$entry";
		    }
		    closedir($dh);
		    my @ldlibpath = ();
		    if (defined $ENV{LD_LIBRARY_PATH}
			&& $ENV{LD_LIBRARY_PATH} ne "") {
			push @ldlibpath, (split /:/, $ENV{LD_LIBRARY_PATH});
		    }
		    # FIXME: workaround allowing installation of systemd should