nixpkgs/pkgs/build-support/build-fhs-userenv/chroot-user.rb

#!/usr/bin/env ruby

# Bind mounts hierarchy: [from, to (relative)]
# If 'to' is nil, path will be the same
mounts = [ ['/nix/store', nil],
           ['/dev', nil],
           ['/proc', nil],
           ['/sys', nil],
           ['/etc', 'host-etc'],
           ['/home', nil],
           ['/var', nil],
           ['/run', nil],
           ['/root', nil],
         ].map! { |x| [ x[0], x[1].nil? ? x[0].sub(/^\/*/, '') : x[1] ] }

# Create directories
mkdirs = ['tmp',
         ]

# Symlinks: [from, to (dir)]
symlinks =
  # /etc symlinks: [file name, prefix in host-etc]
  [ ['passwd', ''],
    ['group', ''],
    ['shadow', ''],
    ['hosts', ''],
    ['resolv.conf', ''],
    ['nsswitch.conf', ''],
    ['pam.d', 'static'],
    ['fonts/fonts.conf', 'static'],
    ['fonts/conf.d/00-nixos.conf', 'static'],
  ].map! { |x| [ "host-etc/#{x[1]}/#{x[0]}", "etc/#{File.dirname x[0]}" ] }

require 'tmpdir'
require 'fileutils'
require 'pathname'
require 'set'
require 'fiddle'

def write_file(path, str)
  File.open(path, 'w') { |file| file.write str }
end

# Import C standard library and several needed calls
$libc = Fiddle.dlopen nil

def make_fcall(name, args, output)
  c = Fiddle::Function.new $libc[name], args, output
  lambda do |*args|
    ret = c.call *args
    raise SystemCallError.new Fiddle.last_error if ret < 0
    return ret
  end
end

$fork = make_fcall 'fork', [], Fiddle::TYPE_INT

CLONE_NEWNS   = 0x00020000
CLONE_NEWUSER = 0x10000000
$unshare = make_fcall 'unshare', [Fiddle::TYPE_INT], Fiddle::TYPE_INT

MS_BIND = 0x1000
MS_REC  = 0x4000
$mount = make_fcall 'mount', [Fiddle::TYPE_VOIDP,
                              Fiddle::TYPE_VOIDP,
                              Fiddle::TYPE_VOIDP,
                              Fiddle::TYPE_LONG,
                              Fiddle::TYPE_VOIDP],
                    Fiddle::TYPE_INT

# Read command line args
abort "Usage: chrootenv swdir program args..." unless ARGV.length >= 2
swdir = Pathname.new ARGV[0]
execp = ARGV.drop 1

# Create temporary directory for root and chdir
root = Dir.mktmpdir 'chrootenv'

# Fork process; we need this to do a proper cleanup because
# child process will chroot into temporary directory.
# We use imported 'fork' instead of native to overcome
# CRuby's meddling with threads; this should be safe because
# we don't use threads at all.
$cpid = $fork.call
if $cpid == 0
  # Save user UID and GID
  uid = Process.uid
  gid = Process.gid

  # Create new mount and user namespaces
  # CLONE_NEWUSER requires a program to be non-threaded, hence
  # native fork above.
  $unshare.call CLONE_NEWNS | CLONE_NEWUSER

  # Map users and groups to the parent namespace
  write_file '/proc/self/setgroups', 'deny'
  write_file '/proc/self/uid_map', "#{uid} #{uid} 1"
  write_file '/proc/self/gid_map', "#{gid} #{gid} 1"

  # Do mkdirs
  mkdirs.each { |x| FileUtils.mkdir_p x }

  # Do rbind mounts.
  mounts.each do |x|
    to = "#{root}/#{x[1]}"
    FileUtils.mkdir_p to
    $mount.call x[0], to, nil, MS_BIND | MS_REC, nil
  end

  # Chroot!
  Dir.chroot root
  Dir.chdir '/'

  # Do symlinks
  symlinks.each do |x|
    FileUtils.mkdir_p x[1]
    FileUtils.ln_s x[0], x[1]
  end

  # Symlink swdir hierarchy
  mount_dirs = Set.new mounts.map { |x| Pathname.new x[1] }
  link_swdir = lambda do |swdir, prefix|
    swdir.find do |path|
      rel = prefix.join path.relative_path_from(swdir)
      # Don't symlink anything in binded or symlinked directories
      Find.prune if mount_dirs.include? rel or rel.symlink?
      if not rel.directory?
        # File does not exist; make a symlink and bail out
        rel.make_symlink path
        Find.prune
      end
      # Recursively follow symlinks
      link_swdir.call path.readlink, rel if path.symlink?
    end
  end
  link_swdir.call swdir, Pathname.new('')

  # New environment
  oldenv = ENV.to_h
  ENV.replace({ 'PS1' => oldenv['PS1'],
                'TERM' => oldenv['TERM'],
                'DISPLAY' => oldenv['DISPLAY'],
                'HOME' => oldenv['HOME'],
                'PATH' => '/bin:/sbin',
                'XDG_RUNTIME_DIR' => oldenv['XDG_RUNTIME_DIR'],
              })

  # Finally, exec!
  exec *execp
end

# Wait for a child. If we catch a signal, resend it to child and continue
# waiting.
def wait_child
  begin
    Process.wait

    # Return child's exit code
    if $?.exited?
      exit $?.exitstatus
      else
      exit 1
    end
  rescue SignalException => e
    Process.kill e.signo, $cpid
    wait_child
  end
end

begin
  wait_child
ensure
  # Cleanup
  FileUtils.rm_rf root, secure: true
end