nsd

#!/usr/bin/env ruby
# frozen_string_literal: true

require_relative '../lib/auto_hck'

module AutoHCK
  # rubocop:disable Metrics/BlockLength
  run do
    # rubocop:enable Metrics/BlockLength

    require 'English'
    require 'etc'

    euid = Process.euid

    argv = if euid.zero?
             # Create namespaces of the following types:
             # - Network
             #   AutoHCK requires several bridges and an isolated network namespace
             #   allows setting up them reliably without requiring privileges and
             #   polluting the root namespace.
             # - Mount
             #   This allows mounting /etc/resolv.conf so that it points to
             #   slirp4netns.
             %w[unshare -m -n]
           else
             # If the current user is not root, create a user namespace as well and
             # map the current user to root. Group mapping is delegated to
             # newgidmap.
             %w[unshare -m -n --map-user=0]
           end

    argv << File.join(__dir__, 'ns_unshared')

    e_read, e_write = IO.pipe
    $stderr.write "[nsd] Running (e pipe): #{e_read.fileno} => #{e_write.fileno}\n"
    r_read, r_write = IO.pipe
    $stderr.write "[nsd] Running (r pipe): #{r_read.fileno} => #{r_write.fileno}\n"
    parent = Process.pid.to_s
    $stderr.write "[nsd] Parent PID: #{parent}\n"

    fork do
      e_write.close
      exit false if e_read.read(1).nil?

      if euid != 0
        pwname = Etc.getpwuid(Process.uid).name

        # getsubids is not available on some supported systems.
        subid_start, subid_count = File.open('/etc/subgid') do |file|
          begin
            file.readline while file.readline(':', chomp: true) != pwname
          rescue EOFError
            abort <<~MESSAGE
              subgid delegation is not set up for the current user.
              See newgidmap(1) for details.
            MESSAGE
          end

          [file.readline(':', chomp: true), file.readline(chomp: true)]
        end

        # Call newgidmap so that virtiofsd can use setgroups() to drop subgroups.
        # unshare command in util-linux 2.38 can call newgidmap by its own, but
        # unfortunately it is not available on some supported systems.
        command = ['newgidmap', parent, '0', Process.gid.to_s, '1', subid_start, subid_start, subid_count]
        $stderr.write "[nsd child] Running (system): #{command.join(' ')}\n"
        system(*command, exception: true, out: :err)
      end

      e_read.close_on_exec = false
      r_write.close_on_exec = false

      command = ['slirp4netns', '-e', e_read.fileno.to_s, '-r', r_write.fileno.to_s,
                 '-a', 'slirp.sock', parent, 'tap_host']
      $stderr.write "[nsd child] Running (exec): #{command.join(' ')}\n"
      exec(*command, out: :err)
    end

    e_read.close
    r_write.close

    $stderr.write "[nsd] Running (exec, 3 => #{e_write.fileno}, 4 => #{r_read.fileno}): #{argv.join(' ')}\n"
    exec(*argv, 3 => e_write, 4 => r_read)
  end
end