pipglr/root/setup.sh

#!/usr/bin/env bash
#
# This script is intended to be run during container-image build.  Any
# other usage outside this context is likely to cause harm.
#
# Setup script for rootless podman container image to run rootless GitLab Runner service.
# Copyright (C) 2023 Christopher C. Evich
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Lesser General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with this program

set -eo pipefail

function die() {
  echo -n '!! ERROR:' >&2
  printf "         %s\n" "$@" >&2
  exit 1
}

function check_vars() {
  for varname in PRUNE_INTERVAL RUNNER_VERSION TARGETARCH; do
    if [[ -z "${!varname}" ]]; then
      die "Env. variable '$varname' must be non-empty."
    fi
  done
}

function main() {
  # Show what's happening to make debugging easier
  set -x

  # Make image smaller by not installing docs.
  dnf=(dnf --setopt=tsflags=nodocs -y)

  install_packages
  setup_user
  setup_service_podman
  setup_service_runner
  setup_gitlab_config

  finalize_ownership
}

function install_packages() {
  readarray xpackages < <(grep -vE '^(# )+' </root/xpackages.txt)
  local exclude_args=()
  for rpm in "${xpackages[@]}"; do
    exclude_args+=("--exclude=$rpm")
  done

  # DNF itself or a dependence may need upgrading, take care of it first.
  "${dnf[@]}" upgrade &&
    "${dnf[@]}" "${exclude_args[@]}" install \
      podman \
      systemd

  # Gitlab-runner package contains scriptlets which do not function properly inside a
  # container-build environment where systemd is not active/running.
  if [[ ${ENABLE_FIPS} == true && $(cat /proc/sys/crypto/fips_enabled) == 1 ]]; then
    PACKAGE_FILE="gitlab-runner_${TARGETARCH}-fips.rpm"
  else
    PACKAGE_FILE="gitlab-runner_${TARGETARCH}.rpm"
  fi

  "${dnf[@]}" "${exclude_args[@]}" \
    --setopt=tsflags=noscripts install \
    "https://gitlab-runner-downloads.s3.amazonaws.com/$RUNNER_VERSION/rpm/${PACKAGE_FILE}"


# Allow removing dnf, sudo, etc. packages.  Also don't start unnecessary or broken
# systemd services, like anything kernel related or login gettys.
rm -rf \
  /etc/dnf/protected.d/* \
  /etc/systemd/system/getty.target.wants/* \
  /etc/systemd/system/multi-user.target.wants/* \
  /etc/systemd/system/sysinit.target.wants/* \
  /etc/systemd/system/timers.target.wants/* \
  /lib/systemd/system/graphical.target.wants/* \
  /lib/systemd/system/multi-user.target.wants/{getty.target,systemd-ask-password-wall.path} \
  /lib/systemd/system/sys-kernel*.mount

  # Allow removing dnf, sudo, etc. packages.
  rm -rf \
    /etc/dnf/protected.d/*
  # Remove unnecessary packages, see xpackages.txt to learn how this list was generated.
  # This makes the image smaller and reduces the attack-surface.
  dnf remove -y "${xpackages[@]}"

  # Wipe out the DNF cache, then remove it entirely, again to make the image smaller.
  "${dnf[@]}" clean all
  rm -rf /var/cache/dnf /var/log/dnf* /var/log/yum.*
  rpm -e dnf

  # Workaround base-image failing to confer capabilties properly on
  # /usr/bin/new{u,g}idmap to `cap_set{u,g}id=ep` in new image layers.
  rpm --setcaps shadow-utils
}

function setup_user() {
  # Guarantee uid/gid 1000 for user 'podman' / 1001 for user 'runner'.
  groupadd -g 1000 podman
  groupadd -g 1001 runner

  # Separate users for services to increase process isolation.
  # The 'podman' user's socket service writes /home/runner/podman.socket
  # Prevent copying of skel since it can interfere with the gitlab-runner
  mkdir -p /home/podman /home/runner
  useradd -M -u 1000 -g podman -G runner podman
  useradd -M -u 1001 -g runner runner

  # Allow only podman in `/home/podman`.
  chmod 700 /home/podman
  # Allow 'podman' user to create socket file under `/home/runner`.
  chmod 770 /home/runner

  # Set permissions.
  chown -R runner:runner /home/runner
  chown -R podman:podman /home/podman

  # Overwrite defaults, only user 'podman' permitted to have a user-namespace
  # Split the namespaced ID's around the containers root (ID 0), podman (ID 1000), and
  # runner (ID 1001) such that the user-namespace of any nested containers cannot
  # read or write any files owned by these users (and/or hijack nested container processes).
  # N/B: The range-end (999+64536) ensures a total of 65535 IDs are available for nested-containers.
  #      This requires the host provide a sufficiently large range, i.e. `pipglr:<start>:65539`
  echo -e "podman:1:999\npodman:1002:64536" | tee /etc/subuid >/etc/subgid
}

function setup_volumes() {
  # Host volume mount necessary for nested-podman to use overlayfs2 for container & volume storage.
  mkdir -p /home/podman/.local/share/containers

  # Nested-container's local container-cache volume mount, recommended by gitlab-runner docs.
  mkdir -p /cache

  # Both the gitlab-runner and podman need access to the cache directory / volume mount.
  chown podman:runner /cache
}

function setup_service_podman() {
  # Setup persistent 'podman' user services to start & run without a login.
  mkdir -p /var/lib/systemd/linger
  touch /var/lib/systemd/linger/podman

  # Setup 'podman' socket and a container-storage pruning service for 'podman' user.
  mkdir -p /home/podman/.config/systemd/user/{sockets.target.wants,default.target.wants}

  cd /home/podman/.config/systemd/user/
  ln -s "$(pwd)/podman.socket" ./sockets.target.wants/ # Added from Containerfile
  ln -s "$(pwd)/prune.timer" ./default.target.wants/   # also from Containerfile

  # Substitute value from --build-arg if specified, otherwise use default from Containerfile.
  sed -i -e "s/@@@PRUNE_INTERVAL@@@/$PRUNE_INTERVAL/" ./prune.timer
}

function setup_service_runner() {
  # Setup persistent 'runner' user services to start & run without a login.
  touch /var/lib/systemd/linger/runner

  # Setup persistent 'runner' user services to start & run without a login.
  mkdir -p /home/runner/.config/systemd/user/default.target.wants

  cd /home/runner/.config/systemd/user/
  # Does not depend on podman.socket file availability, will retry if not present.
  ln -s "$(pwd)/runner.service" ./default.target.wants/
}

function setup_gitlab_config() {
  # gitlab-runner will create side-car '.runner_system_id' file next to 'config.toml'
  # on first startup.  Ensure access is allowed.  Also link to future config file
  # presented as a container-secret.
  mkdir -p /home/runner/.gitlab-runner
  ln -s /var/run/secrets/config.toml /home/runner/.gitlab-runner/config.toml

  chmod -R 700 /home/runner/.gitlab-runner
}

function finalize_ownership() {
  # Adjust ownership to all files created after `setup_user`.
  # and also to the `ADD` instruction in the `Containerfile`.
  chown -R runner:runner /home/runner
  chown -R podman:podman /home/podman

  # Ensure correct permissions of system configuration files.
  # Somehow these can be set incorrectly during Containerfile
  # ADD instruction.
  local path
  for path in "/etc/systemd/system.conf.d" "/etc/systemd/system/user-.slice.d"; do
    chown root:root ${path}/*
    chmod 0644 ${path}/*
  done
}

check_vars
main