kexec-load-grub

#!/usr/bin/env bash

set -eu

PROGNAME=${0##*/}
PROGVERSION="1.0"

GRUBDATA=""
GRUBROOT=""
GRUBPREFIX=""
MENUENTRY=()
INITRDS=()
KERNELS=()
KARGS=()
# Mapping from grubs internal index to our menu index
GRUBINDEX=()
SELECTED=0

main() {
  local showmenu=1

  local args=$(getopt -o v -l version,root:,auto -n "$PROGNAME" -- "$@")
  [ $? != 0 ] && exit 2
  eval set -- $args

  while true; do
    case "$1" in
    '--version'|'-v')
      echo "$PROGNAME version $PROGVERSION"
      exit 0
      ;;
    '--root')
      GRUBROOT="$2"
      shift 2
      continue
      ;;
    '--auto')
      showmenu=0
      shift
      continue
      ;;
    '--')
      shift
      break
      ;;
    *)
      echo 'Internal error!' >&2
      exit 1
    esac
  done

  if [[ -z $GRUBROOT ]]; then
    GRUBROOT=$(grub_root_probe)
  fi

  mountpoint -q /boot && GRUBPREFIX=/boot || GRUBPREFIX=
  GRUBDATA=$(< "$GRUBROOT/grub.cfg")

  grub_cfg_load
  grub_default_load
  [[ 1 = $showmenu ]] && menu_show
  kexec_load
}

grub_root_probe() {
  # Debian and friends
  if [[ -f /boot/grub/grub.cfg ]]; then
    echo "/boot/grub"
  # RHEL and friends
  elif [[ -f /boot/efi/EFI/redhat/grub.cfg ]]; then
    echo "/boot/efi/EFI/redhat"
  elif [[ -f /boot/grub2/grub.cfg ]]; then
    echo "/boot/grub2"
  else
    echo "No grub.cfg could be located." 1>&2
    exit 1
  fi
}

grub_cfg_load() {
  local offsets=($(echo "$GRUBDATA" | grep -n '^[[:space:]]*menuentry[[:space:]]' | cut -d: -f1))
  local menuindex=0
  local begin=0
  local length=0

  for grubindex in ${!offsets[@]}; do
    begin=${offsets[$grubindex]}
    length=$(echo "$GRUBDATA" | tail -n+$begin | grep -n -m 1 '^[[:space:]]*}[[:space:]]*' | cut -d: -f1)

    grub_parse_entry "$(echo "$GRUBDATA" | tail -n+$begin | head -n$length)"

    if [[ ! -z $entry_kernel_image ]]; then
      MENUENTRY+=("$entry_name")
      KERNELS+=("$entry_kernel_image")
      INITRDS+=("$entry_initrd")
      KARGS+=("$entry_append")
      # grub_default_load needs to map from name to menu index; grub index to menu index
      GRUBINDEX[$grubindex]=$menuindex
      menuindex=$(($menuindex + 1))
    fi
  done
}

grub_default_load() {
  # Find the default
  local entry=$(echo "$GRUBDATA" | awk '/set default/ {print $2}' | cut -d'"' -f2 | tail -1)
  if [[ "$entry" = '${saved_entry}' ]]; then
    entry=$(sed -ne 's/^saved_entry=//p' "$GRUBROOT/grubenv")
  fi
  if [[ -z "$entry" ]]; then
    entry=0
  fi

  if [[ $entry =~ ^[0-9]+$ ]]; then
    if [[ "${GRUBINDEX[$entry]+isset}" ]]; then
      SELECTED="${GRUBINDEX[$entry]}"
    fi
  else
    for index in "${!MENUENTRY[@]}"; do
      if [[ "${MENUENTRY[$index]}" = "$entry" ]]; then
        SELECTED=$index
        continue
      fi
    done
  fi
}

grub_parse_entry() {
  local data="$1"
  entry_name=""
  entry_kernel_image=""
  entry_append=""
  entry_initrd=""

   while read command args; do
    case "$command" in
    'menuentry')
      # filter args for $... this is not great.
      eval set -- "$(printf '%s' "$args" | sed 's/\$[^ ]*//g')"
      entry_name="$1"
      ;;
    'linux'|'linux16'|'linuxefi')
      while read image append; do
        entry_kernel_image="${GRUBPREFIX}$image"
        entry_append="$append"
      done <<< "$args"
      ;;
    'initrd'|'initrd16'|'initrdefi')
      entry_initrd="${GRUBPREFIX}$args"
      ;;
    esac
  done <<< "$data"
}

menu_show() {
  local wmenu=()

  for index in "${!MENUENTRY[@]}"; do
    wmenu+=("$index" "${MENUENTRY[$index]}")
  done

  SELECTED=$(whiptail \
  --title 'KEXEC GRUB helper' \
  --menu 'Select a GRUB entry' \
  --notags \
  --default-item "$SELECTED" \
  25 85 16 -- \
  "${wmenu[@]}" 3>&1 1>&2 2>&3)
}

kexec_load() {
  local kernel_image="${KERNELS[$SELECTED]}"
  local initrd="${INITRDS[$SELECTED]}"
  local append="${KARGS[$SELECTED]}"
  test -z "$append" && append="$(cat /proc/cmdline)"

  printf 'Selected "%s"\n\n' "${MENUENTRY[$SELECTED]}"

  if [[ -z "$kernel_image" ]]; then
    echo "No kernel could be located." 1>&2
    exit 1
  fi

  local kexec_args=('-l' "$kernel_image")
  [[ -z "$initrd" ]] || kexec_args+=("--initrd=$initrd")
  [[ -z "$append" ]] || kexec_args+=("--append=$append")

  printf 'kernel  %s\n' "$kernel_image"
  printf 'initrd  %s\n' "$initrd"
  printf 'cmdline %s\n' "$append"

  /sbin/kexec "${kexec_args[@]}"

  echo
  echo 'Ready to boot new kernel using "systemctl kexec"'
}

main "$@"