dotfiles/dot_bin/executable_efi-update

#!/bin/env python3

"""Update EFI boot entries for Linux kernels configured with EFISTUB.

Requires root access.

USAGE: efi-update PATH

PATH points to a JSON file which holds the boot entries to insert. This file is
an array of objects where each object is one boot entry and the order of each
entry determines its order in the boot sequence.

Each entry follows this schema:
- label: the name of this entry (as it should appear in the boot menu)
- device: the path to the device holding the kernel to boot
- partition: the partition on the device (above) holding the kernel
- kernel: the name of the kernel to boot (something like "linux-lts")
- root: and object like:
  - path: the device path of the root partition
  - fstype: the filesytem type of the root partition
  - mountopts: an array of mount options for the root file system
- params: an array of kernel parameters to pass (in addition to those for root)

ASSUMPTIONS:
- linux EFISTUB kernels only
- no secondary bootloader (eg grub)
- root filesystem mounts as rw
- initramfs exists (eg no monolithic kernels)
- initramfs and loader paths are named according to the kernel

"""

import sys
import json
import re
import subprocess as sp
from shutil import which
from itertools import chain

EFIEXE = "efibootmgr"


def call_efibootmgr(args):
    """Call efibootmgr (quietly) with ARGS"""
    return sp.run([EFIEXE, "-q", *args], check=True)


def read_entries():
    """Read boot entries from JSON file"""
    if len(sys.argv) != 2:
        print("must supply entry JSON as single argument")
        sys.exit(1)
    with open(sys.argv[1], encoding="utf-8") as file:
        # hack to remove comments from (nonstandard) json
        out = re.sub("//.*?\n", "", file.read())
        return json.loads(re.sub("/\\*.*?\\*/", "", out))


def delete_entry(bootnum):
    """Remove the entry associated with BOOTNUM"""
    call_efibootmgr(["-b", str(bootnum), "-B"])


def insert_entry(bootnum, entry):
    """Insert ENTRY at position BOOTNUM"""

    def fmt_keyval(key, val):
        return f"{key}={val}"

    def fmt_flag_(flag, arg, quote=False):
        return [f"-{flag}", f"'{arg}'" if quote else f"{arg}"]

    def fmt_flag(flag, key, quote=False):
        return fmt_flag_(flag, entry[key], quote)

    kernel = entry["kernel"]
    root = entry["root"]
    unicode_args = " ".join(
        [
            fmt_keyval("root", root["path"]),
            "rw",
            fmt_keyval("rootfstype", root["fstype"]),
            fmt_keyval("rootflags", ",".join(root["mountopts"])),
            *entry["params"],
            fmt_keyval("initrd", f"/initramfs-{kernel}.img"),
        ]
    )
    args = [
        fmt_flag_("b", bootnum),
        ["-c"],
        fmt_flag("L", "label"),
        fmt_flag("d", "device"),
        fmt_flag("p", "partition"),
        fmt_flag_("l", f"/vmlinuz-{kernel}"),
        fmt_flag_("u", unicode_args),
    ]
    call_efibootmgr([*chain(*args)])


def update_entry(bootnum, entry):
    """Update (delete and insert) ENTRY at position BOOTNUM"""
    print(f"Updating entry '{entry['label']}'")
    try:
        delete_entry(bootnum)
    except sp.CalledProcessError:
        pass
    insert_entry(bootnum, entry)


def remove_dups():
    """Remove duplicate boot entries if they exist"""
    call_efibootmgr("-D")


def set_order(entries):
    """Set the boot order according to the order of ENTRIES"""
    call_efibootmgr("-O")
    order = ",".join([str(i) for i in range(0, len(entries))])
    call_efibootmgr(order)


def main():
    """Update all EFI entries"""
    if not which(EFIEXE):
        print(f"{EFIEXE} not found")
        sys.exit(1)
    entries = read_entries()
    for bootnum, entry in enumerate(entries):
        update_entry(bootnum, entry)
    remove_dups()
    set_order(entries)


main()