nixos-installer/modules/bootloader/extlinux.nix.md

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/*
# Extlinux Bootloader
A simple bootloader for legacy-BIOS environments, like (by default) Qemu.
This uses the same implementation as `boot.loader.generic-extlinux-compatible` to generate the bootloader configuration, but then actually also installs `extlinux` itself, instead of relying on something else (like an externally installed u-boot) to read and execute the configuration.
## Issues
* Updating between package versions is not atomic (and I am not sure it can be).
## Implementation
```nix
#*/# end of MarkDown, beginning of NixOS module:
dirname: inputs: args@{ config, options, pkgs, lib, ... }: let inherit (inputs.self) lib; in let
prefix = inputs.config.prefix;
cfg = config.${prefix}.bootloader.extlinux;
targetMount = let path = lib.findFirst (path: config.fileSystems?${path}) "/" (lib.wip.parentPaths cfg.targetDir); in config.fileSystems.${path};
supportedFSes = [ "vfat" "ntfs" "ext2" "ext3" "ext4" "btrfs" "xfs" "ufs" ]; fsSupported = fs: builtins.elem fs supportedFSes;
in {
options.${prefix} = { bootloader.extlinux = {
enable = lib.mkEnableOption (lib.mdDoc ''
extlinux, a simple bootloader for legacy-BIOS environments, like (by default) Qemu.
This uses the same implementation as `boot.loader.generic-extlinux-compatible` to generate the bootloader configuration, but then actually also installs `extlinux` itself, instead of relying on something else (like an externally installed u-boot) to read and execute the configuration.
Any options affecting the config file generation by `boot.loader.generic-extlinux-compatible` apply, but `boot.loader.generic-extlinux-compatible.enable` should not be set to `true`.
Since the bootloader runs before selecting a generation or specialisation to run, all sub-options, similar to e.g. {option}`boot.loader.timeout`, apply globally to the system, from whichever configuration last applied its bootloader (e.g. by newly `nixos-rebuild switch/boot`ing it or by calling its `.../bin/switch-to-configuration switch/boot`)
'');
package = lib.mkOption { description = lib.mdDoc ''
The `syslinux` package to install `extlinux` from use.
''; type = lib.types.package; default = pkgs.syslinux; defaultText = lib.literalExpression "pkgs.syslinux"; };
targetDir = lib.mkOption { description = lib.mdDoc ''
The path in whose `./extlinux` sub dir `extlinux` will be installed to. When `nixos-rebuild boot/switch` gets called, this or a parent path needs to be mounted from {option}`.targetPart`.
''; type = lib.types.strMatching ''^/.*[^/]$''; default = "/boot"; };
targetPart = lib.mkOption { description = lib.mdDoc ''
The `/dev/disk/by-{id,label,partlabel,partuuid,uuid}/*` path of the *disk partition* holding the filesystem that `extlinux` is installed to. This must be formatted with a filesystem that `extlinux` supports and mounted as (a parent of) {option}`.targetDir`. The disk on which the partition lies will have the bootloader section of its MBR replaced by `extlinux`'s.
''; type = lib.types.strMatching ''^/.*[^/]$''; default = targetMount.device; };
allowInstableTargetPart = lib.mkOption { internal = true; type = lib.types.bool; };
showUI = (lib.mkEnableOption (lib.mdDoc ''
a simple graphical user interface to select the configuration to start during early boot
'')) // { default = true; example = false; };
}; };
config = let
generic-extlinux-compatible = "${inputs.nixpkgs}/nixos/modules/system/boot/loader/generic-extlinux-compatible";
extlinux-conf-builder = (import generic-extlinux-compatible { inherit config pkgs lib; }).config.content.system.build.installBootLoader;
esc = lib.escapeShellArg;
in lib.mkMerge [ (lib.mkIf cfg.enable {
assertions = [ {
assertion = cfg.allowInstableTargetPart || (builtins.match ''^/dev/disk/by-(id|label|partlabel|partuuid|uuid)/.*[^/]$'' cfg.targetPart) != null;
message = ''
`config.${prefix}.bootloader.extlinux.targetPart` is set to `${cfg.targetPart}`, which is not a stable path in `/dev/disk/by-{id,label,partlabel,partuuid,uuid}/`. Not using a unique identifier (or even using a path that can unexpectedly change) is very risky.
'';
} {
assertion = fsSupported targetMount.fsType;
message = ''
`config.${prefix}.bootloader.extlinux.targetPart`'s closest mount (`${targetMount.mountPoint}`) is of type `${targetMount.fsType}`, which is not one of extlinux's supported types (${lib.concatStringsSep ", " supportedFSes}).
'';
} ];
${prefix} = {
fs.boot = { enable = lib.mkDefault true; mountpoint = lib.mkDefault cfg.targetDir; };
bootloader.extlinux.allowInstableTargetPart = lib.mkForce false;
};
system.boot.loader.id = "extlinux";
system.build.installBootLoader = "${pkgs.writeShellScript "install-extlinux.sh" ''
if [[ ! ''${1:-} || $1 != /nix/store/* ]] ; then echo "Usage: $0 TOPLEVEL_PATH" 1>&2 ; exit 1 ; fi
export PATH=$PATH:${pkgs.stdenv}/bin
${extlinux-conf-builder} "$1" -d ${esc cfg.targetDir}
partition=${esc cfg.targetPart}
diskDev=/dev/$( basename "$( readlink -f /sys/class/block/"$( basename "$( realpath "$partition" )" )"/.. )" ) || exit
if [[ $( cat ${esc cfg.targetDir}/extlinux/installedVersion 2>/dev/null || true ) != ${esc cfg.package} ]] ; then
if ! output=$( ${esc cfg.package}/bin/extlinux --install --heads=64 --sectors=32 ${esc cfg.targetDir}/extlinux 2>&1 ) ; then
printf '%s\n' "$output" 1>&2 ; exit 1
fi
printf '%s\n' ${esc cfg.package} >${esc cfg.targetDir}/extlinux/installedVersion
fi
if ! ${pkgs.diffutils}/bin/cmp --quiet --bytes=440 $diskDev ${esc cfg.package}/share/syslinux/mbr.bin ; then
dd bs=440 conv=notrunc count=1 if=${esc cfg.package}/share/syslinux/mbr.bin of=$diskDev status=none || exit
fi
if [[ ${toString cfg.showUI} ]] ; then # showUI
for lib in libutil menu ; do
if ! ${pkgs.diffutils}/bin/cmp --quiet ${esc cfg.targetDir}/extlinux/$lib.c32 ${esc cfg.package}/share/syslinux/$lib.c32 ; then
cp ${esc cfg.package}/share/syslinux/$lib.c32 ${esc cfg.targetDir}/extlinux/$lib.c32
fi
done
if ! ${pkgs.gnugrep}/bin/grep -qP '^UI ' ${esc cfg.targetDir}/extlinux/extlinux.conf ; then # `extlinux-conf-builder` above would have recreated this, so the check should always be true
${pkgs.perl}/bin/perl -i -pe 's/TIMEOUT/UI menu.c32\nTIMEOUT/' ${esc cfg.targetDir}/extlinux/extlinux.conf
fi
else
: # delete library files?
fi
''}";
boot.loader.grub.enable = false;
}) (
(lib.mkIf (options.virtualisation?useDefaultFilesystems) { # (»nixos/modules/virtualisation/qemu-vm.nix« is imported, i.e. we are building a "vmVariant")
${prefix} = {
bootloader.extlinux.enable = lib.mkIf config.virtualisation.useDefaultFilesystems (lib.mkVMOverride false);
bootloader.extlinux.allowInstableTargetPart = lib.mkVMOverride true; # (»/dev/sdX« etc in the VM are stable (if the VM is invoked the same way))
};
})
) ];
}