nixpkgs/nixos/modules/installer/sd-card/sd-image.nix

# This module creates a bootable SD card image containing the given NixOS
# configuration. The generated image is MBR partitioned, with a FAT
# /boot/firmware partition, and ext4 root partition. The generated image
# is sized to fit its contents, and a boot script automatically resizes
# the root partition to fit the device on the first boot.
#
# The firmware partition is built with expectation to hold the Raspberry
# Pi firmware and bootloader, and be removed and replaced with a firmware
# build for the target SoC for other board families.
#
# The derivation for the SD image will be placed in
# config.system.build.sdImage

{ config, lib, pkgs, ... }:

with lib;

let
  rootfsImage = pkgs.callPackage ../../../lib/make-ext4-fs.nix ({
    inherit (config.sdImage) storePaths;
    compressImage = config.sdImage.compressImage;
    populateImageCommands = config.sdImage.populateRootCommands;
    volumeLabel = "NIXOS_SD";
  } // optionalAttrs (config.sdImage.rootPartitionUUID != null) {
    uuid = config.sdImage.rootPartitionUUID;
  });
in
{
  imports = [
    (mkRemovedOptionModule [ "sdImage" "bootPartitionID" ] "The FAT partition for SD image now only holds the Raspberry Pi firmware files. Use firmwarePartitionID to configure that partition's ID.")
    (mkRemovedOptionModule [ "sdImage" "bootSize" ] "The boot files for SD image have been moved to the main ext4 partition. The FAT partition now only holds the Raspberry Pi firmware files. Changing its size may not be required.")
    ../../profiles/all-hardware.nix
  ];

  options.sdImage = {
    imageName = mkOption {
      default = "${config.sdImage.imageBaseName}-${config.system.nixos.label}-${pkgs.stdenv.hostPlatform.system}.img";
      description = ''
        Name of the generated image file.
      '';
    };

    imageBaseName = mkOption {
      default = "nixos-sd-image";
      description = ''
        Prefix of the name of the generated image file.
      '';
    };

    storePaths = mkOption {
      type = with types; listOf package;
      example = literalExpression "[ pkgs.stdenv ]";
      description = ''
        Derivations to be included in the Nix store in the generated SD image.
      '';
    };

    firmwarePartitionOffset = mkOption {
      type = types.int;
      default = 8;
      description = ''
        Gap in front of the /boot/firmware partition, in mebibytes (1024×1024
        bytes).
        Can be increased to make more space for boards requiring to dd u-boot
        SPL before actual partitions.

        Unless you are building your own images pre-configured with an
        installed U-Boot, you can instead opt to delete the existing `FIRMWARE`
        partition, which is used **only** for the Raspberry Pi family of
        hardware.
      '';
    };

    firmwarePartitionID = mkOption {
      type = types.str;
      default = "0x2178694e";
      description = ''
        Volume ID for the /boot/firmware partition on the SD card. This value
        must be a 32-bit hexadecimal number.
      '';
    };

    firmwarePartitionName = mkOption {
      type = types.str;
      default = "FIRMWARE";
      description = ''
        Name of the filesystem which holds the boot firmware.
      '';
    };

    rootPartitionUUID = mkOption {
      type = types.nullOr types.str;
      default = null;
      example = "14e19a7b-0ae0-484d-9d54-43bd6fdc20c7";
      description = ''
        UUID for the filesystem on the main NixOS partition on the SD card.
      '';
    };

    firmwareSize = mkOption {
      type = types.int;
      # As of 2019-08-18 the Raspberry pi firmware + u-boot takes ~18MiB
      default = 30;
      description = ''
        Size of the /boot/firmware partition, in megabytes.
      '';
    };

    populateFirmwareCommands = mkOption {
      example = literalExpression "'' cp \${pkgs.myBootLoader}/u-boot.bin firmware/ ''";
      description = ''
        Shell commands to populate the ./firmware directory.
        All files in that directory are copied to the
        /boot/firmware partition on the SD image.
      '';
    };

    populateRootCommands = mkOption {
      example = literalExpression "''\${config.boot.loader.generic-extlinux-compatible.populateCmd} -c \${config.system.build.toplevel} -d ./files/boot''";
      description = ''
        Shell commands to populate the ./files directory.
        All files in that directory are copied to the
        root (/) partition on the SD image. Use this to
        populate the ./files/boot (/boot) directory.
      '';
    };

    postBuildCommands = mkOption {
      example = literalExpression "'' dd if=\${pkgs.myBootLoader}/SPL of=$img bs=1024 seek=1 conv=notrunc ''";
      default = "";
      description = ''
        Shell commands to run after the image is built.
        Can be used for boards requiring to dd u-boot SPL before actual partitions.
      '';
    };

    compressImage = mkOption {
      type = types.bool;
      default = true;
      description = ''
        Whether the SD image should be compressed using
        {command}`zstd`.
      '';
    };

    expandOnBoot = mkOption {
      type = types.bool;
      default = true;
      description = ''
        Whether to configure the sd image to expand it's partition on boot.
      '';
    };
  };

  config = {
    fileSystems = {
      "/boot/firmware" = {
        device = "/dev/disk/by-label/${config.sdImage.firmwarePartitionName}";
        fsType = "vfat";
        # Alternatively, this could be removed from the configuration.
        # The filesystem is not needed at runtime, it could be treated
        # as an opaque blob instead of a discrete FAT32 filesystem.
        options = [ "nofail" "noauto" ];
      };
      "/" = {
        device = "/dev/disk/by-label/NIXOS_SD";
        fsType = "ext4";
      };
    };

    sdImage.storePaths = [ config.system.build.toplevel ];

    system.build.sdImage = pkgs.callPackage ({ stdenv, dosfstools, e2fsprogs,
    mtools, libfaketime, util-linux, zstd }: stdenv.mkDerivation {
      name = config.sdImage.imageName;

      nativeBuildInputs = [ dosfstools e2fsprogs libfaketime mtools util-linux ]
      ++ lib.optional config.sdImage.compressImage zstd;

      inherit (config.sdImage) imageName compressImage;

      buildCommand = ''
        mkdir -p $out/nix-support $out/sd-image
        export img=$out/sd-image/${config.sdImage.imageName}

        echo "${pkgs.stdenv.buildPlatform.system}" > $out/nix-support/system
        if test -n "$compressImage"; then
          echo "file sd-image $img.zst" >> $out/nix-support/hydra-build-products
        else
          echo "file sd-image $img" >> $out/nix-support/hydra-build-products
        fi

        root_fs=${rootfsImage}
        ${lib.optionalString config.sdImage.compressImage ''
        root_fs=./root-fs.img
        echo "Decompressing rootfs image"
        zstd -d --no-progress "${rootfsImage}" -o $root_fs
        ''}

        # Gap in front of the first partition, in MiB
        gap=${toString config.sdImage.firmwarePartitionOffset}

        # Create the image file sized to fit /boot/firmware and /, plus slack for the gap.
        rootSizeBlocks=$(du -B 512 --apparent-size $root_fs | awk '{ print $1 }')
        firmwareSizeBlocks=$((${toString config.sdImage.firmwareSize} * 1024 * 1024 / 512))
        imageSize=$((rootSizeBlocks * 512 + firmwareSizeBlocks * 512 + gap * 1024 * 1024))
        truncate -s $imageSize $img

        # type=b is 'W95 FAT32', type=83 is 'Linux'.
        # The "bootable" partition is where u-boot will look file for the bootloader
        # information (dtbs, extlinux.conf file).
        sfdisk $img <<EOF
            label: dos
            label-id: ${config.sdImage.firmwarePartitionID}

            start=''${gap}M, size=$firmwareSizeBlocks, type=b
            start=$((gap + ${toString config.sdImage.firmwareSize}))M, type=83, bootable
        EOF

        # Copy the rootfs into the SD image
        eval $(partx $img -o START,SECTORS --nr 2 --pairs)
        dd conv=notrunc if=$root_fs of=$img seek=$START count=$SECTORS

        # Create a FAT32 /boot/firmware partition of suitable size into firmware_part.img
        eval $(partx $img -o START,SECTORS --nr 1 --pairs)
        truncate -s $((SECTORS * 512)) firmware_part.img

        mkfs.vfat --invariant -i ${config.sdImage.firmwarePartitionID} -n ${config.sdImage.firmwarePartitionName} firmware_part.img

        # Populate the files intended for /boot/firmware
        mkdir firmware
        ${config.sdImage.populateFirmwareCommands}

        find firmware -exec touch --date=2000-01-01 {} +
        # Copy the populated /boot/firmware into the SD image
        cd firmware
        # Force a fixed order in mcopy for better determinism, and avoid file globbing
        for d in $(find . -type d -mindepth 1 | sort); do
          faketime "2000-01-01 00:00:00" mmd -i ../firmware_part.img "::/$d"
        done
        for f in $(find . -type f | sort); do
          mcopy -pvm -i ../firmware_part.img "$f" "::/$f"
        done
        cd ..

        # Verify the FAT partition before copying it.
        fsck.vfat -vn firmware_part.img
        dd conv=notrunc if=firmware_part.img of=$img seek=$START count=$SECTORS

        ${config.sdImage.postBuildCommands}

        if test -n "$compressImage"; then
            zstd -T$NIX_BUILD_CORES --rm $img
        fi
      '';
    }) {};

    boot.postBootCommands = lib.mkIf config.sdImage.expandOnBoot ''
      # On the first boot do some maintenance tasks
      if [ -f /nix-path-registration ]; then
        set -euo pipefail
        set -x
        # Figure out device names for the boot device and root filesystem.
        rootPart=$(${pkgs.util-linux}/bin/findmnt -n -o SOURCE /)
        bootDevice=$(lsblk -npo PKNAME $rootPart)
        partNum=$(lsblk -npo MAJ:MIN $rootPart | ${pkgs.gawk}/bin/awk -F: '{print $2}')

        # Resize the root partition and the filesystem to fit the disk
        echo ",+," | sfdisk -N$partNum --no-reread $bootDevice
        ${pkgs.parted}/bin/partprobe
        ${pkgs.e2fsprogs}/bin/resize2fs $rootPart

        # Register the contents of the initial Nix store
        ${config.nix.package.out}/bin/nix-store --load-db < /nix-path-registration

        # nixos-rebuild also requires a "system" profile and an /etc/NIXOS tag.
        touch /etc/NIXOS
        ${config.nix.package.out}/bin/nix-env -p /nix/var/nix/profiles/system --set /run/current-system

        # Prevents this from running on later boots.
        rm -f /nix-path-registration
      fi
    '';
  };
}