# ARM images with Packer

Packer builder ARM

Preface

Packer came to be a pretty good and flexible tool to build system images. Since it has plenty of support for common things such docker etc, I decided to give it a try and unify image building for my ARM machines.
Almost every hardware platform has it's unique quirks, so I had to write a plugin that would be extensible enough to support the variety of devices I currently own.

This post is mostly describing the plugin configuration and features, feel free to checkout code on github.com/mkaczanowski/packer-builder-arm for more details

Packer builder ARM

This plugin allows you to build or extend ARM system image. It operates in two modes:

  • new - creates empty disk image and populates the rootfs on it
  • reuse - uses already existing image as the base

Plugin mimics standard image creation process, such as:

  • builing base empty image (dd)
  • partitioning (sgdisk / sfdisk)
  • filesystem creation (mkfs.type)
  • partition mapping (losetup)
  • filesystem mount (mount)
  • populate rootfs (tar/unzip/xz etc)
  • setup qemu + chroot
  • customize installation within chroot

The virtualization works via binfmt_misc kernel feature and qemu.

Since the setup varies a lot for different hardware types, the example configuration is available per "board". Currently the following boards are supported (feel free to add more):

  • bananapi-r1 (Archlinux ARM)
  • beaglebone-black (Archlinux ARM, Debian)
  • jetson-nano (Ubuntu)
  • odroid-u3 (Archlinux ARM)
  • odroid-xu4 (Archlinux ARM, Ubuntu)
  • parallella (Ubuntu)
  • raspberry-pi (Archlinux ARM, Raspbian)
  • wandboard (Archlinux ARM)

Quick start

git clone https://github.com/mkaczanowski/packer-builder-arm
cd packer-builder-arm
go mod download
go build

sudo packer build boards/odroid-u3/archlinuxarm.json

Dependencies

  • sfdisk / sgdisk
  • e2fsprogs

Configuration

Configuration is split into 3 parts:

  • remote file config
  • image config
  • qemu config

Remote file

Describes the remote file that is going to be used as base image or rootfs archive (depending on image_build_method)

"file_urls" : ["http://os.archlinuxarm.org/os/ArchLinuxARM-odroid-xu3-latest.tar.gz"],
"file_checksum_url": "http://hu.mirror.archlinuxarm.org/os/ArchLinuxARM-odroid-xu3-latest.tar.gz.md5",
"file_checksum_type": "md5",
"file_unarchive_cmd": ["bsdtar", "-xpf", "$ARCHIVE_PATH", "-C", "$MOUNTPOINT"],
"file_target_extension": "tar.gz",

The file_unarchive_cmd is optional and should be used if the standard golang archiver can't handle the archive format.

Image config

The base image description (size, partitions, mountpoints etc).

"image_build_method": "new",
"image_path": "odroid-xu4.img",
"image_size": "2G",
"image_type": "dos",
"image_partitions": [
    {
        "name": "root",
        "type": "8300",
        "start_sector": "4096",
        "filesystem": "ext4",
        "size": "0",
        "mountpoint": "/"
    }
],

The plugin doesn't try to detect the image partitions because that varies a lot. Instead it solely depend on image_partitions specification, so you should set that even if you reuse the image (method = reuse).

Qemu config

Anything qemu related:

"qemu_binary_source_path": "/usr/bin/qemu-arm-static",
"qemu_binary_destination_path": "/usr/bin/qemu-arm-static"

Chroot provisioner

To execute command within chroot environment you should use chroot communicator:

"provisioners": [
 {
   "type": "shell",
   "inline": [
     "pacman-key --init",
     "pacman-key --populate archlinuxarm"
   ]
 }
]

Flashing

To dump image on device you can use custom postprocessor (really wrapper around dd with some sanity checks):

"post-processors": [
 {
     "type": "flasher",
     "device": "/dev/sdX",
     "block_size": "4096",
     "interactive": true
 }
]   

Other

Generating rootfs archive

While image (.img) format is useful for most cases, you might want to use
rootfs for other purposes (ex. export to docker). This is how you can generate
rootfs archive instead of image:

"image_path": "odroid-xu4.img" # generates image
"image_path": "odroid-xu4.img.tar.gz" # generates rootfs archive

Docker

With artifice plugin you can pass rootfs archive to docker plugins

"post-processors": [
    [{
        "type": "artifice",
        "files": ["rootfs.tar.gz"]
    },
    {
        "type": "docker-import",
        "repository": "mkaczanowski/archlinuxarm",
        "tag": "latest"
    }],
    ...
]

CI/CD

This is the live example on how to use github actions to push image to docker image registry:

cat .github/workflows/archlinuxarm-armv7-docker.yml

Examples

For more examples please see:

tree boards/

Demo

asciicast