Build a minimal redpesk rootfs with mkosi

Statement

My goal here is to easily build a rootfs (cpio format) which will be used as a ramdisk (like an initramfs).

At IoT.bzh, we're currently working on rootless builds for redpesk OS thanks to mkosi.

The issue with our build method is that this is not fully integrated on a standard host (outside a redpesk Factory stack).

So even our work with mkosi has not been released yet, I chose mkosi because:

• the redpesk rootfs must be easily customizable (scripts, packages...)
• the build can be done on my Linux host (Fedora 42 + rootless or with root privileges)
• the build method must be fast as possible (more than redpesk Factory + Anaconda/kickstarts)
• the output must be a cpio archive (and compress method must be a standard for U-Boot/Kernel)

What is mkosi?

mkosi is a systemd tool that generates disk images https://mkosi.systemd.io.

For redpesk OS, the main goal is that it uses systemd-repart for making gpt partitionning without any priviledge using libfdisk https://www.freedesktop.org/software/systemd/man/latest/systemd-repart.html.

Have a look on the man page for more information:

mkosi --help

or directly online on mkosi github https://github.com/systemd/mkosi/blob/main/mkosi/resources/man/mkosi.1.md

Build rootfs (cpio format)

NXP S32G2 (my target is a aarch64 architecture so it could be another one)

mkosi --debug --debug-workspace \
--force \
-I mkosi-s32g2.conf \
-E REDPESK_DISTRO=batz-2.0-update \
--profile smack,minimal,localrepo,afb \
--environment=MKOSI_DNF=dnf4

Please note I had issues with dnf5 (on Fedora 42) that's why I moved the environment to dnf4. This concerned some specific options of mkosi INI configuration files.

This build is entirely rootless.

Organisation

The idea is to have a configuration file by board/bsp and have different configuration files into mkosi.conf.d/.

There is a default configuration file, load by everybody and after variations are handled with mkosi profiles.

• mkosi-*board*.conf the entry config file for a specfic board
• mkosi.conf.d the directory of snippet of configuration files, many of then can be activated passing profiles
  • default.conf default/common configuration
• repart.d the directory of snippet of partitions for systemd-repart
• scripts the directory of scripts executed during the image build

Troubleshooting

Dynamic Profiles/Include

In snippet configuration, if a new profile is wanted, it needs to be set and included, indeed conf files are only parsed once so if the minimal profile needs to be append see the example below:

# myconf.conf that append minimal
[Config]
Profiles=minimal # add minimal profile in the profiles list

[Include]
Include=mkosi.conf.d/minimal.conf # load minimal.conf

It also works for conditionnal includes, see the afb example to load the afb-app-manager either for smack nor selinux:

# afb.conf
[Config]
Profiles=afb

[Include]
Include=mkosi.conf.d/afb-smack.conf
Include=mkosi.conf.d/afb-selinux.conf
...

# afb-selinux.conf

...
[TriggerMatch]
Profiles=selinux
Profiles=afb

In this example, the two files afb-smack and afb-selinux are included but as it is written in the TriggerMatch of afb-selinux, it needs the afb and the selinux profile to be triggered.

For development

The following command includes development package:

mkosi --debug --debug-workspace \
--force \
-I mkosi-s32g2.conf \
-E REDPESK_DISTRO=batz-2.0-update \
--profile smack,minimal,localrepo,afb,dev \
--environment=MKOSI_DNF=dnf4