Systemd, read-only rootfs and overlay file system over /etc

Systemd is a popular init system, used to bootstrap user space and manage user processes. It now replaces several Linux utilities with its own components like log management, networking, time management, etc. There is even a bootloader component now. Systemd is obviously ubiquitous nowadays for desktop/server Linux distributions, and is also commonly used on embedded devices to benefit from features such as parallel startup of services, monitoring of services, and more.

In a recent project that uses Buildroot as its build system, we have used systemd with the storage consisting of a read-only root filesystem (SquashFS) and an overlay file system (OverlayFS) mounted on /etc. While doing this, we faced two issues with the use of OverlayFS on /etc:

  • /etc/machine-id file management. This file is created during the first boot by systemd, and if the root filesystem is read-only, it will bind mount it to /run and wait to have read-write access to create it (see more details). In that case, the machine-id file is re-generated at each boot (because /run is a tmpfs, which means that the machine identification changes at each boot, which is not necessarily desirable. On the other hand, we don’t want to machine-id file to be part of the SquashFS filesystem because the SquashFS filesystem is identical on all devices, while the /etc/machine-id file is unique per device. So ideally, we would like this machine-id file to be stored in our OverlayFS, generated during the first boot. The issue is that reading the machine-id file is done very early by systemd, before we get the chance to mount the OverlayFS.
  • We wanted to be able to add or modify systemd services using the OverlayFS. Systemd parses the service files at early init and executes them according to their order and dependencies. The service mounting the filesystems from /etc/fstab and any other services is started after such parsing, which is too late. We could think of running daemon-reload from a custom service once mounting was complete, but this is not really a stable solution, as
    Lennart Poettering commanted on in a short e-mail thread about this issue.

The solution suggested by Lennart, and elsewhere on the wider Internet is to mount the OverlayFS from an initramfs, which allows to have it setup before systemd even starts. As we use Buildroot and using an initramfs adds complexity by requiring a separate configuration to manage multiple images. This was overkill in our case, just for setting up the overlay. The solution we eventually chose was to create an script which is started as init before systemd, by adding init=/sbin/ to the kernel command line:

mount -t proc -o nosuid,nodev,noexec none /proc
mount -t sysfs -o nosuid,nodev,noexec none /sys
mount /dev/mmcblk0p2 /mnt/data
mount -t overlay overlay -o lowerdir=/etc,upperdir=/mnt/data/etc,workdir=/mnt/data/.etc-work /etc
exec /sbin/init

Hopefully, this will be useful to others. Of course, we’re also curious to hear if others faced the same issue, and discover how they solved this. Let us know in the comments.