[systemd-devel] Read-only /etc, machine-id with an overlay - journald failing

[Dimitri John Ledkov]

On Wed, 26 Feb 2020 at 09:59, Andreas Kempe <andreas.kempe at actia.se> wrote:
>
> Hello everyone,
>
> I'm working in a project with an embedded Linux system based on
> Openembedded using Systemd version 241 as our init process. We're
> using a read-only /etc. To facilitate development, we want to use a
> writeable overlay on /etc, but we ran into an issue.
>
> When we start, Systemd detects that there is no machine-id file
> present in /etc so it generates and mounts a /etc/machine-id. When our
> mount unit then applies the overlay on /etc, it hides the mounted
> file. Journald later fails to start because /etc/machine-id isn't
> visible through the overlay.
>

I would expect the /etc/machine-id to exist, and be an empty file on
the RO underlay, then systemd should setup machine-id in /run, and
then after the overlay of /etc is setup to use and is RW, fire the
systemd-machine-id-commit.service unit which will transfer the
machine-id from /run into the RW /etc overlay, after that everything
else should operate "normal".

We do this in Ubuntu live installer images, which use overlayfs across
all of / on top of read-only squashfs rootfs.


> At this point we're considering a number of workarounds, but I thought
> it worthwhile asking the experts before we go patching Systemd or
> similar.

I think didrocks or pitti introduced above for all the cases we had in
ubuntu where we have RO rootfs with a writable overlay which "appears"
later.

No idea if above is suitable for you at all, and/or need tweaking.
I.e. self-transfer machine-id from /run to /etc with like adding
wants=/before= systemd-machine-id-commit.service or some such?

-- 
Regards,

Dimitri.