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

[Andreas Kempe]

On Thu, Feb 27, 2020 at 05:39:27PM +0200, Mantas Mikulėnas wrote:
> On Wed, Feb 26, 2020 at 11:59 AM 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.
> >
> > 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.
> >
> > My gut feeling is that using overlays on /etc can't be that uncommon
> > and it is likely PEBKAC on our end. Is there some canonical way of
> > doing overlays with Systemd and we're screwing things up?
> >
> 
> If you have an initramfs, consider setting up the /etc overlay there
> instead.
> 

For the development assisting part of it, that is unfortunately not an
option for various reasons. We will have some overlays in the final
released product as well, but there we will mount them in the
initramfs so hopefully it won't be a problem then.

Cordially,
Andreas Kempe