Antw: [EXT] Re: Read-only /etc, machine-id with an overlay - journald failing

["Ulrich Windl" <Ulrich.Windl@xxxxxxxxxxxxxxxxxxxx>]

>>> Andreas Kempe <andreas.kempe@xxxxxxxx> schrieb am 27.02.2020 um 16:30 in
Nachricht
<22004_1582817465_5E57E0B9_22004_221_1_20200227153051.GF26693@hitomi.actianordic

se>:
> On Thu, Feb 27, 2020 at 10:04:37AM +0100, Jérémy ROSEN wrote:
>> Le mer. 26 févr. 2020 à 10:59, Andreas Kempe <andreas.kempe@xxxxxxxx> a
>> écrit :
>> 
>> > 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.
Sorry for re-posting: I forgot to reply to the list:

I wonder whether this issue can be reduced to the question whether the machine
ID should be part of initrd. Of course mass-produced embedded devices might end
up all with the same machine-ID, but there are many ways do do things the wrong
way...


>> >
>> > 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?
>> >
>> > Thank you in advance for any help!
>> > Cordially,
>> > Andreas Kempe
>>
>> I had similar problems with a case of booting with the rootfs read‑only
and
>> then becoming read‑write later...
>> 
>> Basically systemd only checks for machine‑id very early (before reading
any
>> config file) and does not deal well with /etc changing status...
>> 
> 
> It is somewhat comforting knowing that others are seeing similar
> issues. :)
> 
>> I did a complete analysis of what's going on, with a patch that improves
>> the situation here : https://github.com/systemd/systemd/pull/14135 
>> I am not sure how to deal with it in your specific case.
>> the simplest approch would be to mount your overlay in a initrd (or in a
>> small script shell that is run before systemd and exec systemd as its last
>> step)
>> 
> 
> I was contemplating whether it could be acceptable having the same
> static machine‑id file pre‑generated for all systems. I'm not 100% sure
> what it's used for, TBH; would it be a really bad idea?
> 
>> My patch wouldn't really help in your case, but maybe you can "cheat" by
>> having the underlying /etc/machine‑id bein a symlink to the overlay
>> directory... that could work.
>> 
> 
> I had a look at your patch and as you said, it doesn't really solve
> our use case. At the moment, we decided to remove the overlay from the
> affected parts and simply require a new system image if one wants to
> change /etc.
> 
> We were planning on having signed read‑only overlays for configuration
> in the future so I guess we'll have to investigate this further at a
> later date.
> 
> Thank you for taking the time to respond!
> Cordially,
> Andreas Kempe
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