[systemd-devel] Mount in volatile mode

[Emmanuel Garette]

Hi,

I am trying to build an immutable image with systemd (v245 on fedora 
32), but I have a problem.

Here's what I'm trying to do:

I would like to build my immutable system with three disk images:

- the first one contains the system, so /usr
- the second one contains the files specific to the instance, so 
/usr/local/lib and it's actually an ISO image
- the third one contains mutable data, so /srv, /var and /var/tmp

The idea is that the first image can be used by several instances. So it 
contains all the common files.

All the specific files (SSL certificates, configuration file with domain 
name, ...) can be found in the second image.

In this usecase, /usr/local/lib must be mounted very early in the boot 
process (before the unit initrd-parse-etc.service).

Without volatile mode, it's easy. We only need to add these lines in the 
/etc/fstab file:

PARTLABEL=srv /srv ext4 defaults,x-systemd.makefs 0 0
PARTLABEL=var /var ext4 defaults,x-systemd.makefs 0 0
PARTLABEL=var-tmp /var/tmp ext4 defaults,x-systemd.makefs 0 0
/dev/sr0 /usr/local/lib iso9660 
ro,x-initrd.mount,nosuid,noexec,uid=0,gid=0,mode=400 0 0

With volatile mode enable (systemd.volatile=yes), it's different.
/etc/fstab file is copied too late by systemd.tmpfiles.

For the 3 first mount point it's not a problem, we just have to creation 
.mount files.

For /usr/local/lib it's different. I'm trying differents way to do that. 
The best way I found is to modify the initramfs (here with dracut):

echo 'add_drivers+="iso9660"' > /etc/dracut.conf.d/cdrom.conf
echo 'add_fstab+=/tmp/fstab' >> /etc/dracut.conf.d/cdrom.conf
echo "/dev/sr0 /sysroot/usr/local/lib iso9660 
ro,x-initrd.mount,nosuid,noexec,uid=0,gid=0,mode=400 0 0" > /tmp/fstab
kernel-install add xxxx /lib/xxxxx

In this case /usr/local/lib is supposed to be mounted in /usr then 
remounted by systemd-volatile-root in read-only mode in 
/run/systemd/volatile-sysroot/usr, which will later become the final /usr.

Unfortunately sometimes it works, sometimes it fails.

systemd-volatile-root does not wait for the partitions to be completely 
mounted before launching.

So sometime /usr/local/lib is present. Sometime it's failed with error: 
mai 21 18:40:22 localhost.localdomain systemd[1]: run.mount: Failed to 
load configuration: No such file or directory

What's the best way to mount extra partition in initramfs in volatile mode?

Maybe it's should be interesting to add a kernerl command line 
parameters to precise the place to volatile fstab. Something like: 
systemd.volatile_fstab=/usr/lib/factory/etc/fstab.

And systemd-volatile-root service should wait that partition with option 
"x-initrd.mount" are mounted before remount in volatile mode.

Regards,

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