How do I mount an overlay over /sysroot?

[oddlama]

I'm trying to create a small dracut module [1] that allows to mount a writable overlay over the whole root partition. The idea is not to create a live-stick environment, but to have a read-only /sysroot with a writable overlay on a second disk, specified separately like rootoverlay=UUID=.... This is useful when spinning up many VMs that share a common system installation but have to be configured slightly differently.

My problem is, I want to allow dracut to setup /sysroot as usual, using whatever it would use normally. Unfortunately, systemd doesn't allow to create a second mount unit with the same Where= as the already existing sysroot.mount unit, as the filename must match the Where option. So I cannot mount an overlayfs on top of /sysroot before switch-root occurs. I also cannot change where /sysroot gets mounted, because a lot of dracut modules hard-code /sysroot (e.g. ZFS).

So the only remaining option that I see is to mount the overlay to e.g. /sysroot-overlay, and tell dracut right after mounting it, that from that point NEWROOT=/sysroot-overlay, so that /sys, /proc, ... are mounted in this directory, and so the switchroot will occur correctly. Unfortunately I cannot find any documentation on how to achieve this rather specific request. I'd be happy about any pointers.

[LaszloGombos]

@oddlama Have you had a chance to try rd.live.overlay=UUID=99440c1f-8daa-41bf-b965-b7240a8996f4 ?

See https://man7.org/linux/man-pages/man7/dracut.cmdline.7.html#:~:text=live.overlay%3DUUID%3D-,99440c1f,-%2D8daa%2D41bf%2Db965

Also relevant - #1744