[systemd-devel] Questions about systemd's "root storage daemon" concept

[Martin Wilck]

On Tue, 2021-01-26 at 11:33 +0100, Lennart Poettering wrote:
> 
> > 
> > [Unit]
> > Description=NVMe Event Monitor for Automatical Subsystem Connection
> > Documentation=man:nvme-monitor(1)
> > DefaultDependencies=false
> > Conflicts=shutdown.target
> > Requires=systemd-udevd-kernel.socket
> > After=systemd-udevd-kernel.socket
> 
> Why do you require this?
> 

Brain fart on my part. I need to connect to the kernel socket, but that
doesn't require the systemd unit.

> My guess: the socket unit gets shutdown, and since you have Requires=
> on it you thus go away too.

That was it, thanks a lot. So obvious in hindsight :-/

Meanwhile I've looked a bit deeper into the problems accessing "/dev"
that I talked about in my other post. scandir on "/" actually returns
an empty directory after switching root, and any path lookups for
absolute paths fail. I didn't expect that, because I thought systemd
removed the contents of the old root, and stopped on (bind) mounts.
Again, this is systemd-234.

If I chdir("/run") before switching root and chroot("..") afterwards
(*), I'm able to access everything just fine (**). However, if I do
this, I end up in the real root file system, which is what I wanted to
avoid in the first place.

So, I guess I'll have to create bind mounts for /dev, /sys etc. in the
old root, possibly after entering a private mount namespace?

The other option would be to save fd's for the file systems I need to
access and use opendirat() only. Right?

Regards,
Martin

(*) Michal suggested to simply do chroot(".") instead. That might as
well work, I haven't tried it yet.

(**) For notification about switching root, I used epoll(EPOLLPRI) on
/proc/self/mountinfo, because I read that inotify doesn't work on proc.
polling for EPOLLPRI works just fine.