Bug#845161: udev: "mount: invalid option --" when busybox is not installed

[Simon McVittie]

On Mon, 21 Nov 2016 at 09:50:39 +0100, Martin Pitt wrote:
> Michael Biebl [2016-11-21  1:15 +0100]:
> > Am 21.11.2016 um 00:50 schrieb Simon McVittie:
> > > Because udev's initramfs hook now does "mount --move", it doesn't work with
> > > klibc mount, which is used when busybox isn't installed:
> > 
> > 
> > This was changed as a response to
> > https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=844775
> > 
> > Luca, you wrote that initramfs-tools is now using mount from util-linux,
> > but I can't find anything in the initramfs-tools changelog

For what it's worth, initramfs-tools in Debian hasn't changed since April,
and appears to use "mount -o move" unconditionally in its own files
(for example in /init, to move /run to /root/run). So it might be best to
just revert that change and stick to "mount -o move", which is known to
work in both busybox and klibc mount.

I can't help wondering why we support three different mount
implementations here, rather than mandating a preferred implementation.

For what it's worth, dracut appears to install the "full-fat" util-linux
mount binary from the host system rather than trying to use statically
linked or otherwise minimal binaries.

> -# move the /dev tmpfs to the rootfs
> -mount -n --move /dev ${rootmnt}/dev
> +# move the /dev tmpfs to the rootfs; fall back to klibc mount that does not
> +# understand --move
> +mount -n --move /dev ${rootmnt}/dev || mount -n -o move /dev ${rootmnt}/dev

This looks fine as a short-term answer.

Longer-term, it would be great if we had a specification for how a
suitable mount implementation for the initramfs must behave, or a consensus
on which mount implementations had to be supported, or even a requirement
that potential mount implementors (klibc or busybox or util-linux) define
a mount_move() shell function or something.

(An alternative long-term direction would be defaulting to dracut, which
has the advantage of not being Debian-specific.)

    S