[libvirt] Mount events are not propagated to namespaces

[Daniel P. Berrange]

On Fri, Jun 16, 2017 at 10:01:26AM +0200, Michal Privoznik wrote:
> Dear list,
> 
> while trying to fix a bug of mine, I've realized the mounts are not
> being propagated into qemu namespaces once domains are running. That is,
> imagine you have a domain running and then you plug in a flash disk,
> mount it into host, because you have a file there that you want to
> hotplug as a disk to your domain. You're not hotplugging the whole flash
> disk, but a file on it. Problem is, that the mount event of the flash
> disk is not propagated to qemu namespace even though the root is
> remounted as MS_SLAVE|MS_REC right after the namespace is created.
> 
> After some digging. I am able to reproduce this even without libvirt at all:
> 
> # mount --make-rshared /
> # mount | grep floppy
>   <empty/>
> # unshare -m /bin/bash
> # mount --make-rslave /

The problem here is your 'unshare' command, which mounts '/' as
private, and it appears the later '--make-rslave' is unable to
undo this.

If you use

   unshare -m --propagation slave /bin/sh

then the example works correctly.

Also, you shouldn't need the initial '--make-rshare /' as (at least on
systemd host, this is already the case).

You can look in /proc/self/mountinfo to view current propagation
status

Regards,
Daniel
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