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Re: [libvirt] [PATCH] LXC: mount a fresh /run directory for container



On 08/22/2013 05:41 PM, Daniel P. Berrange wrote:
> On Thu, Aug 22, 2013 at 08:57:49AM +0800, Gao feng wrote:
>> On 08/21/2013 05:31 PM, Daniel P. Berrange wrote:
>>> On Wed, Aug 21, 2013 at 04:22:29PM +0800, Gao feng wrote:
>>>> The unix socket file /run/systemd/private is used to
>>>> send reboot/shutdown messages. and since this type of
>>>> unix sockets are not per net namespace , they are
>>>> global resources. systemctl in container can use
>>>> this unix socket to send shutdown message to the
>>>> systemd-shutdownd running on host. finally the
>>>> host will be poweroff.
>>>>
>>>> this problem occurs when container shares the same
>>>> root directory with host.
>>>>
>>>> this patch umount host's /run directory and mount
>>>> the /run directory of container as tmpfs.
>>>>
>>>> Signed-off-by: Gao feng <gaofeng cn fujitsu com>
>>>> ---
>>>>  src/lxc/lxc_container.c | 5 +++++
>>>>  1 file changed, 5 insertions(+)
>>>
>>> I don't think we should be doing this by default. IMHO this is something
>>> the mgmt app / admin should take care of it they want to have separate
>>> /run.
>>>
>>> You may be preventing access to the systemd socket by doing this, but
>>> equally you can be breaking any number of other valid use cases by
>>> hiding the host's /run
>>
>> We can't assume user know the root reason why shutdown in container will
>> shut down the host. they don't know it's because of container shares the
>> /run/ directory with host. This will confuse them and bring bad image to
>> them. We have lxcContainerHasReboot in libvirt, and it did tell user that
>> "Containerized reboot support is available", but the fact is reboot in
>> container will reboot host.
>>
>> and the /run directory is mounted as tmpfs on host. it means the files
>> under /run are temporary, I don't think it's meaningful to share these
>> files with container.
>>
>> If someone really want to share host's /run directory with container, he
>> should add this filesystem configuration to the domain xml.
> 
> Quite simply, no.
> 
> If the user asks for '/', then that's what they'll get. If they want
> to hide /run they can do so.
> 

Users don't know why sharing root directory with host will cause host
can be rebooted from container. pid namespace is enabled by libvirt lxc and
actually libvirt did say that "Containerized reboot support is available".
it's hard for user to find out that container shouldn't share /run directory
with host. it's easy for them to find out some files are leaked under /run
directory for container, and then add this filesystem configuration to the
domain xml file.


And I still think it really make no sense for container to share /run
directory with host.

> What you're describing is a usability policy issue, solution to which
> belongs in the tools.
> 
> If you are editting XML directly to configure guests, it is expected
> that you know what you are doing.
> 
>>> Ultimately user namespace should prevent access to the systemd
>>> sockets for people wanting a secure setup without replacing /run
>>>
>>
>> Some people may think user namespace is too strict, they may dislike
>> to enable user namespace, just like they may want share net namespace
>> with host. They have rights to start a container which shares same
>> user namespace with host.
> 
> They have the ability to specify a new mount of /run if they so desire.
> 

Yep, but they don't know how to fix this reboot-problem until they read
this thread or some documents somewhere.

I think though users know sharing root directory with host will bring bad influence.
I guess they must don't know this will make their host can be powered off by the
user in container.

Thanks


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