[lxc-users] Establish a bind mount to a running container

[Jäkel, Guido]

>-----Original Message-----
>From: lxc-users [mailto:lxc-users-bounces at lists.linuxcontainers.org] On Behalf Of Stéphane Graber
>Sent: Friday, October 07, 2016 4:26 PM
>To: LXC users mailing-list
>Subject: Re: [lxc-users] Establish a bind mount to a running container
>
>On Fri, Oct 07, 2016 at 02:19:53PM +0000, Jäkel, Guido wrote:
>> >-----Original Message-----
>> >From: lxc-users [mailto:lxc-users-bounces at lists.linuxcontainers.org] On Behalf Of Stéphane Graber
>> >Sent: Friday, October 07, 2016 3:38 PM
>> >To: LXC users mailing-list
>> >Subject: Re: [lxc-users] Establish a bind mount to a running container
>> >
>> >On Fri, Oct 07, 2016 at 01:26:26PM +0000, Jäkel, Guido wrote:
>> >> >-----Original Message-----
>> >> >From: lxc-users [mailto:lxc-users-bounces at lists.linuxcontainers.org] On Behalf Of Stéphane Graber
>> >> >Sent: Friday, October 07, 2016 11:46 AM
>> >> >To: LXC users mailing-list
>> >> >Subject: Re: [lxc-users] Establish a bind mount to a running container
>> >> >
>> >> >On Fri, Oct 07, 2016 at 07:03:21AM +0000, Jäkel, Guido wrote:
>> >> >> Dear experts,
>> >> >>
>> >> >> I wonder if it's possible to establish a bind mount filesystem resource from the LXC host to an already running
>> >container in
>> >> >an manual way, but analogous as it is done at startup time.
>> >> >>
>> >> >> I already figured out that the releasing an existing link is no thing; just umount it from inside the container. But is
>> >there a
>> >> >way to establish one while shifting the destination of a bind mount into the right namespace?
>> >> >>
>> >> >> I ask about, because in a couple of days I have to change a (NFS) filesystem source (because of an hardware
>migration)
>> >> >that is common to a large number of running containers but not frequently used and I want to avoid to restart all
>the
>> >> >containers with it services.
>> >> >>
>> >> >> thank you for advice
>> >> >>
>> >> >> Guido
>> >> >
>> >> >It's very difficult due to a number of restrictions in place in the kernel.
>> >> >
>> >> >The only way of doing this that I'm aware of is what we do in LXD. We
>> >> >create a path on the host before the container starts, put that on a
>> >> >rshared mountpoint, then bind-mount that directory into the container
>> >> >under some arbitrary path.
>> >> >
>> >> >Then when you want to inject a new mount in the container, you can mount
>> >> >it in a sub-directory of that path you create on the host, which will
>> >> >then have the container inherit the mount entry thanks to the host
>> >> >mountpoint being rshared and the container's mountpoint being rslave.
>> >> >
>> >> >Once the mountpoint shows up in the container, you can then move it to
>> >> >whatever path you actually want it on.
>> >>
>> >>
>> >> Dear Stéphane,
>> >>
>> >> I sorry, but I don't get it yet; some of your terms and where to do it are dubious to me. Maybe an example may light it
>up
>> >to me:
>> >>
>> >> Let say, I want to inject the path  host:/mnt/some_host_mountpoint/some_directory  as a bind mount to a running
>> >container; it should end up on  container:/import/some_container_moutpoint  . On the host, the mountpoint
>> >host:/mnt/some_host_mountpoint is mouted to a NFS source, let say  nfshost:/some_export
>> >>
>> >> Now please, where to issue which commands?
>> >
>> >You'll have to do some research yourself or hope that someone can give
>> >you step by step instructions :)
>> >
>> >
>> >A guestimate (completely untested) would be:
>> >
>> >Setup steps, before you first start the container:
>> >
>> > 1) mkdir /tmp/shared-c1
>> > 2) mount --bind /tmp/shared-c1 /tmp/shared-c1
>> > 3) mount --make-rshared /tmp/shared-c1
>> > 4) Add to /var/lib/lxc/c1/config => lxc.mount.entry=/tmp/shared-c1 /.shared none bind,create=dir 0 0
>> > 5) lxc-start -n c1
>> >
>> >At which point, you could inject a new mount with:
>> >
>> > 1) mkdir /tmp/share-c1/some_directory
>> > 2) mount -- bind /mnt/some_host_mountpoint/some_directory /tmp/share-c1/some_directory
>> > 3) lxc-attach -n c1 -- mkdir -p /import/some_container_mountpoint
>> > 4) lxc-attach -n c1 -- mount --move /.shared/some_directory /import/some_container_mountpoint
>> >
>> >
>> >Again, that's a very rough approximation from what I remember the LXD
>> >code is doing (though we're doing it in a mix of Go and C).
>>
>>
>> Thanks again. I don't read through it but the first thing I read: "Steps to to before first start of the container". Does that
>mean, that there's no chance in my case, because all the containers are already running?
>>
>> Or may I (miss)use an established bind mount (or even the rootfs mount) to create the path /tmp/shared "here" at it's
>source path on the host , then "over-bind-mount-and-rshared" it on the host and then continue the injection?
>
>If you're lucky enough that one of your established bind-mounts is
>rshared, then you could. But given that it's not the default, your
>chances are pretty slim.
>
>It's easy enough to try though, just create a directory on the host side
>of your existing bind-mount, bind-mount the filesystem you want over
>that and see if you can see it in the container.

There is no tags stating a shared state in /proc/self/mountinfo,neigther on the host nor in the container. As expected, in the simple way you described there's nothing to see inside the container.