Date:   Tue, 25 May 2021 13:49:48 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     menglong8.dong@...il.com
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Subject: Re: [PATCH v2 2/3] init/do_cmounts.c: introduce 'user_root' for initramfs

menglong8.dong@...il.com writes:

> From: Menglong Dong <dong.menglong@....com.cn>
>
> If using container platforms such as Docker, upon initialization it
> wants to use pivot_root() so that currently mounted devices do not
> propagate to containers. An example of value in this is that
> a USB device connected prior to the creation of a containers on the
> host gets disconnected after a container is created; if the
> USB device was mounted on containers, but already removed and
> umounted on the host, the mount point will not go away until all
> containers unmount the USB device.
>
> Another reason for container platforms such as Docker to use pivot_root
> is that upon initialization the net-namspace is mounted under
> /var/run/docker/netns/ on the host by dockerd. Without pivot_root
> Docker must either wait to create the network namespace prior to
> the creation of containers or simply deal with leaking this to each
> container.
>
> pivot_root is supported if the rootfs is a initrd or block device, but
> it's not supported if the rootfs uses an initramfs (tmpfs). This means
> container platforms today must resort to using block devices if
> they want to pivot_root from the rootfs. A workaround to use chroot()
> is not a clean viable option given every container will have a
> duplicate of every mount point on the host.
>
> In order to support using container platforms such as Docker on
> all the supported rootfs types we must extend Linux to support
> pivot_root on initramfs as well. This patch does the work to do
> just that.
>
> pivot_root will unmount the mount of the rootfs from its parent mount
> and mount the new root to it. However, when it comes to initramfs, it
> donesn't work, because the root filesystem has not parent mount, which
> makes initramfs not supported by pivot_root.
>
> In order to support pivot_root on initramfs we introduce a second
> "user_root" mount which is created before we do the cpio unpacking.
> The filesystem of the "user_root" mount is the same the rootfs.
>
> While mounting the 'user_root', 'rootflags' is passed to it, and it means
> that we can set options for the mount of rootfs in boot cmd now.
> For example, the size of tmpfs can be set with 'rootflags=size=1024M'.

What is the flow where docker uses an initramfs?

Just thinking about this I am not being able to connect the dots.

The way I imagine the world is that an initramfs will be used either
when a linux system boots for the first time, or an initramfs would
come from the distribution you are running inside a container.  In
neither case do I see docker being in a position to add functionality
to the initramfs as docker is not responsible for it.

Is docker doing something creating like running a container in a VM,
and running some directly out of the initramfs, and wanting that code
to exactly match the non-VM case?

If that is the case I think the easy solution would be to actually use
an actual ramdisk where pivot_root works.

I really don't see why it makes sense for docker to be a special
snowflake and require kernel features that no other distribution does.

It might make sense to create a completely empty filesystem underneath
an initramfs, and use that new rootfs as the unchanging root of the
mount tree, if it can be done with a trivial amount of code, and
generally make everything cleaner.

As this change sits it looks like a lot of code to handle a problem
in the implementation of docker.   Which quite frankly will be a pain
to have to maintain if this is not a clean general feature that
other people can also use.

Eric

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