[Buildroot] add option to run docker with initramfs

[Buildroot] add option to run docker with initramfs

[Fragkiskos Sigalas]

Hi,

By default docker uses pivot_root which is not available in initramfs. The
way to override the default behaviour is to define an environment variable
DOCKER_RAMDISK when starting the daemon (weirdly enough there appears to be
no respective command line option).

Would it make sense to add such an option in the docker package?

Regards,
Frank
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[Buildroot] add option to run docker with initramfs

[Yann E. MORIN]

Frank, All,

On 2019-06-12 19:18 +0300, Fragkiskos Sigalas spake thusly:
> By default docker uses pivot_root which is not available in initramfs.

pivot_root is provided by either util-linux or busybox; it has (AFAICT)
no relation to whether running from an initramfs or any other filesystem).

Our default configuration for busybox eeven enables pivot_root.

So, it looks like your could very well add pivot_root to your initramfs,
in fact.

Regards,
Yann E. MORIN.

> The way to override the default behaviour is to define an
> environment variable DOCKER_RAMDISK when starting the daemon (weirdly enough there appears to be no respective command line option).
> Would it make sense to add such an option in the docker package?



-- 
.-----------------.--------------------.------------------.--------------------.
|  Yann E. MORIN  | Real-Time Embedded | /"\ ASCII RIBBON | Erics' conspiracy: |
| +33 662 376 056 | Software  Designer | \ / CAMPAIGN     |  ___               |
| +33 561 099 427 `------------.-------:  X  AGAINST      |  \e/  There is no  |
| http://ymorin.is-a-geek.org/ | _/*\_ | / \ HTML MAIL    |   v   conspiracy.  |
'------------------------------^-------^------------------^--------------------'

[Buildroot] add option to run docker with initramfs

[Christian Stewart]

Frank, all,

Frank is referring to a docker environment variable that must be set in
order to change the root pivot step to be compatible with a ramfs

I do this in my SkiffOS project,

system/docker.service.d/ramdisk.conf

[Service]
Environment=DOCKER_RAMDISK=true

On Wed, Jun 12, 2019, 1:19 PM Yann E. MORIN <yann.morin.1998@free.fr> wrote:

> > By default docker uses pivot_root which is not available in initramfs.
>
> pivot_root is provided by either util-linux or busybox; it has (AFAICT)
> no relation to whether running from an initramfs or any other filesystem).
>
> Our default configuration for busybox eeven enables pivot_root.
>

This is more related to docker behavior than busybox or anything else.

Best,
Christian
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[Buildroot] add option to run docker with initramfs

[Fragkiskos Sigalas]

Yann, All,

In order to use pivot_root a real device is needed, alas initramfs space is
not a real one.
Config.in for util-linux in busybox states:

The pivot_root utility swaps the mount points for the root filesystem
        with some other mounted filesystem. This allows you to do all sorts
        of wild and crazy things with your Linux system and is far more
        powerful than 'chroot'.

        Note: This is for initrd in linux 2.4. Under initramfs (introduced
        in linux 2.6) use switch_root instead.

So in case of initramfs switch_root should be used instead. I think that
dockerd
uses MS_MOVE with chroot, but this is of less or no importance.

On Wed, Jun 12, 2019 at 11:19 PM Yann E. MORIN <yann.morin.1998@free.fr>
wrote:

> Frank, All,
>
> On 2019-06-12 19:18 +0300, Fragkiskos Sigalas spake thusly:
> > By default docker uses pivot_root which is not available in initramfs.
>
> pivot_root is provided by either util-linux or busybox; it has (AFAICT)
> no relation to whether running from an initramfs or any other filesystem).
>
> Our default configuration for busybox eeven enables pivot_root.
>
> So, it looks like your could very well add pivot_root to your initramfs,
> in fact.
>
> Regards,
> Yann E. MORIN.
>
> > The way to override the default behaviour is to define an
> > environment variable DOCKER_RAMDISK when starting the daemon (weirdly
> enough there appears to be no respective command line option).
> > Would it make sense to add such an option in the docker package?
>
>
>
> --
>
> .-----------------.--------------------.------------------.--------------------.
> |  Yann E. MORIN  | Real-Time Embedded | /"\ ASCII RIBBON | Erics'
> conspiracy: |
> | +33 662 376 056 | Software  Designer | \ / CAMPAIGN     |  ___
>      |
> | +33 561 099 427 `------------.-------:  X  AGAINST      |  \e/  There is
> no  |
> | http://ymorin.is-a-geek.org/ | _/*\_ | / \ HTML MAIL    |   v
>  conspiracy.  |
>
> '------------------------------^-------^------------------^--------------------'
>
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