Re: [PATCH 0/15] Add preliminary support for Linux containers

[ludo@gnu.org (Ludovic Courtès)]

"Thompson, David" <dthompson2@worcester.edu> skribis:

> On Tue, Jul 7, 2015 at 6:28 AM, Ludovic Courtès <ludo@gnu.org> wrote:

[...]

>>>       (lambda ()
>>>         (sethostname "guix-0.8.3"))
>>
>> Surprisingly, calling ‘getpid’ in the thunk returns the PID of the
>> parent (I was expecting it to return 1.)  Not sure why that is the
>> case.  I’m still amazed that this works as non-root, BTW.
>
> The first process created inside the PID namespace gets the honor of
> being PID 1, not the process created with the 'clone' call.
>
> For more information, see: https://lwn.net/Articles/532748/

To me, the thunk above is just like ‘childFunc’ in
<https://lwn.net/Articles/533492/>–i.e., it’s the procedure that ‘clone’
calls in the first child process of the new PID name space.

What am I missing?

>> There’s an issue when the parent’s Guile is not mapped into the
>> container’s file system: ‘use-modules’ forms and auto-loading will fail.
>> For instance, I did (use-modules (ice-9 ftw)) in the parent and called
>> ‘scandir’ in the child, but that failed because of an attempt to
>> auto-load (ice-9 i18n), which is unavailable in the container.
>
> Hmm, I don't know of a way to deal with that other than the user being
> careful to bind-mount in the Guile modules they need.

Right.  Maybe the best we can do is to add a word of caution in the
docstring or something.

> Hmm, there's various reasons that EINVAL would be thrown.  Could you
> readlink "those" files, that is /proc/<pid-outside-container>/ns/user
> and /proc/<pid-inside-container>/ns/user, and tell me if the contents
> are the same?  They shouldn't be, but this will eliminate one of the
> possible causes of EINVAL.

It turns out I was targeting the wrong PID.

>> Also, I think we should add --expose and --share as for ‘guix system’,
>> though that can come later.
>
> Yes, I also really want that, but it's a task for another time.

Sure.

>>> Here's how you build it:
>>>
>>>     guix system container container.scm
>>
>> Very neat.  I wonder if that should automatically override the
>> ‘file-systems’ field to be ‘%container-file-systems’, so that one can
>> reuse existing OS declarations unmodified.  WDYT?
>
> This would be a better user experience, for sure.  I thought about
> this, but I don't know how to do it in a way that isn't surprising or
> just broken.  Ideas?

IMO it’d be fine to simply override the subset of ‘file-systems’ that
clashes with ‘%container-file-systems’, similar to what
‘virtualized-operating-system’ does in (gnu system vm).

WDYT?

Thanks,
Ludo’.