Re: Update on GuixSD containers

[Thompson, David]

On Sat, Jun 13, 2015 at 4:19 PM, Ludovic Courtès <address@hidden> wrote:
> "Thompson, David" <address@hidden> skribis:
>
>> On Sat, Jun 13, 2015 at 9:06 AM, Ludovic Courtès <address@hidden> wrote:
>>> "Thompson, David" <address@hidden> skribis:
>>>
>>>> On Fri, Jun 12, 2015 at 11:12 AM, Ludovic Courtès <address@hidden> wrote:
>>>>> "Thompson, David" <address@hidden> skribis:
>>>>>
>>>>>> Yeah, our daemon would do the same thing.  We could maybe even have a
>>>>>> little Guile library that allows one to evaluate arbitrary scheme code
>>>>>> from within the container. :)
>>>>>
>>>>> Actually, something quite easily feasible would be this:
>>>>>
>>>>>   (eval-in-container #~(system* #$evil-program
>>>>>                                 #$(local-file "important-data.txt"))
>>>>>                      #:networking? #f)
>>>>>
>>>>> ... where the container’s store would be populated with just
>>>>> EVIL-PROGRAM and the local file.
>>>>>
>>>>> Food for thought...
>>>>
>>>> Ooooh yeah!  That would be cool.  Though I think we should still spawn
>>>> a dmd process as PID 1 to deal with reaping zombie processes.  We
>>>> could generate a single service that runs the gexp script.  How does
>>>> that sound?
>>>
>>> Wouldn’t it be enough to have the Guile process that evaluates the
>>> expression be PID 1 in the container, as is the case in guix-daemon
>>> containers?
>>
>> Sure, it would work, but my concern is that a long-running process on
>> a user's machine could create and orphan tons of child processes and
>> nothing would be able to clean them up until the PID namespace is
>> garbage collected.
>
> My understanding was that killing a container’s PID 1 (from the outside)
> effectively killed all the processes of that PID name space.  Isn’t it
> the case?

Yes, that is the case.  That triggers the "garbage collection" of that
namespace, if you will.  My point is that, without a proper PID 1 that
can DTRT with orphaned processes, a long running process in a
container could potentially create a ton of orphaned child processes
with no way for them to be reaped without killing PID 1.  I wouldn't
be very happy if a program that I was running in a sandbox was
polluting the process list.  I don't think this is a concern for the
build daemon because the build process is a (relatively) short-lived
process, but running something like a web browser could go on for
days, weeks, etc.

> (The daemon works around that by running processes under a separate UID
> and doing kill(-1, SIGKILL) under that UID.)

So, PID 1 in the build container forks and changes the UID or
something?  Sorry, I'm a bit lost right now.

Thanks for trying to explain.

- Dave