Re: Update on GuixSD containers

[ludo@gnu.org (Ludovic Courtès)]

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

> On Sat, Jun 13, 2015 at 4:19 PM, Ludovic Courtès <ludo@gnu.org> wrote:
>> "Thompson, David" <dthompson2@worcester.edu> skribis:
>>
>>> On Sat, Jun 13, 2015 at 9:06 AM, Ludovic Courtès <ludo@gnu.org> wrote:
>>>> "Thompson, David" <dthompson2@worcester.edu> skribis:
>>>>
>>>>> On Fri, Jun 12, 2015 at 11:12 AM, Ludovic Courtès <ludo@gnu.org> wrote:
>>>>>> "Thompson, David" <dthompson2@worcester.edu> 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.

Yes, I understand.  This is definitely an important concern for full
GuixSD containers.

However, ‘eval-in-container’ would be much simpler, synchronous, and
typically for short-lived processes.  So I guess the process that runs
‘eval-in-container’ would clone(2) (via ‘call-with-container’) and
simply waitpid(2) the child process (which is PID 1 in its container).

When the parent process gets a SIGINT or SIGHUP, it could send SIGKILL
to the child, thereby terminating the container.

Does that make sense?

>> (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?

Yes, with setuid (see build.cc:2180.)

Thanks,
Ludo’.