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From: ludo@gnu.org (Ludovic =?utf-8?Q?Court=C3=A8s?=)
Subject: Re: [PATCH 0/15] Add preliminary support for Linux containers
Date: Wed, 08 Jul 2015 23:59:52 +0200
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	(David Thompson's message of "Wed, 8 Jul 2015 09:00:33 -0400")
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To: "Thompson, David" <dthompson2@worcester.edu>
Cc: guix-devel <guix-devel@gnu.org>

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

> On Wed, Jul 8, 2015 at 8:46 AM, Ludovic Court=C3=A8s <ludo@gnu.org> wrote:
>> "Thompson, David" <dthompson2@worcester.edu> skribis:
>>
>>> On Tue, Jul 7, 2015 at 6:28 AM, Ludovic Court=C3=A8s <ludo@gnu.org> wro=
te:
>>
>> [...]
>>
>>>>>       (lambda ()
>>>>>         (sethostname "guix-0.8.3"))
>>>>
>>>> Surprisingly, calling =E2=80=98getpid=E2=80=99 in the thunk returns th=
e PID of the
>>>> parent (I was expecting it to return 1.)  Not sure why that is the
>>>> case.  I=E2=80=99m 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 =E2=80=98childFunc=E2=80=99 in
>> <https://lwn.net/Articles/533492/>=E2=80=93i.e., it=E2=80=99s the proced=
ure that =E2=80=98clone=E2=80=99
>> calls in the first child process of the new PID name space.
>>
>> What am I missing?
>
> It's non-intuitive because PID namespaces are given special treatment.
> The cloned process is like PID 1 in the sense that if you fork, the
> new process is PID 2.  However, if you call 'getpid' in the cloned
> process, it returns the PID in the context of the parent PID
> namespace, and you are expecting PID 1.
>
> In that example from LWN, 'childFunc' calls 'execvp', and *that* new
> process becomes PID 1 (and 'getpid' agrees).  This is the usual
> pattern I see in all container implementations:  The process that
> calls clone sets up the environment and then execs the real init
> system.
>
> Is it more clear now?

Yes, indeed.  The weird part is that =E2=80=98exec=E2=80=99 does not create=
 a new
process, so it=E2=80=99s not supposed to change the return value of =E2=80=
=98getpid=E2=80=99.
But I guess it=E2=80=99s just an artifact of the whole name space hack.  ;-)

Thanks!

Ludo=E2=80=99.