CLONE_PARENT in a container
Eric W. Biederman
ebiederm at xmission.com
Thu Mar 19 20:25:29 PDT 2009
Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com> writes:
> Cc: Oleg, Eric
> Oren Laadan [orenl at cs.columbia.edu] wrote:
> | What happens when a container-init calls clone() with the
> | CLONE_PARENT flag set ?
> | Since CLONE_PARENT can be used to create a sibling, I'd
> | think that this will create a sibling, in particular, a
> | new task in the same container whose parent is the parent
> | of the container. From a quick look in the code I can't
> | see why this would be impossible.
> | Is this so ? Is this the desired behavior ?
> Good question. CLONE_PARENT was discussed recently on lkml but did
> not look obvious to me who uses it or what the semantics are.
> Some observations.
> - the "reaper" for this sibling would be the reaper of the
> parent container, not the init of the new container.
> - if container-init exits, this sibling will also be killed since
> it has a pid in this container.
> Not sure if it needs to be prevented though. An using CLONE_PARENT
> may want to run as a container-init :-) And if CLONE_PARENT is used
> with CLONE_THREAD, we don't want to preclude threaded container-inits.
Fascinating. We have a way of generating processes that breaks the unix
In the initial pid namespace a init that calls CLONE_PARENT will be the
reaper of that child, because the idle thread is in the same pid namespace.
I hadn't even thought about CLONE_PARENT in the context of a pid namespace.
CLONE_PARENT is a rare uncommon case a left over from the first experiments of
threading in linux. So I would not work hard at getting it to do the right thing.
If it is a problem I would kill it.
However if we can support processes who don't have init as their parent it makes
entering a pid namespace a much more realistic proposition.
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