linux-kernel - Re: [RFC][PATCH 0/3] fork: Add the ability to create tasks with given pids

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Message-ID: <20111117154936.GB12325@redhat.com>
Date:	Thu, 17 Nov 2011 16:49:36 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...ux.intel.com>,
	Roland McGrath <roland@...k.frob.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	James Bottomley <jbottomley@...allels.com>
Subject: Re: [RFC][PATCH 0/3] fork: Add the ability to create tasks with
	given pids

On 11/17, Pavel Emelyanov wrote:
>
> Gentlemen, please, find some time for this, your ACK/NACK on the API proposal
> is required badly.

Please.

> The proposal is to introduce the CLONE_CHILD_USEPIDS flag for clone() syscall
> and pass the pids values in the child_tidptr. In order not to introduce the
> hole for the pid-reuse attack, using this flag will result in EPERM in case
> the pid namespace we're trying to create pid in has at least one pid (except
> for the init's one) generated with regular fork()/clone().
>
> Currently Tejun and Oleg are worrying only about the intrusiveness of this
> approach, although Oleg agrees, that it solves all the problems it should. The
> previous attempts to implement the similar stuff stopped, but no objections
> against this were expressed. So the decision of whether it's OK to go this
> way or not is required.

Yes, personally I'd prefer /proc/set_last_pid (or something similar) which
simply writes to pid_ns->last_pid. Perhaps it is less convenient from the
user-space pov (serialization, security) but it is much simpler.

OTOH, I do not pretend I understand the user-space needs, so I won't argue.
This series seems correct, the bugs we discussed are fixed.

But. Speaking of API, it differs a bit compared to the previous version...

> The API will be used like in the code below
>
> 	/* restore new pid namespace with an init in it */
> 	pid = clone(CLONE_NEWPID);

Yes, CLONE_NEWPID | CLONE_CHILD_USEPIDS is not possible.

Then how the array of pids in child_tidptr[] can be useful? If CLONE_NEWPID
can't restore the pid_nr's in the parent namespaces, then probably this
doesn't makes sense at all?

IOW. I think we should either allow CLONE_NEWPID | CLONE_CHILD_USEPIDS
(with additional check in set_pidmap() to ensure that CLONE_NEWPID
 comes with child_tidptr[0] == 1), or we should treat the "overloaded"
child_tidptr as a simple pid_t.

Again, I won't insist. Just I want to be sure we do not miss something
adding the new API.

Oleg.

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