linux-kernel - Re: [PATCH 3/3] pids: Make it possible to clone tasks with given pids

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Date:	Sun, 13 Nov 2011 20:28:48 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Glauber Costa <glommer@...allels.com>,
	Nathan Lynch <ntl@...ox.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Serge Hallyn <serue@...ibm.com>,
	Daniel Lezcano <dlezcano@...ibm.com>
Subject: Re: [PATCH 3/3] pids: Make it possible to clone tasks with given
	pids

On 11/11, Pavel Emelyanov wrote:
>
> Hm, so intrusiveness is your main concern here, I see.

To me, this is important.

> OK, let's assume we go with sysctl setting the last_pid.
>
> One of the major concerns with previous attempts have been - someone creates
> a process with a pid that was in use by some app recently and screws things
> up with pid reuse.

Good point.

> My approach solves this,

Yes. Although, can't resist, in a subtle way (imho). CLONE_CHILD_USEPIDS
stops working after a clone() without this flag.

> how can sysctl handle it? Allowing
> the last_pid change by the CAP_SYA_ADMIN

Yes, when I suggested set_last_pid I assumed that it needs CAP_SYS_ADMIN.

> only is not an option, since people
> are looking forward to non-root restore.

But CLONE_NEWPID needs CAP_SYS_ADMIN too ?

Anyway, I do not pretend I understand the problem space. And probably
it would be more convenient to change the creds before forking some
children with the predefined pids, I do not know.

So yes, I agree, CLONE_CHILD_USEPIDS wins here. Perhaps set_last_pid
needs another sysctl(set_last_pid_allowed)/whatever, or another idea.
Or we should use CLONE_CHILD_USEPIDS ;)

Let me repeat. It is not that I strongly against CLONE_CHILD_USEPIDS
(although yes, I can't say personally I like it very much ;). Just
I am trying to ensure we can't make something more clear/clean/simple,
at least from the kernel pov. Especially because you are trying to
establish the new user-visible API.

Oleg.

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