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

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Message-ID: <20111117160011.GC12325@redhat.com>
Date:	Thu, 17 Nov 2011 17:00:11 +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: [PATCH 3/3] pids: Make it possible to clone tasks with given
	pids

On 11/17, Pavel Emelyanov wrote:
>
> On 11/17/2011 07:32 PM, Oleg Nesterov wrote:
> > On 11/17, Pavel Emelyanov wrote:
> >>
> >> +static int set_pidmap(struct pid_namespace *pid_ns, int pid)
> >> +{
> >> +	int offset;
> >> +	struct pidmap *map;
> >> +
> >> +	/*
> >> +	 * When creating a new pid namespace we must make its init
> >> +	 * have pid == 1 in it.
> >> +	 */
> >> +	if (pid_ns->child_reaper == NULL)
> >> +		return 0;
> >
> > Do we really need this check? please see below...
> >
> >> +	/*
> >> +	 * Don't allow to create a task with a pid which has recently
> >> +	 * belonged to some other (dead already) task. Only init (of
> >> +	 * a freshly created namespace) and his clones can do this.
> >> +	 */
> >> +	if (pid_ns->last_pid != 1)
> >> +		return -EPERM;
> >
> > ->last_pid == 1. This means that pid_nr == 1 was already created
> > in this namespace via CLONE_NEWPID, and the child with this pid
> > must be ->child_reaper, no?
>
> If you use the CLONE_NEWPID | CLONE_CHILD_USEPIDS

Ah wait... I misread the check above as if it returns the error if
->child_reaper == NULL.

So, CLONE_NEWPID simply ignores child_tidptr[0], alloc_pid()
fallbacks to alloc_pidmap() after set_pidmap() returns 0.

> > Cough. I really think 45a68628 should be reverted ;) IMHO it
> > complicates the understanding of CLONE_NEWPID logic.
>
> If we remove it, then it's OK to remove the check above, but in this case we
> make it possible to have an init with pid != 1. This is flexible, but ... strange.

No, I didn't mean we should allow ->child_reaper with pid != 1, sorry
for confusion.

Oleg.

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