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

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Date:	Thu, 17 Nov 2011 19:49:40 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Oleg Nesterov <oleg@...hat.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 11/17/2011 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 then you should provide the 1st
pid in array is 1. Otherwise init in this new pid namespace will have pid != 1 and
the child_reaper assignment (yes, the 45a68628 commit) will be lost :(

> IOW. if copy_process() allocs the first pid in the new pid_ns, it
> always sets ->child_reaper.

Fixup - if it allocates pid == 1, then it sets the child reaper.

> 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.

> Oleg.
> 
> .
> 

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