linux-kernel - Re: For review: pid_namespaces(7) man page

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Date:	Tue, 05 Mar 2013 18:23:57 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Rob Landley <rob@...dley.net>
Cc:	mtk.manpages@...il.com, linux-man <linux-man@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: For review: pid_namespaces(7) man page

Rob Landley <rob@...dley.net> writes:

> On 03/04/2013 11:52:19 AM, Eric W. Biederman wrote:
>> > How about this:
>> >
>> >        The  point  here is that unshare(2) and setns(2) change the  
>> PID
>> >        namespace for processes subsequently created by the caller,  
>> but
>> >        not  for the calling process, while clone(2) CLONE_VM  
>> specifies
>> >        the creation of a new thread in the same process.
>> 
>> Hmm.  How about this.
>> 
>>          The point here is that unshare(2) and setns(2) change the PID
>>          namespace that will be used by in all subsequent calls to  
>> clone
>>          and fork by the caller, but not for the calling process, and
>>          that all threads in a process must share the same PID
>>          namespace.  Which makes a subsequent clone(2) CLONE_VM
>>          specify the creation of a new thread in the a different PID
>>          namespace but in the same process which is impossible.
>
> CLONE_VM and CLONE_NEWPID are incompatible because all threads of the  
> same process must be in the same PID namespace. Since unshare(2) and  
> setns(2) change the PID namespace for subsequent calls to clone(2),  
> those subsequent calls cannot create new threads (unless you setns(2)  
> back to the original namespace first).
>
> That last bit's a guess. :)

Good wording thank you, and the last bit is right.  You can restore
the pid namespace  with setns(2), and that will allow thread and process
creation creation again.

Eric

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