linux-kernel - Re: [PATCH] Make access to task's nsproxy liter

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Date:	Fri, 10 Aug 2007 18:15:01 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	Pavel Emelyanov <xemul@...nvz.org>, Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.osdl.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, devel@...nvz.org
Subject: Re: [PATCH] Make access to task's nsproxy liter

On 08/10, Oleg Nesterov wrote:
>
> On 08/10, Serge E. Hallyn wrote:
> >
> > Quoting Pavel Emelyanov (xemul@...nvz.org):
> > > +/*
> > > + * the namespaces access rules are:
> > > + *
> > > + *  1. only current task is allowed to change tsk->nsproxy pointer or
> > > + *     any pointer on the nsproxy itself
> > > + *
> > > + *  2. when accessing (i.e. reading) current task's namespaces - no
> > > + *     precautions should be taken - just dereference the pointers
> > > + *
> > > + *  3. the access to other task namespaces is performed like this
> > > + *     rcu_read_lock();
> > > + *     nsproxy = task_nsproxy(tsk);
> > > + *     if (nsproxy != NULL) {
> > > + *             / *
> > > + *               * work with the namespaces here
> > > + *               * e.g. get the reference on one of them
> > > + *               * /
> > > + *     } / *
> > > + *         * NULL task_nsproxy() means that this task is
> > > + *         * almost dead (zombie)
> > > + *         * /
> > > + *     rcu_read_unlock();
> > 
> > And lastly, I guess that the caller to switch_task_namespaces() has
> > to ensure that new_nsproxy either (1) is the init namespace, (2) is a
> > brand-new namespace to which noone else has a reference, or (3) the
> > caller has to hold a reference to the new_nsproxy across the call to
> > switch_task_namespaces().
> > 
> > As it happens the current calls fit (1) or (2).  Again if we happen to
> > jump into the game of switching a task into another task's nsproxy,
> > we'll need to be mindful of (3) so that new_nsproxy can't be tossed into
> > the bin between
> > 
> > 	if (new)
> > 		get_nsproxy(new);
> 
> 4) Unless tsk == current, get_task_namespaces(tsk) and get_nsproxy(tsk)
>    are racy even if done under rcu_read_lock().

(sorry for noise, but I'm afraid I was not clear again...)

This looks OK, we don't do get_nsproxy(not_a_current), but perhaps it is
not immediately obvious that we shouldn't.

Oleg.

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