[Devel] Re: [PATCH] Make access to task's nsproxy liter

[Oleg Nesterov]

On 08/10, Serge E. Hallyn wrote:
>
> Quoting Pavel Emelyanov (xemul at openvz.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().

Oleg.