On 08/10, Serge E. Hallyn wrote:
>
> Quoting Pavel Emelyanov (xemul@xxxxxxxxxx):
> > +/*
> > + * 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.
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