Oleg Nesterov <oleg@tv-sign.ru> writes:

> On 08/10, Pavel Emelyanov wrote:
>>
>> Oleg Nesterov wrote:
>> >On 08/10, Serge E. Hallyn wrote:
>> >>Quoting Pavel Emelyanov (xemul@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().
>> 
>> Yup :)
>> 
>> It is already written in comment that only the current is allowed
>> to change its nsproxy. I.e. when switch_task_nsproxy() is called
>> for tsk other than current it's a BUG
>
> Yes, but what I meant is that this code
>
>         rcu_read_lock();
>         nsproxy = task_nsproxy(tsk);
>         if (nsproxy != NULL)
>                 get_nsproxy(nsproxy);
>         rcu_read_unlock();
>
> 	if (nsproxy) {
> 		use_it(nsproxy);
> 		put_nsproxy(nsproxy);
> 	}
>
> is not safe despite the fact we are _not_ changing tsk->nsproxy.
>
> The patch itself is correct because we don't do that, and the comment
> is right. Just it is not immediately obvious.

Ugh.  That is nasty, non obvious and almost a problem.  I don't want
to do get_net(nsproxy->net_ns) from another task so I can migrate
network between namespaces.

But thinking about it because we don't do the other decrements
until later we can still increment the counts on the individual
namespaces.  We just can't share nsproxy.

So if you did want to do an enter thing you could copy the
nsproxy object of a task under the rcu_read_lock(), and
you would be fine.

Eric