Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of ns_last_pid's on ns hierarhy

[ebiederm@xmission.com (Eric W. Biederman)]

Kirill Tkhai <ktkhai@virtuozzo.com> writes:

> On 02.05.2017 19:33, Oleg Nesterov wrote:
>> sorry for delay, vacation...
>> 
>> On 04/28, Kirill Tkhai wrote:
>>>
>>> On 27.04.2017 19:22, Oleg Nesterov wrote:
>>>>
>>>> Ah, OK, I didn't notice the ns->child_reaper check in pidns_for_children_get().
>>>>
>>>> But note that it doesn't need tasklist_lock too.
>>>
>>> Hm, are there possible strange situations with memory ordering, when we see
>>> ns->child_reaper of already died ns, which was placed in the same memory?
>>> Do we have to use some memory barriers here?
>> 
>> Could you spell please? I don't understand your concerns...
>> 
>> I don't see how, say,
>> 
>> 	static struct ns_common *pidns_for_children_get(struct task_struct *task)
>> 	{
>> 		struct ns_common *ns = NULL;
>> 		struct pid_namespace *pid_ns;
>> 
>> 		task_lock(task);
>> 		if (task->nsproxy) {
>> 			pid_ns = task->nsproxy->pid_ns_for_children;
>> 			if (pid_ns->child_reaper) {
                            ^^^^^^^^^^^^^^^^^^^^
                            Oleg my apologies I missed this line earlier.
                            This does look like a valid way to skip read_lock(&tasklist_lock);
>> 				ns = &pid_ns->ns;
>> 				get_pid_ns(ns);
                                ^^^^^^^^^^^^^ This needs to be:
                                get_pid_ns(pid_ns);
                                
>> 			}
>> 		}
>> 		task_unlock(task);
>> 
>> 		return ns;
>> 	}
>> 
>> can be wrong. It also looks more clean to me.
>> 
>> ->child_reaper is not stable without tasklist, it can be dead/etc, but
>> we do not care?
>
> I mean the following. We had a pid_ns1 with a child_reaper set. Then
> it became dead, and a new pid_ns2 were allocated in the same memory.

task->nsproxy->pid_ns_for_children is always changed with
task_lock(task) held.  See switch_task_namespaces (used by unshare and
setns).  This also gives us the guarantee that the pid_ns reference
won't be freed/reused in any for until task_lock(task) is dropped.

> A task on another cpu opens the pid_for_children file, and because
> of there is no memory ordering, it sees pid_ns1->child_reaper,
> when it opens pid_ns2.
>
> I forgot, what guarantees this situation is impossible? What guarantees,
> the renewed content of pid_ns2 on another cpu is seen not later, than
> we can't open it?

Eric