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

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> writes:

> 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) {
> 				ns = &pid_ns->ns;
> 				get_pid_ns(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?

It breaks a number of assumptions if you can join a pid namespace before
an init process is created in that pid namespace.  Checking for
child_reaper is a bit heavy handed but appears to ensure all of the
assumptions of initial pid namespace creation have been met.

Which means your simplified pidns_for_children_get is a bit insufficient.

Eric