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Subject: Re: [PATCH 3/7] ns proc: Add support for the network namespace.
From: Nathan Lynch <ntl@pobox.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        jamal <hadi@cyberus.ca>, Daniel Lezcano <daniel.lezcano@free.fr>,
        Linux Containers <containers@lists.osdl.org>,
        Renato Westphal <renatowestphal@gmail.com>
Date: Wed, 11 May 2011 16:42:33 -0500
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On Wed, 2011-05-11 at 14:34 -0700, Eric W. Biederman wrote:
> Nathan Lynch <ntl@pobox.com> writes:
> 
> > On Fri, 2011-05-06 at 19:24 -0700, Eric W. Biederman wrote:
> >> diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
> >> index 3f86026..bf7707e 100644
> >> --- a/net/core/net_namespace.c
> >> +++ b/net/core/net_namespace.c
> >> @@ -573,3 +573,34 @@ void unregister_pernet_device(struct pernet_operations *ops)
> >>  	mutex_unlock(&net_mutex);
> >>  }
> >>  EXPORT_SYMBOL_GPL(unregister_pernet_device);
> >> +
> >> +#ifdef CONFIG_NET_NS
> >> +static void *netns_get(struct task_struct *task)
> >> +{
> >> +	struct net *net;
> >> +	rcu_read_lock();
> >> +	net = get_net(task->nsproxy->net_ns);
> >
> > This should use task_nsproxy() and check the result before grabbing the
> > net_ns, but I think you fix that in a later patch.
> >
> > Regardless, it looks as if all the proc_ns_ops->get() implementations
> > really just want the nsproxy, so maybe the get() methods should take
> > that instead of the task_struct, and proc_ns_instantiate() should do
> > something like:
> >
> > struct nsproxy *nsproxy;
> > ...
> >
> > ei->ns_ops = ns_ops;
> > error = -ESRCH;
> > rcu_read_lock();
> > nsproxy = task_nsproxy(task);
> > rcu_read_unlock();
> > if (!nsproxy)
> > 	got out;
> > ei->ns = ns_ops->get(nsproxy);
> >
> >
> > So then the zombie check is consolidated in one place instead of having
> > to do it in every get() method.
> 
> For the pid namespace at least I want the task not the nsproxy,
> so I can use task_active_pid_namespace().
> 
> I admit that is a little asymmetrical with the install, but at
> least until the details of getting the pid namespace working in
> this context are worked out I don't want to reconsider the
> current design.
> 
> There is also the user namespace that does not even exist in
> nsproxy to consider.  I will worry about that namespace when
> it happens.
> 
> Ultimately nsproxy is an space/time optimization that not all
> namespaces use so forcing it in the design is probably not
> what we want.

Okay.


> >> +	rcu_read_unlock();
> >> +	return net;
> >> +}
> >> +
> >> +static void netns_put(void *ns)
> >> +{
> >> +	put_net(ns);
> >> +}
> >> +
> >> +static int netns_install(struct nsproxy *nsproxy, void *ns)
> >> +{
> >> +	put_net(nsproxy->net_ns);
> >> +	nsproxy->net_ns = get_net(ns);
> >> +	return 0;
> >> +}
> >
> > This introduces a window where, potentially, nsproxy->net_ns is stale
> > before it is updated with the namespace which is being attached, no? 
> > (Same concern applies to other install methods in the patch set).  It
> > seems possible to oops the kernel in this window by looking up
> > /proc/$PID/ns/net while $PID is in the midst of setns().
> 
> Except the nsproxy being referred to is a brand new nsproxy, with an
> extra reference count on every namespace.  current->nsproxy still
> contains the reference counts of the current process.

Ahh, yeah.  Got it.  Thanks.


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