netdev - Re: sock_create_kern() and network namespace reference counts

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Date:   Wed, 1 Feb 2017 09:19:39 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: sock_create_kern() and network namespace reference counts

On Tue, Jan 31, 2017 at 9:57 AM, David Laight <David.Laight@...lab.com> wrote:
> From: Cong Wang
>> Sent: 31 January 2017 17:38
>> On Tue, Jan 31, 2017 at 7:41 AM, David Laight <David.Laight@...lab.com> wrote:
>> > Commit 26abe1437 changed sock_create_kern() so that it stopped
>> > holding a reference to the network namespace.
>> > The rational seemed to be 'to allow to stop it' (presumably 'be deleted').
>> > Prior to this change some kernel paths used sk_change_net() (etc) to
>> > change the namespace after the socket was created.
>> >
>> > If the socket doesn't hold a reference to the namespace, what actually
>> > happens when the namespace is deleted?
>>
>> Kernel socket should have the same lifetime with the net namespace,
>> that is, created in net_init and released in net_exit. Think about it, if it
>> really held a refcnt to this netns, how could this netns be teared down?
>
> That rather depends on what they are being used for.
> Consider something like an in kernel ftp client, it doesn't really care
> about namespaces except in as much as the connections it creates must
> be inside the correct namespace.
> The namespace shouldn't be torn down while that connection exists any more
> than it should be torn down while a user process has an open connection.
> (Listening sockets are likely to be more of a problem.)

If you don't care about netns, why not just use init_net which is never
torn down and make your kernel socket global so that each netns
can access it too?