netdev - Re: [PATCH net-next] kcm: do not attach sockets if sk_user

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Date:   Thu, 18 Jan 2018 17:30:16 +0000
From:   James Chapman <jchapman@...alix.com>
To:     Guillaume Nault <g.nault@...halink.fr>
Cc:     David Miller <davem@...emloft.net>,
        Tom Herbert <tom@...bertland.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] kcm: do not attach sockets if sk_user_data is
 already used

On 18 January 2018 at 16:29, Guillaume Nault <g.nault@...halink.fr> wrote:
> On Thu, Jan 18, 2018 at 03:40:52PM +0000, James Chapman wrote:
>> On 18 January 2018 at 15:18, Guillaume Nault <g.nault@...halink.fr> wrote:
>> > On Wed, Jan 17, 2018 at 02:25:38PM -0500, David Miller wrote:
>> >> If all else was equal, even though it doesn't make much sense to KCM
>> >> attach L2TP sockets to KCM, I would suggest to change L2TP to store
>> >> it's private stuff elsewhere.
>> >>
>> >> But that is not the case.  Anything using the generic UDP
>> >> encapsulation layer is going to make use of sk->sk_user_data like this
>> >> (see setup_udp_tunnel_sock).
>> >>
>> > Most UDP encapsulations only use kernel sockets though. It seems that
>> > only L2TP and GTP use setup_udp_tunnel_sock() with userpsace sockets.
>> > So it might be feasible to restrict usage of sk_user_data to kernel
>> > sockets only.
>> >
>> > For L2TP, we probably can adapt l2tp_sock_to_tunnel() so that it does
>> > a lookup in a hashtable indexed by the socket pointer, rather than
>> > dereferencing sk_user_data. That doesn't look very satisfying to me,
>> > but that's the only way I found so far.
>>
>> L2TP needs a way to get at its local data from the socket in the data path.
>>
> Did I miss something? On xmit, the session is provided by l2tp_ppp or
> l2tp_eth, which is enough to get access to the parent tunnel.
> For reception, l2tp_udp_encap_recv() receives the socket pointer as
> parameter and could get enough information from the headers to retrieve the
> tunnel structure anymay (l2tp_ip and l2tp_ip6 use the headers).

It's the receive side I was thinking about. It would be a little more
involved to derive the tunnel and session from the packet with UDP
since we'd have to handle L2TPv2 and L2TPv3.