Re: [PATCH net-next] kcm: do not attach sockets if sk_user_data is already used

[Guillaume Nault <g.nault@alphalink.fr>]

On Thu, Jan 18, 2018 at 03:40:52PM +0000, James Chapman wrote:
> On 18 January 2018 at 15:18, Guillaume Nault <g.nault@alphalink.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).

l2tp_ppp also uses sk_user_data for its PPPOX sockets, but we probably
can handle this case more easily.