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SubjectRe: [PATCH] l2tp: Add protocol field decompression
Hi Guillaume,

On Sun, Dec 16, 2018 at 6:29 PM Guillaume Nault <g.nault@alphalink.fr> wrote:
>
> On Fri, Dec 14, 2018 at 07:59:21PM +0200, Sam Protsenko wrote:
> > When Protocol Field Compression (PFC) is enabled, the "Protocol" field
> > in PPP packet will be received without leading 0x00. See section 6.5 in
> > RFC 1661 for details. So let's decompress protocol field if needed, the
> > same way it's done in drivers/net/ppp/pptp.c.
> >
> > In case when "nopcomp" pppd option is not enabled, PFC (pcomp) can be
> > negotiated during LCP handshake, and L2TP driver in kernel will receive
> > PPP packets with compressed Protocol field, which in turn leads to next
> > error:
> >
> > Protocol Rejected (unsupported protocol 0x2145)
> >
> > because instead of Protocol=0x0021 in PPP packet there will be
> > Protocol=0x21. This patch unwraps it back to 0x0021, which fixes the
> > issue.
> >
> > Sending the compressed Protocol field will be implemented in subsequent
> > patch, this one is self-sufficient.
> >
> > Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
> > ---
> > net/l2tp/l2tp_ppp.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
> > index 04d9946dcdba..c03c6461f236 100644
> > --- a/net/l2tp/l2tp_ppp.c
> > +++ b/net/l2tp/l2tp_ppp.c
> > @@ -236,6 +236,10 @@ static void pppol2tp_recv(struct l2tp_session *session, struct sk_buff *skb, int
> > skb->data[1] == PPP_UI)
> > skb_pull(skb, 2);
> >
> > + /* Decompress protocol field if PFC is enabled */
> > + if ((*skb->data) & 0x1)
> > + *(u8 *)skb_push(skb, 1) = 0;
> > +
> Sorry, but PFC is PPP stuff. It's absolutely independent from the
> transport layer. Therefore, it belongs to ppp_generic.c and L2TP has
> nothing do with it.
>
> I know some other transports got it wrong, but let's stop
> re-implementing PPP features in every lower layer. Teaching ppp_input()
> about PFC is all that is needed to get the feature work correctly on
> all transports. As a bonus, it'd bring PFC support to PPPoE and would
> let us drop the duplicated code from pptp.c, ppp_async.c and
> ppp_synctty.c.
>
> As an example of the problems caused by mixing the PPP and L2TP
> layers, consider the case of L2TP sockets that aren't PPPOX_BOUND.
> They're supposed to receive the original PPP frames. Now with your
> patch if the protocol was compressed, the L2TP socket receives a fake
> header. User space didn't ask for that and has no way to figure out
> what was the original packet. To make things worse, that behaviour now
> changes depending on the kernel version.
>
> If you all agree, can we please revert this patch and properly
> implement PFC in ppp_generic.c?

How about instead of reverting I will try to provide the patch
extracting duplicated PFC code (from L2TP, PPTP, etc) to
ppp_generic.c? This one fixes actual problem, and if we are going to
add it to ppp_generic anyway, why not do it in one patch? I'm willing
to do this work in next few days. Hope there is no rush with revert?

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