l2tp: Add protocol field decompression
authorSam Protsenko <semen.protsenko@linaro.org>
Fri, 14 Dec 2018 17:59:21 +0000 (19:59 +0200)
committerDavid S. Miller <davem@davemloft.net>
Sun, 16 Dec 2018 07:28:19 +0000 (23:28 -0800)
commitc151acc6e9ffa08972aa20d8bc07a66081922c1e
tree60e9e0d9fc2b5f78eb3102197580254f9aa65479
parent63de273f34b5eeb5ead8440f20516fae9b7b1165
l2tp: Add protocol field decompression

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>
Signed-off-by: David S. Miller <davem@davemloft.net>
net/l2tp/l2tp_ppp.c