tree 1f3944a665e981d0161c3b60d469a00a4728d799
parent 9532aa6528c3d7d64ee4b1076dc1e2c825a47ea7
author Sam Protsenko <semen.protsenko@linaro.org> 1544810361 +0200
committer Alistair Strachan <astrachan@google.com> 1548276235 +0000

UPSTREAM: 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>

(cherry picked from commit c151acc6e9ffa08972aa20d8bc07a66081922c1e)
Bug: 116424816
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Change-Id: I623803405a9beb0371aca56d8446e793f4db3beb