netdev - net/l2tp/l2tp_ppp.c should take pmtu from tunnel socket

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Date:	Sun, 6 Apr 2014 00:58:27 +0600
From:	Dmitry Petukhov <dmgenp@...il.com>
To:	netdev@...r.kernel.org
Subject: net/l2tp/l2tp_ppp.c should take pmtu from tunnel socket

currently when it tries to get PMTU, it does

dst = sk_dst_get(sk);
...
u32 pmtu = dst_mtu(__sk_dst_get(sk));

where sk represents PPPoX socket

it should use tunnel UDP socket instead, tunnel->sock

This bug bite me when I tried to use l2tp vpn over the link with mtu <
1500 but where fragments were dropped somewere along the path. I tried
to use this mechanism with

ip route add <dst> via ... mtu <lower_mtu>

but it didn't work.

After the fix I was able to use this feature of l2tp_ppp driver, it
picked up mtu set by ip route and used it.

patch against https://github.com/torvalds/linux master:

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index d276e2d..950909f 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -753,9 +753,9 @@ static int pppol2tp_connect(struct socket *sock,
struct sockaddr *uservaddr,
        session->deref = pppol2tp_session_sock_put;

        /* If PMTU discovery was enabled, use the MTU that was discovered */
-       dst = sk_dst_get(sk);
+       dst = sk_dst_get(tunnel->sock);
        if (dst != NULL) {
-               u32 pmtu = dst_mtu(__sk_dst_get(sk));
+               u32 pmtu = dst_mtu(__sk_dst_get(tunnel->sock));
                if (pmtu != 0)
                        session->mtu = session->mru = pmtu -
                                PPPOL2TP_HEADER_OVERHEAD;
--
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