l2tp: ignore L2TP_ATTR_MTU

This attribute's handling is broken. It can only be used when creating
Ethernet pseudo-wires, in which case its value can be used as the
initial MTU for the l2tpeth device.
However, when handling update requests, L2TP_ATTR_MTU only modifies
session->mtu. This value is never propagated to the l2tpeth device.
Dump requests also return the value of session->mtu, which is not
synchronised anymore with the device MTU.

The same problem occurs if the device MTU is properly updated using the
generic IFLA_MTU attribute. In this case, session->mtu is not updated,
and L2TP_ATTR_MTU will report an invalid value again when dumping the
session.

It does not seem worthwhile to complexify l2tp_eth.c to synchronise
session->mtu with the device MTU. Even the ip-l2tp manpage advises to
use 'ip link' to initialise the MTU of l2tpeth devices (iproute2 does
not handle L2TP_ATTR_MTU at all anyway). So let's just ignore it
entirely.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/uapi/linux/l2tp.h b/include/uapi/linux/l2tp.h
index 8bb8c7cfabe5..61158f5a1a5b 100644
--- a/include/uapi/linux/l2tp.h
+++ b/include/uapi/linux/l2tp.h
@@ -119,7 +119,7 @@ enum {
         L2TP_ATTR_IP_DADDR,             /* u32 */
         L2TP_ATTR_UDP_SPORT,            /* u16 */
         L2TP_ATTR_UDP_DPORT,            /* u16 */
-        L2TP_ATTR_MTU,                  /* u16 */
+        L2TP_ATTR_MTU,                  /* u16 (not used) */
         L2TP_ATTR_MRU,                  /* u16 (not used) */
         L2TP_ATTR_STATS,                /* nested */
         L2TP_ATTR_IP6_SADDR,            /* struct in6_addr */