Re: [PATCH net-next v3 1/2]L2TP:Adjust intf MTU, add underlay L3, L2 hdrs

[R Parameswaran]

[Posting a v4 patch-set shortly based on additional code review
comments received in internal review, please disregard the v3 patches]

On Thu, Mar 16, 2017 at 9:33 PM, R. Parameswaran
<parameswaran.r7@xxxxxxxxx> wrote:
>
>
> In existing kernel code, when setting up the L2TP interface, all of the
> tunnel encapsulation headers are not taken into account when setting
> up the MTU on the  L2TP logical interface device. Due to this, the
> packets created by the applications on top of the L2TP layer are larger
> than they ought to be, relative to the underlay MTU, which leads to
> needless fragmentation once the L2TP packet is encapsulated in an outer IP
> packet.
>
> Specifically, the MTU calculation  does not take into account the (outer)
> IP header imposed on the encapsulated L2TP packet, and the Layer 2 header
> imposed on the inner L2TP packet prior to encapsulation. The patch posted
> here takes care of these.
>
> Existing code also seems to assume an Ethernet (non-jumbo) underlay. The
> patch uses the PMTU mechanism and the dst entry in the L2TP tunnel socket
> to directly pull up the underlay MTU (as the baseline number on top of
> which the encapsulation headers are factored in).  Ethernet MTU is
> assumed as a fallback only if this fails.
>
> Picked up review comments from James Chapman, added a function
> to compute ip header + ip option overhead on a socket, and factored it
> into L2TP change-set.
>
> Signed-off-by: R. Parameswaran <rparames@xxxxxxxxxxx>
> ---
>  include/linux/net.h |  3 +++
>  net/socket.c        | 41 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 44 insertions(+)
>
> diff --git a/include/linux/net.h b/include/linux/net.h
> index 0620f5e..a42fab2 100644
> --- a/include/linux/net.h
> +++ b/include/linux/net.h
> @@ -298,6 +298,9 @@ int kernel_sendpage(struct socket *sock, struct page *page, int offset,
>  int kernel_sock_ioctl(struct socket *sock, int cmd, unsigned long arg);
>  int kernel_sock_shutdown(struct socket *sock, enum sock_shutdown_cmd how);
>
> +/* Following routine returns the IP overhead imposed by a socket.  */
> +u32 kernel_sock_ip_overhead(struct sock *sk);
> +
>  #define MODULE_ALIAS_NETPROTO(proto) \
>         MODULE_ALIAS("net-pf-" __stringify(proto))
>
> diff --git a/net/socket.c b/net/socket.c
> index e034fe4..af54b12 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -3345,3 +3345,44 @@ int kernel_sock_shutdown(struct socket *sock, enum sock_shutdown_cmd how)
>         return sock->ops->shutdown(sock, how);
>  }
>  EXPORT_SYMBOL(kernel_sock_shutdown);
> +
> +/*     This routine returns the IP overhead imposed by a socket i.e.
> + *     the length of the underlying IP header, depending on whether
> + *     this is an IPv4 or IPv6 socket and the length from IP options turned
> + *     on at the socket.
> + */
> +u32 kernel_sock_ip_overhead(struct sock *sk)
> +{
> +       struct inet_sock *inet;
> +       struct ipv6_pinfo *np;
> +       struct ip_options_rcu *opt = NULL;
> +       struct ipv6_txoptions *optv6 = NULL;
> +       u32 overhead = 0;
> +       bool owned_by_user = sock_owned_by_user(sk);
> +
> +       if (!sk)
> +               return overhead;
> +       switch (sk->sk_family) {
> +       case AF_INET:
> +               inet = inet_sk(sk);
> +               overhead += sizeof(struct iphdr);
> +               if (inet)
> +                       opt = rcu_dereference_protected(inet->inet_opt,
> +                                                       owned_by_user);
> +               if (opt)
> +                       overhead += opt->opt.optlen;
> +               return overhead;
> +       case AF_INET6:
> +               np = inet6_sk(sk);
> +               overhead += sizeof(struct ipv6hdr);
> +               if (np)
> +                       optv6 = rcu_dereference_protected(np->opt,
> +                                                         owned_by_user);
> +               if (optv6)
> +                       overhead += (optv6->opt_flen + optv6->opt_nflen);
> +               return overhead;
> +       default: /* Returns 0 overhead if the socket is not ipv4 or ipv6 */
> +               return overhead;
> +       }
> +}
> +EXPORT_SYMBOL(kernel_sock_ip_overhead);
> --
> 2.1.4
>