netdev - Re: MPLS outbound packets being dropped

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Date:	Mon, 7 Dec 2015 18:56:51 +1300
From:	Sam Russell <sam.h.russell@...il.com>
To:	netdev@...r.kernel.org
Subject: Re: MPLS outbound packets being dropped

I can confirm that MPLS packets are sent correctly if the conditional
at mpls_iptunnel.c:57-65 is removed and just replaced with lines
58-59:

ttl = ip_hdr(skb)->ttl;
rt = (struct rtable *)dst;

Obviously this isn't sufficient for IPv6, but is a workaround that
enables MPLS-encapsulated packets to be sent from Linux in the
meantime

On 6 December 2015 at 23:20, Sam Russell <sam.h.russell@...il.com> wrote:
> tl;dr mpls_output expects skb->protocol to be set to correct
> ethertype, but it isn't
>
> https://github.com/torvalds/linux/blob/ede2059dbaf9c6557a49d466c8c7778343b208ff/net/mpls/mpls_iptunnel.c#L64
>
> Problem:
>
> I set up two interfaces pointed at each other, and added a static arp
> entry to minimise complexity
>
> ifconfig enp0s8 10.0.0.1/24 up
> ifconfig enp0s9 up
> arp -s 10.0.0.5 00:12:34:56:78:90
>
> I then added an MPLS route
>
> ./dev/iproute2/ip/ip route add 192.168.2.0/24 encap mpls 100 via inet
> 10.0.0.5 dev enp0s8
>
> I then tried to ping an IP in this route but got errors back
>
> ping 192.168.2.1
> * PING 192.168.2.1 (192.168.2.1) 56(84) bytes of data.
> * ping: sendmsg: Invalid argument
>
> I then recorded calls to skb_kfree
>
> ./tools/perf/perf record -e skb:kfree_skb -g -a
>
> This gave me the following packet trace:
>
>    100.00%   100.00%  ping     [kernel.kallsyms]  [k] kfree_skb
>                |
>                ---kfree_skb
>                   mpls_output
>                   lwtunnel_output
>                   ip_local_out_sk
>                   ip_send_skb
>                   ip_push_pending_frames
>                   raw_sendmsg
>                   inet_sendmsg
>                   sock_sendmsg
>                   ___sys_sendmsg
>                   __sys_sendmsg
>                   sys_sendmsg
>                   entry_SYSCALL_64_fastpath
>                   sendmsg
>                   0
>
> I then went through mpls_output.c and put printk() at every call to
> "goto drop" and found that this was being hit after failing to match
> skb->protocol
>
> https://github.com/torvalds/linux/blob/ede2059dbaf9c6557a49d466c8c7778343b208ff/net/mpls/mpls_iptunnel.c#L64
>
> My understanding is that skb->protocol is normally set after
> dst_output. For example, a ping packet hitting a normal IPv4 route
> should follow something like:
>
> raw_sendmsg
> ip_push_pending_frames
> ip_send_skb
> ip_local_out_sk
> dst_output
> ip_output
>
> ip_output() is the first place where skb->protocol gets set
>
> https://github.com/torvalds/linux/blob/dbd3393c56a8794fe596e7dd20d0efa613b9cf61/net/ipv4/ip_output.c#L356
>
> The path that a packet follows when hitting an MPLS route is as follows:
>
> raw_sendmsg
> ip_push_pending_frames
> ip_send_skb
> ip_local_out_sk
> dst_output
> lwtunnel_output
> mpls_output
>
> lwtunnel_output merely routes to the correct output function (mpls_output)
> mpls_output expects skb->protocol to be set, but nothing has set it
> yet, so it drops the packet!
>
> Any suggestions on how mpls_output should detect the protocol?
>
> Setup:
>
> Ubuntu 15.10
>
> iproute2 built from head
>
> Kernel 4.3, both ubuntu mainline and home-built:
> make menuconfig, add lwtunnel, mpls-router, mpls-gso and mpls-iptunnel
>
> Running virtualbox on OSX
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