On 06/12/15 10:20, Sam Russell 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?
Thanks for reporting this and for your analysis.
We could write wrappers to lwtunnel_output for the v4 and v6 cases that
set the protocol accordingly and then call lwtunnel_output, but since
mpls_output relies on the AF-specific type of dst I think the simpler
fix is to just test the type of the dst in mpls_output rather than
skb->protocol.
Thanks,
Rob
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