Re: [PATCH] net: fix wrong network header length

[Paolo Abeni]

Hello,

On Tue, 2022-02-08 at 10:55 +0800, Lina Wang wrote:
> When clatd starts with ebpf offloaing, and NETIF_F_GRO_FRAGLIST is enable,
> several skbs are gathered in skb_shinfo(skb)->frag_list. The first skb's
> ipv6 header will be changed to ipv4 after bpf_skb_proto_6_to_4,
> network_header\transport_header\mac_header have been updated as ipv4 acts,
> but other skbs in frag_list didnot update anything, just ipv6 packets.
> 
> udp_queue_rcv_skb will call skb_segment_list to traverse other skbs in
> frag_list and make sure right udp payload is delivered to user space.
> Unfortunately, other skbs in frag_list who are still ipv6 packets are
> updated like the first skb and will have wrong transport header length.
> 
> e.g.before bpf_skb_proto_6_to_4,the first skb and other skbs in frag_list
> has the same network_header(24)& transport_header(64), after
> bpf_skb_proto_6_to_4, ipv6 protocol has been changed to ipv4, the first
> skb's network_header is 44,transport_header is 64, other skbs in frag_list
> didnot change.After skb_segment_list, the other skbs in frag_list has
> different network_header(24) and transport_header(44), so there will be 20
> bytes difference,that is difference between ipv6 header and ipv4 header.

> Actually, there are two solutions to fix it, one is traversing all skbs
> and changing every skb header in bpf_skb_proto_6_to_4, the other is
> modifying frag_list skb's header in skb_segment_list. 

I don't think the above should be addressed into the GSO layer. The
ebpf program is changing the GRO packet in arbitrary way violating the
GSO packet constraint - arguably, it's corrupting the packet.

I think it would be better change the bpf_skb_proto_6_to_4() to
properly handle FRAGLIST GSO packets.

If traversing the segments become too costly, you can try replacing
GRO_FRAGLIST with GRO_UDP_FWD.

Thanks!

Paolo