Re: [PATCH v2] ipvs: align inner_mac_header for encapsulation

[Simon Horman <horms@kernel.org>]

On Fri, Jun 09, 2023 at 10:58:42PM +0200, Terin Stock wrote:
> When using encapsulation the original packet's headers are copied to the
> inner headers. This preserves the space for an inner mac header, which
> is not used by the inner payloads for the encapsulation types supported
> by IPVS. If a packet is using GUE or GRE encapsulation and needs to be
> segmented, flow can be passed to __skb_udp_tunnel_segment() which
> calculates a negative tunnel header length. A negative tunnel header
> length causes pskb_may_pull() to fail, dropping the packet.
> 
> This can be observed by attaching probes to ip_vs_in_hook(),
> __dev_queue_xmit(), and __skb_udp_tunnel_segment():
> 
>     perf probe --add '__dev_queue_xmit skb->inner_mac_header \
>     skb->inner_network_header skb->mac_header skb->network_header'
>     perf probe --add '__skb_udp_tunnel_segment:7 tnl_hlen'
>     perf probe -m ip_vs --add 'ip_vs_in_hook skb->inner_mac_header \
>     skb->inner_network_header skb->mac_header skb->network_header'
> 
> These probes the headers and tunnel header length for packets which
> traverse the IPVS encapsulation path. A TCP packet can be forced into
> the segmentation path by being smaller than a calculated clamped MSS,
> but larger than the advertised MSS.
> 
>     probe:ip_vs_in_hook: inner_mac_header=0x0 inner_network_header=0x0 mac_header=0x44 network_header=0x52
>     probe:ip_vs_in_hook: inner_mac_header=0x44 inner_network_header=0x52 mac_header=0x44 network_header=0x32
>     probe:dev_queue_xmit: inner_mac_header=0x44 inner_network_header=0x52 mac_header=0x44 network_header=0x32
>     probe:__skb_udp_tunnel_segment_L7: tnl_hlen=-2
> 
> When using veth-based encapsulation, the interfaces are set to be
> mac-less, which does not preserve space for an inner mac header. This
> prevents this issue from occurring.
> 
> In our real-world testing of sending a 32KB file we observed operation
> time increasing from ~75ms for veth-based encapsulation to over 1.5s
> using IPVS encapsulation due to retries from dropped packets.
> 
> This changeset modifies the packet on the encapsulation path in
> ip_vs_tunnel_xmit() and ip_vs_tunnel_xmit_v6() to remove the inner mac
> header offset. This fixes UDP segmentation for both encapsulation types,
> and corrects the inner headers for any IPIP flows that may use it.
> 
> Fixes: 84c0d5e96f3a ("ipvs: allow tunneling with gue encapsulation")
> Signed-off-by: Terin Stock <terin@cloudflare.com>

Acked-by: Simon Horman <horms@kernel.org>