netdev - Re: [PATCH] ipvs: align inner_mac

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Date: Fri, 9 Jun 2023 22:03:39 +0300 (EEST)
From: Julian Anastasov <ja@....bg>
To: Terin Stock <terin@...udflare.com>
cc: horms@...ge.net.au, netdev@...r.kernel.org, lvs-devel@...r.kernel.org,
        kernel-team@...udflare.com
Subject: Re: [PATCH] ipvs: align inner_mac_header for encapsulation


	Hello,

On Fri, 9 Jun 2023, 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() 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@...udflare.com>
> ---
>  net/netfilter/ipvs/ip_vs_xmit.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
> index c7652da78c88..4d20b89dd765 100644
> --- a/net/netfilter/ipvs/ip_vs_xmit.c
> +++ b/net/netfilter/ipvs/ip_vs_xmit.c
> @@ -1207,6 +1207,7 @@ ip_vs_tunnel_xmit(struct sk_buff *skb, struct ip_vs_conn *cp,
>  	skb->transport_header = skb->network_header;
>  
>  	skb_set_inner_ipproto(skb, next_protocol);
> +	skb_set_inner_mac_header(skb, skb_inner_network_offset(skb));

	Can you send v2 after including the same line also in 
ip_vs_tunnel_xmit_v6?

>  
>  	if (tun_type == IP_VS_CONN_F_TUNNEL_TYPE_GUE) {
>  		bool check = false;
> -- 
> 2.40.1

Regards

--
Julian Anastasov <ja@....bg>