linux-kernel - Re: [PATCH] bpf: in bpf_skb_adjust_room correct inner protocol for vxlan

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Date:   Mon, 8 Feb 2021 08:06:59 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     huangxuesen <hxseverything@...il.com>
Cc:     David Miller <davem@...emloft.net>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        huangxuesen <huangxuesen@...ishou.com>,
        chengzhiyong <chengzhiyong@...ishou.com>,
        wangli <wangli09@...ishou.com>
Subject: Re: [PATCH] bpf: in bpf_skb_adjust_room correct inner protocol for vxlan

On Mon, Feb 8, 2021 at 7:16 AM huangxuesen <hxseverything@...il.com> wrote:
>
> From: huangxuesen <huangxuesen@...ishou.com>
>
> When pushing vxlan tunnel header, set inner protocol as ETH_P_TEB in skb
> to avoid HW device disabling udp tunnel segmentation offload, just like
> vxlan_build_skb does.
>
> Drivers for NIC may invoke vxlan_features_check to check the
> inner_protocol in skb for vxlan packets to decide whether to disable
> NETIF_F_GSO_MASK. Currently it sets inner_protocol as the original
> skb->protocol, that will make mlx5_core disable TSO and lead to huge
> performance degradation.
>
> Signed-off-by: huangxuesen <huangxuesen@...ishou.com>
> Signed-off-by: chengzhiyong <chengzhiyong@...ishou.com>
> Signed-off-by: wangli <wangli09@...ishou.com>
> ---
>  net/core/filter.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 255aeee72402..f8d3ba3fe10f 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3466,7 +3466,12 @@ static int bpf_skb_net_grow(struct sk_buff *skb, u32 off, u32 len_diff,
>                 skb->inner_mac_header = inner_net - inner_mac_len;
>                 skb->inner_network_header = inner_net;
>                 skb->inner_transport_header = inner_trans;
> -               skb_set_inner_protocol(skb, skb->protocol);
> +
> +               if (flags & BPF_F_ADJ_ROOM_ENCAP_L4_UDP &&
> +                   inner_mac_len == ETH_HLEN)
> +                       skb_set_inner_protocol(skb, htons(ETH_P_TEB));

This may be used by vxlan, but it does not imply it.

Adding ETH_HLEN bytes likely means pushing an Ethernet header, but same point.

Conversely, pushing an Ethernet header is not limited to UDP encap.

This probably needs a new explicit BPF_F_ADJ_ROOM_.. flag, rather than
trying to infer from imprecise heuristics.