netdev - Re: [PATCH bpf-next 3/9] bpf: add bpf helper bpf_skb_set

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Date:   Tue, 19 Feb 2019 10:30:19 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     brakmo <brakmo@...com>, netdev <netdev@...r.kernel.org>
Cc:     Martin Lau <kafai@...com>, Alexei Starovoitov <ast@...com>
Subject: Re: [PATCH bpf-next 3/9] bpf: add bpf helper bpf_skb_set_ecn



On 02/18/2019 09:38 PM, brakmo wrote:
> This patch adds a new bpf helper BPF_FUNC_skb_set_ecn
> "int bpf_skb_set_Ecn(struct sk_buff *skb)". It is added to
> BPF_PROG_TYPE_CGROUP_SKB typed bpf_prog which currently can
> be attached to the ingress and egress path. This type of
> bpf_prog cannot modify the skb directly.
> 
> This helper is used to set the ECN bits (2) of the IPv6 or IPv4
> header in skb. It can be used by a bpf_prog to manage egress
> network bandwdith limit per cgroupv2 by inducing an ECN
> response in the TCP sender (when the packet is ECN enabled).
> This works best when using DCTCP.


> +
> +BPF_CALL_2(bpf_skb_set_ecn, struct sk_buff *, skb, u32, val)
> +{
> +	struct ipv6hdr *ip6h = ipv6_hdr(skb);
> +
> +	if ((val & ~0x3) != 0)
> +		return -EINVAL;
> +
> +	if (ip6h->version == 6) {
> +		ip6h->flow_lbl[0] = (ip6h->flow_lbl[0] & ~0x30) | (val << 4);
> +		return 0;
> +	} else if (ip6h->version == 4) {
> +		struct iphdr *ip4h = (struct iphdr *)ip6h;
> +
> +		ip4h->tos = (ip4h->tos & ~0x3) | val;

Why is not the IPv4 checksum recomputed here ?

If you leave this task to the caller, this should be documented.

These hard coded constants are not really nice.

Why not simply using INET_ECN_set_ce() which is IPv4/IPv6 ready ?

Do you really need to set anything else than CE ?