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

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Message-ID: <509F6C41-FA0B-4688-AFA0-BB9E5FB2904C@fb.com>
Date:   Thu, 21 Feb 2019 03:43:20 +0000
From:   Lawrence Brakmo <brakmo@...com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>
CC:     Martin Lau <kafai@...com>, Alexei Starovoitov <ast@...com>,
        "Daniel Borkmann" <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next 3/9] bpf: add bpf helper bpf_skb_set_ecn

On 2/19/19, 10:30 AM, "Eric Dumazet" <eric.dumazet@...il.com> wrote:

    
    
    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 ?

Good point, thank you. I will use it.