netdev - Re: [PATCH] Bpf Helper Function BPF_FUNC_skb_change

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Date:   Mon, 20 Dec 2021 22:16:52 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Tyler Wear <quic_twear@...cinc.com>
CC:     Yonghong Song <yhs@...com>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>
Subject: Re: [PATCH] Bpf Helper Function BPF_FUNC_skb_change_dsfield

On Mon, Dec 20, 2021 at 07:18:42PM -0800, Yonghong Song wrote:
> 
> 
> On 12/20/21 12:40 PM, Tyler Wear wrote:
> > New bpf helper function BPF_FUNC_skb_change_dsfield
> > "int bpf_skb_change_dsfield(struct sk_buff *skb, u8 mask, u8 value)".
> > BPF_PROG_TYPE_CGROUP_SKB typed bpf_prog which currently can
> > be attached to the ingress and egress path. The helper is needed
> > because this type of bpf_prog cannot modify the skb directly.
> > 
> > Used by a bpf_prog to specify DS field values on egress or
> > ingress.
> 
> Maybe you can expand a little bit here for your use case?
> I know DS field might help but a description of your actual
> use case will make adding this helper more compelling.
+1.  More details on the use case is needed.
Also, having an individual helper for each particular header field
is too specific.

For egress, there is bpf_setsockopt() for IP_TOS and IPV6_TCLASS
and it can be called in other cgroup hooks. e.g.
BPF_PROG_TYPE_SOCK_OPS during tcp ESTABLISHED event.
There is an example in tools/testing/selftests/bpf/progs/test_tcpbpf_kern.c.
Is it enough for egress?