netdev - Re: [PATCH bpf-next 07/10] bpf: Add helpers to query conntrack info

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Date:   Wed, 20 Oct 2021 16:55:28 -0600
From:   David Ahern <dsahern@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Florian Westphal <fw@...len.de>
Cc:     Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Maxim Mikityanskiy <maximmi@...dia.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Brendan Jackman <jackmanb@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Joe Stringer <joe@...ium.io>,
        Lorenz Bauer <lmb@...udflare.com>,
        Tariq Toukan <tariqt@...dia.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH bpf-next 07/10] bpf: Add helpers to query conntrack info

On 10/20/21 2:54 PM, Toke Høiland-Jørgensen wrote:
>> Sure, but I'm not sure I understand the use case.
>>
>> Insertion at XDP layer turns off netfilters NAT capability, so its
>> incompatible with the classic forwarding path.
>>
>> If thats fine, why do you need to insert into the conntrack table to
>> begin with?  The entire infrastructure its designed for is disabled...
> One of the major selling points of XDP is that you can reuse the
> existing kernel infrastructure instead of having to roll your own. So
> sure, one could implement their own conntrack using BPF maps (as indeed,
> e.g., Cilium has done), but why do that when you can take advantage of
> the existing one in the kernel? Same reason we have the bpf_fib_lookup()
> helper...
> 

Exactly, and a key point is that it allows consistency between XDP fast
path and full stack slow path. e.g., the BPF program is removed or
defers a flow to full stack for some reason.