netdev - Re: [PATCH bpf-next 02/10] tcp: bpf: Parse BPF experimental header option

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Date:   Sun, 28 Jun 2020 17:45:45 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Eric Dumazet <edumazet@...gle.com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        kernel-team <kernel-team@...com>,
        Lawrence Brakmo <brakmo@...com>,
        Neal Cardwell <ncardwell@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH bpf-next 02/10] tcp: bpf: Parse BPF experimental header
 option

On Sat, Jun 27, 2020 at 10:17:26AM -0700, Eric Dumazet wrote:
[ ... ]

> It seems strange that we want to add code in TCP stack only to cover a
> limited use case (kind 254 and 0xEB9F magic)
> 
> For something like the work Petar Penkov did (to be able to generate
> SYNCOOKIES from XDP), we do not go through tcp_parse_options() and BPF
> program
> would have to implement its own parsing (without having an SKB at
> hand), probably calling a helper function, with no
> TCP_SKB_CB(skb)->bpf_hdr_opt_off.
> 
> This patch is hard coding a specific option and will prevent anyone
> using private option(s) from using this infrastructure in the future,
> yet paying the extra overhead.
There is a discussion in patch 4 about not limiting this patch set
to option kind 254.  That will affect the usefulness of bpf_hdr_opt_off.

> 
> TCP_SKB_CB(skb) is tight, I would prefer keeping the space in it for
> standard TCP stack features.
> 
> If an optional BPF program needs to re-parse the TCP options to find a
> specific option, maybe the extra cost is noise (especially if this is
> only for SYN & SYNACK packets) ?
> 
> Thanks