netdev - Re: [PATCH bpf v2 4/4] bpf: Fix documentation of th_len in bpf_tcp_{gen,check}

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Date:   Tue, 1 Feb 2022 17:02:51 +0000
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Maxim Mikityanskiy <maximmi@...dia.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Petar Penkov <ppenkov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH bpf v2 4/4] bpf: Fix documentation of th_len in bpf_tcp_{gen,check}_syncookie

On Mon, 31 Jan 2022 at 13:38, Maxim Mikityanskiy <maximmi@...dia.com> wrote:
>
> On 2022-01-26 11:45, Lorenz Bauer wrote:
> > On Mon, 24 Jan 2022 at 15:13, Maxim Mikityanskiy <maximmi@...dia.com> wrote:
> >>
> >> bpf_tcp_gen_syncookie and bpf_tcp_check_syncookie expect the full length
> >> of the TCP header (with all extensions). Fix the documentation that says
> >> it should be sizeof(struct tcphdr).
> >
> > I don't understand this change, sorry. Are you referring to the fact
> > that the check is len < sizeof(*th) instead of len != sizeof(*th)?
> >
> > Your commit message makes me think that the helpers will access data
> > in the extension headers, which isn't true as far as I can tell.
>
> Yes, they will. See bpf_tcp_gen_syncookie -> tcp_v4_get_syncookie ->
> tcp_get_syncookie_mss -> tcp_parse_mss_option, which iterates over the
> TCP options ("extensions" wasn't the best word I used here). Moreover,
> bpf_tcp_gen_syncookie even checks that th_len == th->doff * 4.
>
> Although bpf_tcp_check_syncookie doesn't need the TCP options and
> doesn't enforce them to be passed, it's still allowed.

Sorry, I was only looking at bpf_tcp_check_syncookie indeed.
Unfortunate, it would be better if that function had a th->doff check
as well. :(

Acked-by: Lorenz Bauer <lmb@...udflare.com>

-- 
Lorenz Bauer  |  Systems Engineer
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