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

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Message-ID: <CACAyw9_5-T5Y9AQpAmCe=aj9A0Q=SMyx1cMz6TRQvnW=NU9ygA@mail.gmail.com>
Date:   Wed, 26 Jan 2022 09:45:00 +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, 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. That
would be a problem in fact, since it could be used to read memory that
the verifier hasn't deemed safe.

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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