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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Wed, 12 May 2021 10:13:32 -0400
Message-ID: <CA+FuTSdOvfdeEPffem6ZDyMzu7yqWxBZrVi2S2wgwjBwqpqquA@mail.gmail.com>
Subject: Re: [PATCH bpf-next v3] bpf: check for BPF_F_ADJ_ROOM_FIXED_GSO when bpf_skb_change_proto
To:     Dongseok Yi <dseok.yi@samsung.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Wed, May 12, 2021 at 3:41 AM Dongseok Yi <dseok.yi@samsung.com> wrote:
>
> In the forwarding path GRO -> BPF 6 to 4 -> GSO for TCP traffic, the
> coalesced packet payload can be > MSS, but < MSS + 20.
> bpf_skb_proto_6_to_4 will upgrade the MSS and it can be > the payload
> length. After then tcp_gso_segment checks for the payload length if it
> is <= MSS. The condition is causing the packet to be dropped.
>
> tcp_gso_segment():
>         [...]
>         mss = skb_shinfo(skb)->gso_size;
>         if (unlikely(skb->len <= mss))
>                 goto out;
>         [...]
>
> Allow to upgrade/downgrade MSS only when BPF_F_ADJ_ROOM_FIXED_GSO is
> not set.
>
> Signed-off-by: Dongseok Yi <dseok.yi@samsung.com>

Acked-by: Willem de Bruijn <willemb@google.com>