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Date:   Tue, 20 Sep 2022 15:42:31 +0100
From:   "Lorenz Bauer" <oss@lmb.io>
To:     shaozhengchao <shaozhengchao@huawei.com>,
        "Stanislav Fomichev" <sdf@google.com>
Cc:     ast@kernel.org, daniel@iogearbox.net, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, yuehaibing@huawei.com
Subject: Re: [PATCH v4,bpf-next] bpf: Don't redirect packets with invalid pkt_len
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On Mon, 19 Sep 2022, at 11:55, shaozhengchao wrote:
> Sorry for the delay. I'm busy testing the TC module recently. I'm very 
> sorry for the user-space breakage.
>
> The root cause of this problem is that eth_type_trans() is called when
> the protocol type of the SKB is parsed. The len value of the SKB is
> reduced to 0. If the user mode requires that the forwarding succeed, or
>   if the MAC header is added again after the MAC header is subtracted, 
> is this appropriate?

We don't require forwarding to succeed with a 14 byte input buffer. We also don't look at the MAC header.

I think refusing to forward 0 length packets would be OK. Not 100% certain I understood you correctly, let me know if this helps.

Best
Lorenz