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Date:   Fri, 14 Oct 2022 09:55:40 -0700
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Subject: Re: [PATCH v4,bpf-next] bpf: Don't redirect packets with invalid pkt_len
From:   sdf@google.com
To:     Lorenz Bauer <oss@lmb.io>
Cc:     shaozhengchao <shaozhengchao@huawei.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, yuehaibing@huawei.com
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On 10/14, Lorenz Bauer wrote:
> On Thu, 13 Oct 2022, at 11:44, shaozhengchao wrote:
> > 	Sorry, I haven't fully understood your intentions yet.
> > Can you explain it more detail?

> I'll try! Roughly, we do the following:

> 1. Create a BPF_PROG_TYPE_SOCKET_FILTER program that just returns 0
> 2. Load the program into the kernel
> 3. Call BPF_PROG_RUN with data_size_in == 14

> After your bugfix, it seems like step 3 is rejected due to data_size_in  
> == 14. We had to increase data_size_in to 15 to
> avoid this, see [0].

> This breaks user space, so it would be great if you could fix this in a  
> way that doesn't refuse BPF_PROG_RUN with

[..]

> data_size_in == 14. Since I don't understand the original problem very  
> well I can't tell you what the best fix is however.

The problem was that we were able to generate skb with len=0 via
BPF_PROG_RUN. Prohibiting those cases breaks backwards compatibility, so
we either have to:

a) (preferred?) accept inputs with <14, but maybe internally pad to 14
bytes to make the core stack happy
b) revert the patch and instead have length checks at runtime; doesn't seem  
to
be worth the penalty in the forwarding path because of some corner cases
like these ?


> 0:  
> https://github.com/cilium/ebpf/commit/a38fb6b5a46ab3b5639ea4d421232a10013596c0

> Thanks
> Lorenz