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Message-ID: <Y0mUjLXEpVuheSUc@google.com>
Date: Fri, 14 Oct 2022 09:55:40 -0700
From: sdf@...gle.com
To: Lorenz Bauer <oss@....io>
Cc: shaozhengchao <shaozhengchao@...wei.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>, bpf@...r.kernel.org,
linux-kernel@...r.kernel.org, yuehaibing@...wei.com
Subject: Re: [PATCH v4,bpf-next] bpf: Don't redirect packets with invalid pkt_len
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
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