Re: [PATCH v4,bpf-next] bpf: Don't redirect packets with invalid pkt_len

[Stanislav Fomichev]

On Wed, Sep 21, 2022 at 1:48 AM shaozhengchao <shaozhengchao@xxxxxxxxxx> wrote:
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>
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> On 2022/9/20 22:42, Lorenz Bauer wrote:
> > 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
> Hi Lorenz
>         Sorry. But how does the rejection of the 0 length affect the
> test case? Is the return value abnormal, send packet failure or some
> others?

Sorry for the late reply, I'm still traveling.
What we want to avoid is creating invalid skbs via prog_test_run.
We can reject <14 byte packets with ENIVAL, but that might break some
of the existing apps (as Lorenz reported).
So it seems like a safer alternative would be to accept those packets,
but just append missing bytes in the kernel to create the valid
packets? Padding with zeros seems fine?

> Zhengchao Shao