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

[shaozhengchao]

On 2022/10/15 0:55, sdf@xxxxxxxxxx wrote:
> 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 ?
Hi sdf:
	a） looks better and I'll put up a patch as soon as possible to
fix it.

Zhengchao Shao
> > 0: 
> > https://github.com/cilium/ebpf/commit/a38fb6b5a46ab3b5639ea4d421232a10013596c0
>
> > Thanks
> > Lorenz