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

[shaozhengchao <shaozhengchao@huawei.com>]

On 2022/10/15 0:55, sdf@google.com 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