Re: [PATCH net-next 2/2] tun: allow to attach ebpf socket filter

[Willem de Bruijn]

On Thu, Jan 4, 2018 at 8:28 AM, Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
>
> On 2018å01æ02æ 17:19, Willem de Bruijn wrote:
>>>>
>>>> More importantly, should this program just return a boolean pass or
>>>> drop. Taking a length and trimming may introduce bugs later on if the
>>>> stack parses the packet unconditionally, expecting a minimum size
>>>> to be present.
>>>>
>>>> This was the reason for introducing sk_filter_trim_cap and using that
>>>> in other sk_filter sites.
>>>>
>>>> A quick scan shows that tun_put_user expects a full vlan tag to exist
>>>> if skb_vlan_tag_present(skb), for instance. If trimmed to below this
>>>> length the final call to skb_copy_datagram_iter may have negative
>>>> length.
>>>>
>>>> This is an issue with the existing sk_filter call as much as with the
>>>> new run_ebpf_filter call.
>>>
>>> Good point, so consider it was used by sk_filter too, we need to fix it
>>> anyway. Actually, I've considered the boolean return value but finally I
>>> decide to obey the style of sk filter. Maybe the trimming has real user.
>>> e.g
>>> high speed header recoding/analysis? Consider it's not hard to fix, how
>>> about just keep that?
>>
>> I don't see an obvious use case, but sure. We'll just need to look
>> at what the minimum trim length needs to be.
>
>
> Try to reproduce the possible issue, but looks like we are safe since we may
> hit -EFAULT which is returned by skb_copy_datagram_iter() before.

Ah, indeed, it can handle short lengths. Great, thanks for checking.

> So in V2,
> I will keep the code as is except trim 4 more bytes if vlan tag is present.

This is do not follow. Perhaps the patch will make it clear.