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

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Date:   Wed, 3 Jan 2018 10:53:00 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next 2/2] tun: allow to attach ebpf socket filter



On 2018年01月02日 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.

It looks to me that the minimum length is:

skb_vlan_tag_present(skb) ? offsetof(struct vlan_ethhdr, h_vlan_proto) : 0

And consider the vlan tag insertion done in tun_put_user(), we need trim 
4 more bytes if vlan tag is present.

Thanks