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Date:   Tue, 13 Mar 2018 17:26:34 -0700
From:   Eric Dumazet <>
To:     Alexei Starovoitov <>, Yonghong Song <>,
        Steffen Klassert <>
Cc:     Daniel Borkmann <>,
        netdev <>, Martin Lau <>,,
Subject: Re: BUG_ON triggered in skb_segment

On 03/13/2018 05:04 PM, Alexei Starovoitov wrote:
> On 3/13/18 4:27 PM, Eric Dumazet wrote:
>> On 03/13/2018 04:09 PM, Alexei Starovoitov wrote:
>>> we have bpf_skb_proto_6_to_4() that was used by cilium for long time.
>>> It's not clear why it's not crashing there, but we cannot just
>>> reject changing proto in bpf programs now.
>>> We have to fix whatever needs to be fixed in skb_segment
>>> (if bug is there) or fix whatever necessary on mlx5 side.
>>> In bpf helper we mark it as SKB_GSO_DODGY just like packets coming
>>> through virtio would do, so if skb_segment() needs to do something
>>> special with skb the SKB_GSO_DODGY flag is already there.
>> 'Fixing' skb_segment(), I did that a long time ago and Herbert Xu was
>> not happy with the fix and provided something else.
> any link to your old patches and discussion?
> I think since mlx4 can do tso on them and the packets came out
> correct on the wire, there is nothing fundamentally wrong with
> changing gso_size. Just tricky to teach skb_segment.

The world is not mlx4 only. Some NIC will ask skb_segment() fallback 
segmentation for various reasons (like skb->len above a given limit like 


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