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Date:   Tue, 13 Mar 2018 17:04:59 -0700
From:   Alexei Starovoitov <>
To:     Eric Dumazet <>, Yonghong Song <>,
        Steffen Klassert <>
CC:     Daniel Borkmann <>,
        netdev <>, Martin Lau <>,
        <>, <>
Subject: Re: BUG_ON triggered in skb_segment

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.

> GSO_DODGY has nothing to do with the problem really.
> Changing gso_size is breaking GRO since it ends up changing the number
> of segments on the wire. TCP is not going to be happy, so you'll also
> have to fix TCP eventually.

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