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Date:   Tue, 13 Mar 2018 17:35:42 -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 05:26 PM, Eric Dumazet wrote:
> 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 
> 16KB)
> Maybe

Herbert patch :

commit 9d8506cc2d7ea1f911c72c100193a3677f6668c3
Author: Herbert Xu <>
Date:   Thu Nov 21 11:10:04 2013 -0800

     gso: handle new frag_list of frags GRO packets

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