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Date:   Thu, 16 Jul 2020 10:42:11 +0800
From:   wenxu <>
To:     Florian Westphal <>,
        "David S. Miller" <>
Cc:     Jakub Kicinski <>,,,
        Cong Wang <>
Subject: Re: [PATCH net-next v2 0/3] make nf_ct_frag/6_gather elide the skb CB

On 7/16/2020 5:17 AM, Florian Westphal wrote:
> Jakub Kicinski <> wrote:
>> On Tue,  7 Jul 2020 12:55:08 +0800 wrote:
>>> From: wenxu <>
>>> Add nf_ct_frag_gather and Make nf_ct_frag6_gather elide the CB clear 
>>> when packets are defragmented by connection tracking. This can make
>>> each subsystem such as br_netfilter, openvswitch, act_ct do defrag
>>> without restore the CB. 
>>> This also avoid serious crashes and problems in  ct subsystem.
>>> Because Some packet schedulers store pointers in the qdisc CB private
>>> area and parallel accesses to the SKB.
>>> This series following up
>>> patch1: add nf_ct_frag_gather elide the CB clear
>>> patch2: make nf_ct_frag6_gather elide the CB clear
>>> patch3: fix clobber qdisc_skb_cb in act_ct with defrag
>>> v2: resue some ip_defrag function in patch1
>> Florian, Cong - are you willing to venture an ack on these? Anyone?
> Nope, sorry.  Reason is that I can't figure out the need for this series.
> Taking a huge step back:
> That patch looks ok to me:
> I understand the problem statement/commit message and I can see how its addressed.
> I don't understand why the CB clearing must be avoided.
> defrag assumes skb ownership -- e.g. it may realloc skb->data
> (calls pskb_may_pull), it calls skb_orphan(), etc.
> AFAICS, tcf_classify makes same assumption -- exclusive ownership
> and no parallel skb accesses.
> So, if in fact the "only" problem is the loss of
> qdisc_skb_cb(skb)->pkt_len, then the other patch looks ok to me.
> If we indeed have parallel access, then I do not understand how
> avoiding the memsets in the defrag path makes things any better
> (see above wrt. skb pull and the like).

Hi David,

What case for the parallel access the skb in tcf_classify?

If there indeed have this. Maybe it can't do defrag which also

access and modify the skb?



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