netdev - Re: [PATCH net-next v2 0/3] make nf_ct_frag/6

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Message-ID: <CAM_iQpUw+D=6aQc0Dxfy8bUuk_vz8JKtWV8GvhKOKJvyQ-a=dg@mail.gmail.com>
Date:   Fri, 17 Jul 2020 08:32:25 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Jakub Kicinski <kuba@...nel.org>, wenxu <wenxu@...oud.cn>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        NetFilter <netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH net-next v2 0/3] make nf_ct_frag/6_gather elide the skb CB clear

On Wed, Jul 15, 2020 at 2:17 PM Florian Westphal <fw@...len.de> wrote:
>
> Jakub Kicinski <kuba@...nel.org> wrote:
> > On Tue,  7 Jul 2020 12:55:08 +0800 wenxu@...oud.cn wrote:
> > > From: wenxu <wenxu@...oud.cn>
> > >
> > > 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
> > > http://patchwork.ozlabs.org/project/netdev/patch/1593422178-26949-1-git-send-email-wenxu@ucloud.cn/
> > >
> > > 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:
>
> http://patchwork.ozlabs.org/project/netdev/patch/1593422178-26949-1-git-send-email-wenxu@ucloud.cn/
>
> 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).

+1

I don't see parallel access here either. skb can be cloned for packet
socket or act_mirred, but its CB is cloned at the same time.

Thanks.