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Date:   Mon, 11 Jan 2021 09:43:22 +0100
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Dongseok Yi <dseok.yi@...sung.com>
CC:     "'David S. Miller'" <davem@...emloft.net>,
        <namkyu78.kim@...sung.com>,
        'Alexey Kuznetsov' <kuznet@....inr.ac.ru>,
        'Hideaki YOSHIFUJI' <yoshfuji@...ux-ipv6.org>,
        'Jakub Kicinski' <kuba@...nel.org>,
        "'Willem de Bruijn'" <willemb@...gle.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH net] udp: check sk for UDP GRO fraglist

On Mon, Jan 11, 2021 at 11:02:42AM +0900, Dongseok Yi wrote:
> On 2021-01-08 22:35, Steffen Klassert wrote:
> > On Fri, Jan 08, 2021 at 09:52:28PM +0900, Dongseok Yi wrote:
> > > It is a workaround patch.
> > >
> > > UDP/IP header of UDP GROed frag_skbs are not updated even after NAT
> > > forwarding. Only the header of head_skb from ip_finish_output_gso ->
> > > skb_gso_segment is updated but following frag_skbs are not updated.
> > >
> > > A call path skb_mac_gso_segment -> inet_gso_segment ->
> > > udp4_ufo_fragment -> __udp_gso_segment -> __udp_gso_segment_list
> > > does not try to update any UDP/IP header of the segment list.
> > >
> > > It might make sense because each skb of frag_skbs is converted to a
> > > list of regular packets. Header update with checksum calculation may
> > > be not needed for UDP GROed frag_skbs.
> > >
> > > But UDP GRO frag_list is started from udp_gro_receive, we don't know
> > > whether the skb will be NAT forwarded at that time. For workaround,
> > > try to get sock always when call udp4_gro_receive -> udp_gro_receive
> > > to check if the skb is for local.
> > >
> > > I'm still not sure if UDP GRO frag_list is really designed for local
> > > session only. Can kernel support NAT forward for UDP GRO frag_list?
> > > What am I missing?
> > 
> > The initial idea when I implemented this was to have a fast
> > forwarding path for UDP. So forwarding is a usecase, but NAT
> > is a problem, indeed. A quick fix could be to segment the
> > skb before it gets NAT forwarded. Alternatively we could
> > check for a header change in __udp_gso_segment_list and
> > update the header of the frag_skbs accordingly in that case.
> 
> Thank you for explaining.
> Can I think of it as a known issue?

No, it was not known before you reported it.

> I think we should have a fix
> because NAT can be triggered by user. Can I check the current status?
> Already planning a patch or a new patch should be written?

We have to do a new patch to fix that issue. If you want do
do so, go ahead.

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