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Date:   Mon, 11 Jan 2021 11:02:42 +0900
From:   "Dongseok Yi" <>
To:     "'Steffen Klassert'" <>
Cc:     "'David S. Miller'" <>,
        "'Alexey Kuznetsov'" <>,
        "'Hideaki YOSHIFUJI'" <>,
        "'Jakub Kicinski'" <>,
        "'Willem de Bruijn'" <>,
        <>, <>
Subject: RE: [RFC PATCH net] udp: check sk for UDP GRO fraglist

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? 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?

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