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Date:   Fri, 8 Jan 2021 14:35:02 +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 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.

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