Re: [RFC PATCH net] udp: check sk for UDP GRO fraglist

[Alexander Lobakin <alobakin@pm.me>]

From: Steffen Klassert <steffen.klassert@secunet.com>
Date: Mon, 11 Jan 2021 09:43:22 +0100

> 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.

This patch is incorrect. I do NAT UDP GRO Fraglists via nftables
(both with and without flow offload) with no issues since March'20.
Packet loss rates are always +/- 0, so I can say it works properly.
I can share any details / dump any runtime data if needed.

Thanks,
Al