Re: [PATCH net v2] udp: ipv4: manipulate network header of NATed UDP GRO fraglist

[Alexander Lobakin]

> From: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>
> Date: Mon, 18 Jan 2021 07:37:59 +0100
>
> On Fri, Jan 15, 2021 at 05:12:33PM +0000, Alexander Lobakin wrote:
>> From: Dongseok Yi <dseok.yi@xxxxxxxxxxx>
>> Date: Fri, 15 Jan 2021 22:20:35 +0900
>>
>>> 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 UDP/IP header of the segment list but copy
>>> only the MAC header.
>>>
>>> Update dport, daddr and checksums of each skb of the segment list
>>> in __udp_gso_segment_list. It covers both SNAT and DNAT.
>>>
>>> Fixes: 9fd1ff5d2ac7 (udp: Support UDP fraglist GRO/GSO.)
>>> Signed-off-by: Dongseok Yi <dseok.yi@xxxxxxxxxxx>
>>> ---
>>> v1:
>>> Steffen Klassert said, there could be 2 options.
>>> https://lore.kernel.org/patchwork/patch/1362257/
>>> I was trying to write a quick fix, but it was not easy to forward
>>> segmented list. Currently, assuming DNAT only.
>>>
>>> v2:
>>> Per Steffen Klassert request, move the procedure from
>>> udp4_ufo_fragment to __udp_gso_segment_list and support SNAT.
>>>
>>> To Alexander Lobakin, I've checked your email late. Just use this
>>> patch as a reference. It support SNAT too, but does not support IPv6
>>> yet. I cannot make IPv6 header changes in __udp_gso_segment_list due
>>> to the file is in IPv4 directory.
>>
>> I used another approach, tried to make fraglist GRO closer to plain
>> in terms of checksummming, as it is confusing to me why GSO packet
>> should have CHECKSUM_UNNECESSARY.
>
> This is intentional. With fraglist GRO, we don't mangle packets
> in the standard (non NAT) case. So the checksum is still correct
> after segmentation. That is one reason why it has good forwarding
> performance when software segmentation is needed. Checksuming
> touches the whole packet and has a lot of overhead, so it is
> heplfull to avoid it whenever possible.
>
> We should find a way to do the checksum only when we really
> need it. I.e. only if the headers of the head skb changed.

I suggest to do memcmp() between skb_network_header(skb) and
skb_network_header(skb->frag_list) with the len of
skb->data - skb_network_header(skb). This way we will detect changes
in IPv4/IPv6 and UDP headers.
If so, copy the full headers and fall back to the standard checksum,
recalculation, else use the current path.

Al