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.
