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

From: "Dongseok Yi" <[email-masked]>
Date: Thu, 21 Jan 2021 21:47:08 +0900

On 2021-01-21 21:28, Steffen Klassert wrote:
> On Wed, Jan 20, 2021 at 03:55:42PM +0900, Dongseok Yi wrote:
> > On 2021-01-18 22:27, Steffen Klassert wrote:
> > > On Fri, Jan 15, 2021 at 10:20:35PM +0900, Dongseok Yi wrote:
> > > > 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 <[email-masked]>
> > > > ---
> > > > 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.
> > > >
> > > >  include/net/udp.h      |  2 +-
> > > >  net/ipv4/udp_offload.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++----
> > > >  net/ipv6/udp_offload.c |  2 +-
> > > >  3 files changed, 59 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/include/net/udp.h b/include/net/udp.h
> > > > index 877832b..01351ba 100644
> > > > --- a/include/net/udp.h
> > > > +++ b/include/net/udp.h
> > > > @@ -178,7 +178,7 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
> > > >  int udp_gro_complete(struct sk_buff *skb, int nhoff, udp_lookup_t lookup);
> > > >
> > > >  struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
> > > > -				  netdev_features_t features);
> > > > +				  netdev_features_t features, bool is_ipv6);
> > > >
> > > >  static inline struct udphdr *udp_gro_udphdr(struct sk_buff *skb)
> > > >  {
> > > > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > > > index ff39e94..c532d3b 100644
> > > > --- a/net/ipv4/udp_offload.c
> > > > +++ b/net/ipv4/udp_offload.c
> > > > @@ -187,8 +187,57 @@ struct sk_buff *skb_udp_tunnel_segment(struct sk_buff *skb,
> > > >  }
> > > >  EXPORT_SYMBOL(skb_udp_tunnel_segment);
> > > >
> > > > +static void __udpv4_gso_segment_csum(struct sk_buff *seg,
> > > > +				     __be32 *oldip, __be32 *newip,
> > > > +				     __be16 *oldport, __be16 *newport)
> > > > +{
> > > > +	struct udphdr *uh = udp_hdr(seg);
> > > > +	struct iphdr *iph = ip_hdr(seg);
> > > > +
> > > > +	if (uh->check) {
> > > > +		inet_proto_csum_replace4(&uh->check, seg, *oldip, *newip,
> > > > +					 true);
> > > > +		inet_proto_csum_replace2(&uh->check, seg, *oldport, *newport,
> > > > +					 false);
> > > > +		if (!uh->check)
> > > > +			uh->check = CSUM_MANGLED_0;
> > > > +	}
> > > > +	uh->dest = *newport;
> > > > +
> > > > +	csum_replace4(&iph->check, *oldip, *newip);
> > > > +	iph->daddr = *newip;
> > > > +}
> > >
> > > Can't we just do the checksum recalculation for this case as it is done
> > > with standard GRO?
> >
> > If I understand standard GRO correctly, it calculates full checksum.
> > Should we adopt the same method to UDP GRO fraglist? I did not find
> > a simple method for the incremental checksum update.
> >
> > >
> > > > +
> > > > +static struct sk_buff *__udpv4_gso_segment_list_csum(struct sk_buff *segs)
> > > > +{
> > > > +	struct sk_buff *seg;
> > > > +	struct udphdr *uh, *uh2;
> > > > +	struct iphdr *iph, *iph2;
> > > > +
> > > > +	seg = segs;
> > > > +	uh = udp_hdr(seg);
> > > > +	iph = ip_hdr(seg);
> > > > +
> > > > +	while ((seg = seg->next)) {
> > > > +		uh2 = udp_hdr(seg);
> > > > +		iph2 = ip_hdr(seg);
> > > > +
> > > > +		if (uh->source != uh2->source || iph->saddr != iph2->saddr)
> > > > +			__udpv4_gso_segment_csum(seg,
> > > > +						 &iph2->saddr, &iph->saddr,
> > > > +						 &uh2->source, &uh->source);
> > > > +
> > > > +		if (uh->dest != uh2->dest || iph->daddr != iph2->daddr)
> > > > +			__udpv4_gso_segment_csum(seg,
> > > > +						 &iph2->daddr, &iph->daddr,
> > > > +						 &uh2->dest, &uh->dest);
> > > > +	}
> 
> 
> > >
> > > You don't need to check the addresses and ports of all packets in the
> > > segment list. If the addresses and ports are equal for the first and
> > > second packet in the list, then this also holds for the rest of the
> > > packets.
> >
> > I think we can try this with an additional flag (seg_csum).
> >
> > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > index 36b7e30..3f892df 100644
> > --- a/net/ipv4/udp_offload.c
> > +++ b/net/ipv4/udp_offload.c
> > @@ -213,25 +213,36 @@ static struct sk_buff *__udpv4_gso_segment_list_csum(struct sk_buff *segs)
> >         struct sk_buff *seg;
> >         struct udphdr *uh, *uh2;
> >         struct iphdr *iph, *iph2;
> > +       bool seg_csum = false;
> >
> >         seg = segs;
> >         uh = udp_hdr(seg);
> >         iph = ip_hdr(seg);
> 
> Why not
> 
>        if ((udp_hdr(seg)->dest == udp_hdr(seg->next)->dest) &&
>            (udp_hdr(seg)->source == udp_hdr(seg->next)->source) &&
>            (ip_hdr(seg)->daddr == ip_hdr(seg->next)->daddr) &&
>            (ip_hdr(seg)->saddr == ip_hdr(seg->next)->saddr))
> 	   	return segs;
> 
> Then you don't need to test inside the loop. Just update all
> packets if there is a header mismatch.

The test inside the loop would be needed to distinguish SNAT
and DNAT. I will try your approach on v3. Thanks.

> 
> >
> > -       while ((seg = seg->next)) {
> > +       seg = seg->next;
> > +       do {
> > +               if (!seg)
> > +                       break;
> > +
> >                 uh2 = udp_hdr(seg);
> >                 iph2 = ip_hdr(seg);
> >
> > -               if (uh->source != uh2->source || iph->saddr != iph2->saddr)
> > +               if (uh->source != uh2->source || iph->saddr != iph2->saddr) {
> >                         __udpv4_gso_segment_csum(seg,
> >                                                  &iph2->saddr, &iph->saddr,
> >                                                  &uh2->source, &uh->source);
> > +                       seg_csum = true;
> > +               }
> >
> > -               if (uh->dest != uh2->dest || iph->daddr != iph2->daddr)
> > +               if (uh->dest != uh2->dest || iph->daddr != iph2->daddr) {
> >                         __udpv4_gso_segment_csum(seg,
> >                                                  &iph2->daddr, &iph->daddr,
> >                                                  &uh2->dest, &uh->dest);
> > -       }
> > +                       seg_csum = true;
> > +               }
> > +
> > +               seg = seg->next;
> > +       } while (seg_csum);
> >
> >         return segs;
> >  }