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From:   "Dongseok Yi" <dseok.yi@samsung.com>
To:     "'Steffen Klassert'" <steffen.klassert@secunet.com>
Cc:     "'David S. Miller'" <davem@davemloft.net>,
        "'Alexander Lobakin'" <alobakin@pm.me>, <namkyu78.kim@samsung.com>,
        "'Jakub Kicinski'" <kuba@kernel.org>,
        "'Hideaki YOSHIFUJI'" <yoshfuji@linux-ipv6.org>,
        "'Willem de Bruijn'" <willemb@google.com>,
        <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>
In-Reply-To: <20210118132736.GM3576117@gauss3.secunet.de>
Subject: RE: [PATCH net v2] udp: ipv4: manipulate network header of NATed
 UDP GRO fraglist
Date:   Wed, 20 Jan 2021 15:55:42 +0900
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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 <dseok.yi@samsung.com>
> > ---
> > 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);

-       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;
 }