[PATCH net-next 1/2] net: udp: local checksum offload for encapsulation

[Edward Cree <ecree@solarflare.com>]

The arithmetic properties of the ones-complement checksum mean that a
 correctly checksummed inner packet, including its checksum, has a ones
 complement sum depending only on whatever value was used to initialise
 the checksum field before checksumming (in the case of TCP and UDP,
 this is the ones complement sum of the pseudo header, complemented).
Consequently, if we are going to offload the inner checksum with
 CHECKSUM_PARTIAL, we can compute the outer checksum based only on the
 packed data not covered by the inner checksum, and the initial value of
 the inner checksum field.

Signed-off-by: Edward Cree <ecree@solarflare.com>
---
 net/ipv4/udp.c | 34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 8841e98..3e63c3d 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -767,13 +767,37 @@ void udp_set_csum(bool nocheck, struct sk_buff *skb,
 {
 	struct udphdr *uh = udp_hdr(skb);
 
-	if (nocheck)
+	if (nocheck) {
 		uh->check = 0;
-	else if (skb_is_gso(skb))
+	} else if (skb_is_gso(skb)) {
 		uh->check = ~udp_v4_check(len, saddr, daddr, 0);
-	else if (skb_dst(skb) && skb_dst(skb)->dev &&
-		 (skb_dst(skb)->dev->features &
-		  (NETIF_F_IP_CSUM | NETIF_F_HW_CSUM))) {
+	} else if (skb->ip_summed == CHECKSUM_PARTIAL &&
+		   skb_dst(skb) && skb_dst(skb)->dev &&
+		   (skb_dst(skb)->dev->features &
+		    (NETIF_F_IP_CSUM | NETIF_F_HW_CSUM))) {
+		/* Everything from csum_start onwards will be
+		 * checksummed and will thus have a sum of whatever
+		 * we previously put in the checksum field (eg. sum
+		 * of pseudo-header)
+		 */
+		__wsum csum;
+
+		/* Fill in our pseudo-header checksum */
+		uh->check = ~udp_v4_check(len, saddr, daddr, 0);
+		/* Start with complement of inner pseudo-header checksum */
+		csum = ~skb_checksum(skb, skb_checksum_start_offset(skb) + skb->csum_offset,
+				     2, 0);
+		/* Add in checksum of our headers (incl. pseudo-header
+		 * checksum filled in above)
+		 */
+		csum = skb_checksum(skb, 0, skb_checksum_start_offset(skb), csum);
+		/* The result is the outer checksum */
+		uh->check = csum_fold(csum);
+		if (uh->check == 0)
+			uh->check = CSUM_MANGLED_0;
+	} else if (skb_dst(skb) && skb_dst(skb)->dev &&
+		   (skb_dst(skb)->dev->features &
+		    (NETIF_F_IP_CSUM | NETIF_F_HW_CSUM))) {
 
 		BUG_ON(skb->ip_summed == CHECKSUM_PARTIAL);
 
-- 
2.4.3