Add csum_udp6() helper for calculating UDP over IPv6 checksums

Add a helper for calculating UDP checksums when used over IPv6
For future flexibility, the new helper takes parameters for the fields in
the IPv6 pseudo-header, so an IPv6 header or pseudo-header doesn't need to
be explicitly constructed.  It also allows the UDP header and payload to
be in separate buffers, although we don't use this yet.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

checksum.c

@@ -52,6 +52,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <linux/udp.h>
 #include <linux/icmp.h>
 #include <linux/icmpv6.h>
 
@@ -123,6 +124,27 @@ void csum_icmp4(struct icmphdr *icmp4hr, const void *payload, size_t len)
 	icmp4hr->checksum = csum_unaligned(payload, len, psum);
 }
 
+/**
+ * csum_udp6() - Calculate and set checksum for a UDP over IPv6 packet
+ * @udp6hr:	UDP header, initialised apart from checksum
+ * @payload:	UDP packet payload
+ * @len:	Length of @payload (not including UDP header)
+ */
+void csum_udp6(struct udphdr *udp6hr,
+	       const struct in6_addr *saddr, const struct in6_addr *daddr,
+	       const void *payload, size_t len)
+{
+	/* Partial checksum for the pseudo-IPv6 header */
+	uint32_t psum = sum_16b(saddr, sizeof(*saddr)) +
+		        sum_16b(daddr, sizeof(*daddr)) +
+		        htons(len + sizeof(*udp6hr)) + htons(IPPROTO_UDP);
+
+	udp6hr->check = 0;
+	/* Add in partial checksum for the UDP header alone */
+	psum += sum_16b(udp6hr, sizeof(*udp6hr));
+	udp6hr->check = csum_unaligned(payload, len, psum);
+}
+
 /**
  * csum_icmp6() - Calculate and set checksum for an ICMPv6 packet
  * @icmp6hr:	ICMPv6 header, initialised apart from checksum

checksum.h

@@ -6,6 +6,7 @@
 #ifndef CHECKSUM_H
 #define CHECKSUM_H
 
+struct udphdr;
 struct icmphdr;
 struct icmp6hdr;
 
@@ -13,6 +14,9 @@ uint32_t sum_16b(const void *buf, size_t len);
 uint16_t csum_fold(uint32_t sum);
 uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init);
 void csum_icmp4(struct icmphdr *ih, const void *payload, size_t len);
+void csum_udp6(struct udphdr *udp6hr,
+	       const struct in6_addr *saddr, const struct in6_addr *daddr,
+	       const void *payload, size_t len);
 void csum_icmp6(struct icmp6hdr *icmp6hr,
 		const struct in6_addr *saddr, const struct in6_addr *daddr,
 		const void *payload, size_t len);

tap.c

@@ -183,9 +183,8 @@ void tap_ip_send(const struct ctx *c, const struct in6_addr *src, uint8_t proto,
 		} else if (proto == IPPROTO_UDP) {
 			struct udphdr *uh = (struct udphdr *)(ip6h + 1);
 
-			uh->check = 0;
+			csum_udp6(uh, &ip6h->saddr, &ip6h->daddr,
-			uh->check = csum_unaligned(ip6h, len + sizeof(*ip6h),
+				  uh + 1, len - sizeof(*uh));
-						   0);
 		} else if (proto == IPPROTO_ICMPV6) {
 			struct icmp6hdr *ih = (struct icmp6hdr *)(ip6h + 1);