Add csum_icmp6() helper for calculating ICMPv6 checksums

At least two places in passt calculate ICMPv6 checksums, ndp() and
tap_ip_send().  Add a helper to handle this calculation in both places.
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 ICMPv6 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 +52,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <linux/icmpv6.h>
+
 /**
  * sum_16b() - Calculate sum of 16-bit words
  * @buf:	Input buffer
@@ -105,6 +107,29 @@ uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init)
 	return (uint16_t)~csum_fold(sum_16b(buf, len) + init);
 }
 
+/**
+ * csum_icmp6() - Calculate and set checksum for an ICMPv6 packet
+ * @icmp6hr:	ICMPv6 header, initialised apart from checksum
+ * @saddr:	IPv6 source address
+ * @daddr:	IPv6 destination address
+ * @payload:	ICMP packet payload
+ * @len:	Length of @payload (not including ICMPv6 header)
+ */
+void csum_icmp6(struct icmp6hdr *icmp6hr,
+		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(*icmp6hr)) + htons(IPPROTO_ICMPV6);
+
+	icmp6hr->icmp6_cksum = 0;
+	/* Add in partial checksum for the ICMPv6 header alone */
+	psum += sum_16b(icmp6hr, sizeof(*icmp6hr));
+	icmp6hr->icmp6_cksum = csum_unaligned(payload, len, psum);
+}
+
 /**
  * csum_tcp4() - Calculate TCP checksum for IPv4 and set in place
  * @iph:	Packet buffer, IP header

checksum.h

@@ -6,9 +6,14 @@
 #ifndef CHECKSUM_H
 #define CHECKSUM_H
 
+struct icmp6hdr;
+
 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_icmp6(struct icmp6hdr *icmp6hr,
+		const struct in6_addr *saddr, const struct in6_addr *daddr,
+		const void *payload, size_t len);
 void csum_tcp4(struct iphdr *iph);
 uint16_t csum(const void *buf, size_t len, uint32_t init);
 

ndp.c

@@ -189,10 +189,7 @@ dns_done:
 		ip6hr->saddr = c->ip6.addr_ll;
 
 	ip6hr->payload_len = htons(sizeof(*ihr) + len);
-	ip6hr->hop_limit = IPPROTO_ICMPV6;
+	csum_icmp6(ihr, &ip6hr->saddr, &ip6hr->daddr, ihr + 1, len);
-	ihr->icmp6_cksum = 0;
-	ihr->icmp6_cksum = csum_unaligned(ip6hr, sizeof(*ip6hr) +
-						 sizeof(*ihr) + len, 0);
 
 	ip6hr->version = 6;
 	ip6hr->nexthdr = IPPROTO_ICMPV6;

tap.c

@@ -191,10 +191,8 @@ void tap_ip_send(const struct ctx *c, const struct in6_addr *src, uint8_t proto,
 		} else if (proto == IPPROTO_ICMPV6) {
 			struct icmp6hdr *ih = (struct icmp6hdr *)(ip6h + 1);
 
-			ih->icmp6_cksum = 0;
+			csum_icmp6(ih, &ip6h->saddr, &ip6h->daddr,
-			ih->icmp6_cksum = csum_unaligned(ip6h,
+				   ih + 1, len - sizeof(*ih));
-							 len + sizeof(*ip6h),
-							 0);
 		}
 		ip6h->version = 6;
 		ip6h->nexthdr = proto;