Add csum_ip4_header() helper to calculate IPv4 header checksums

We calculate IPv4 header checksums in at least two places, in dhcp() and
in tap_ip_send.  Add a helper to handle this calculation in both places.

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

checksum.c

@@ -115,6 +115,16 @@ uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init)
 	return (uint16_t)~csum_fold(sum_16b(buf, len) + init);
 }
 
+/**
+ * csum_ip4_header() - Calculate and set IPv4 header checksum
+ * @ip4h:	IPv4 header
+ */
+void csum_ip4_header(struct iphdr *ip4h)
+{
+	ip4h->check = 0;
+	ip4h->check = csum_unaligned(ip4h, (size_t)ip4h->ihl * 4, 0);
+}
+
 /**
  * csum_udp4() - Calculate and set checksum for a UDP over IPv4 packet
  * @udp4hr:	UDP header, initialised apart from checksum

checksum.h

@@ -13,6 +13,7 @@ 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_ip4_header(struct iphdr *ip4h);
 void csum_udp4(struct udphdr *udp4hr, in_addr_t saddr, in_addr_t daddr,
 	       const void *payload, size_t len);
 void csum_icmp4(struct icmphdr *ih, const void *payload, size_t len);

dhcp.c

@@ -371,8 +371,7 @@ int dhcp(const struct ctx *c, const struct pool *p)
 	iph->tot_len = htons(len += sizeof(*iph));
 	iph->daddr = c->ip4.addr;
 	iph->saddr = c->ip4.gw;
-	iph->check = 0;
+	csum_ip4_header(iph);
-	iph->check = csum_unaligned(iph, (intptr_t)(iph->ihl * 4), 0);
 
 	len += sizeof(*eh);
 	memcpy(eh->h_dest, eh->h_source, ETH_ALEN);

tap.c

@@ -135,8 +135,7 @@ void tap_ip_send(const struct ctx *c, const struct in6_addr *src, uint8_t proto,
 		iph->daddr = c->ip4.addr_seen;
 		memcpy(&iph->saddr, &src->s6_addr[12], 4);
 
-		iph->check = 0;
+		csum_ip4_header(iph);
-		iph->check = csum_unaligned(iph, (size_t)iph->ihl * 4, 0);
 
 		memcpy(data, in, len);