Add csum_udp4() helper for calculating UDP over IPv4 checksums

At least two places in passt fill in UDP over IPv4 checksums, although
since UDP checksums are optional with IPv4 that just amounts to storing
a 0 (in tap_ip_send()) or leaving a 0 from an earlier initialization (in
dhcp()).  For consistency, add a helper for this "calculation".

Just for the heck of it, add the option (compile time disabled for now) to
calculate real UDP checksums.

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

checksum.c

@@ -56,6 +56,12 @@
 #include <linux/icmp.h>
 #include <linux/icmpv6.h>
 
+/* Checksums are optional for UDP over IPv4, so we usually just set
+ * them to 0.  Change this to 1 to calculate real UDP over IPv4
+ * checksums
+ */
+#define UDP4_REAL_CHECKSUMS	0
+
 /**
  * sum_16b() - Calculate sum of 16-bit words
  * @buf:	Input buffer
@@ -109,6 +115,33 @@ uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init)
 	return (uint16_t)~csum_fold(sum_16b(buf, len) + init);
 }
 
+/**
+ * csum_udp4() - Calculate and set checksum for a UDP over IPv4 packet
+ * @udp4hr:	UDP header, initialised apart from checksum
+ * @saddr:	IPv4 source address
+ * @daddr:	IPv4 destination address
+ * @payload:	ICMPv4 packet payload
+ * @len:	Length of @payload (not including UDP)
+ */
+void csum_udp4(struct udphdr *udp4hr, in_addr_t saddr, in_addr_t daddr,
+	       const void *payload, size_t len)
+{
+	/* UDP checksums are optional, so don't bother */
+	udp4hr->check = 0;
+
+	if (UDP4_REAL_CHECKSUMS) {
+		/* UNTESTED: if we did want real UDPv4 checksums, this
+		 * is roughly what we'd need */
+		uint32_t psum = csum_fold(htonl(saddr))
+			+ csum_fold(htonl(daddr))
+			+ htons(len + sizeof(*udp4hr))
+			+ htons(IPPROTO_UDP);
+		/* Add in partial checksum for the UDP header alone */
+		psum += sum_16b(udp4hr, sizeof(*udp4hr));
+		udp4hr->check = csum_unaligned(payload, len, psum);
+	}
+}
+
 /**
  * csum_icmp4() - Calculate and set checksum for an ICMP packet
  * @icmp4hr:	ICMP header, initialised apart from checksum

checksum.h

@@ -13,6 +13,8 @@ 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_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);
 void csum_udp6(struct udphdr *udp6hr,
 	       const struct in6_addr *saddr, const struct in6_addr *daddr,

dhcp.c

@@ -364,9 +364,9 @@ int dhcp(const struct ctx *c, const struct pool *p)
 		opt_set_dns_search(c, sizeof(m->o));
 
 	uh->len = htons(len = offsetof(struct msg, o) + fill(m) + sizeof(*uh));
-	uh->check = 0;
 	uh->source = htons(67);
 	uh->dest = htons(68);
+	csum_udp4(uh, c->ip4.gw, c->ip4.addr, uh + 1, len - sizeof(*uh));
 
 	iph->tot_len = htons(len += sizeof(*iph));
 	iph->daddr = c->ip4.addr;

tap.c

@@ -145,7 +145,7 @@ void tap_ip_send(const struct ctx *c, const struct in6_addr *src, uint8_t proto,
 		} else if (iph->protocol == IPPROTO_UDP) {
 			struct udphdr *uh = (struct udphdr *)(iph + 1);
 
-			uh->check = 0;
+			csum_udp4(uh, iph->saddr, iph->daddr, uh + 1, len - sizeof(*uh));
 		} else if (iph->protocol == IPPROTO_ICMP) {
 			struct icmphdr *ih = (struct icmphdr *)(iph + 1);
 			csum_icmp4(ih, ih + 1, len - sizeof(*ih));