tap: Split tap_ip4_send() into UDP and ICMP variants

tap_ip4_send() has special case logic to compute the checksums for UDP
and ICMP packets, which is a mild layering violation.  By using a suitable
helper we can split it into tap_udp4_send() and tap_icmp4_send() functions
without greatly increasing the code size, this removing that layering
violation.

We make some small changes to the interface while there.  In both cases
we make the destination IPv4 address a parameter, which will be useful
later.  For the UDP variant we make it take just the UDP payload, and it
will generate the UDP header.  For the ICMP variant we pass in the ICMP
header as before.  The inconsistency is because that's what seems to be
the more natural way to invoke the function in the callers in each case.

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

icmp.c

@@ -124,7 +124,8 @@ void icmp_sock_handler(const struct ctx *c, union epoll_ref ref,
 			icmp_id_map[V4][id].seq = seq;
 		}
 
-		tap_ip4_send(c, sr4->sin_addr.s_addr, IPPROTO_ICMP, buf, n);
+		tap_icmp4_send(c, sr4->sin_addr.s_addr, tap_ip4_daddr(c),
+			       buf, n);
 	}
 }
 

tap.c

@@ -132,19 +132,19 @@ static void *tap_push_l2h(const struct ctx *c, void *buf, uint16_t proto)
 }
 
 /**
- * tap_ip4_send() - Send IPv4 packet, with L2 headers, calculating L3/L4 checksums
+ * tap_push_ip4h() - Build IPv4 header for inbound packet, with checksum
  * @c:		Execution context
- * @src:	IPv4 source address
- * @proto:	L4 protocol number
- * @in:		Payload
+ * @src:	IPv4 source address, network order
+ * @dst:	IPv4 destination address, network order
  * @len:	L4 payload length
+ * @proto:	L4 protocol number
+ *
+ * Return: pointer at which to write the packet's payload
  */
-void tap_ip4_send(const struct ctx *c, in_addr_t src, uint8_t proto,
-		  const char *in, size_t len)
+static void *tap_push_ip4h(char *buf, in_addr_t src, in_addr_t dst,
+			   size_t len, uint8_t proto)
 {
-	char buf[USHRT_MAX];
-	struct iphdr *ip4h = (struct iphdr *)tap_push_l2h(c, buf, ETH_P_IP);
-	char *data = (char *)(ip4h + 1);
+	struct iphdr *ip4h = (struct iphdr *)buf;
 
 	ip4h->version = 4;
 	ip4h->ihl = sizeof(struct iphdr) / 4;
@@ -155,21 +155,62 @@ void tap_ip4_send(const struct ctx *c, in_addr_t src, uint8_t proto,
 	ip4h->ttl = 255;
 	ip4h->protocol = proto;
 	ip4h->saddr = src;
-	ip4h->daddr = tap_ip4_daddr(c);
+	ip4h->daddr = dst;
 	csum_ip4_header(ip4h);
+	return ip4h + 1;
+}
 
+/**
+ * tap_udp4_send() - Send UDP over IPv4 packet
+ * @c:		Execution context
+ * @src:	IPv4 source address
+ * @sport:	UDP source port
+ * @dst:	IPv4 destination address
+ * @dport:	UDP destination port
+ * @in:		UDP payload contents (not including UDP header)
+ * @len:	UDP payload length (not including UDP header)
+ */
+/* cppcheck-suppress unusedFunction */
+void tap_udp4_send(const struct ctx *c, in_addr_t src, in_port_t sport,
+		   in_addr_t dst, in_port_t dport,
+		   const void *in, size_t len)
+{
+	size_t udplen = len + sizeof(struct udphdr);
+	char buf[USHRT_MAX];
+	void *ip4h = tap_push_l2h(c, buf, ETH_P_IP);
+	void *uhp = tap_push_ip4h(ip4h, src, dst, udplen, IPPROTO_UDP);
+	struct udphdr *uh = (struct udphdr *)uhp;
+	char *data = (char *)(uh + 1);
+
+	uh->source = htons(sport);
+	uh->dest = htons(dport);
+	uh->len = htons(udplen);
+	csum_udp4(uh, src, dst, in, len);
 	memcpy(data, in, len);
 
-	if (ip4h->protocol == IPPROTO_UDP) {
-		struct udphdr *uh = (struct udphdr *)(ip4h + 1);
-
-		csum_udp4(uh, ip4h->saddr, ip4h->daddr,
-			  uh + 1, len - sizeof(*uh));
-	} else if (ip4h->protocol == IPPROTO_ICMP) {
-		struct icmphdr *ih = (struct icmphdr *)(ip4h + 1);
-		csum_icmp4(ih, ih + 1, len - sizeof(*ih));
+	if (tap_send(c, buf, len + (data - buf)) < 0)
+		debug("tap: failed to send %lu bytes (IPv4)", len);
 }
 
+/**
+ * tap_icmp4_send() - Send ICMPv4 packet
+ * @c:		Execution context
+ * @src:	IPv4 source address
+ * @dst:	IPv4 destination address
+ * @in:		ICMP packet, including ICMP header
+ * @len:	ICMP packet length, including ICMP header
+ */
+void tap_icmp4_send(const struct ctx *c, in_addr_t src, in_addr_t dst,
+		    void *in, size_t len)
+{
+	char buf[USHRT_MAX];
+	void *ip4h = tap_push_l2h(c, buf, ETH_P_IP);
+	char *data = tap_push_ip4h(ip4h, src, dst, len, IPPROTO_ICMP);
+	struct icmphdr *icmp4h = (struct icmphdr *)data;
+
+	memcpy(data, in, len);
+	csum_icmp4(icmp4h, icmp4h + 1, len - sizeof(*icmp4h));
+
 	if (tap_send(c, buf, len + (data - buf)) < 0)
 		debug("tap: failed to send %lu bytes (IPv4)", len);
 }

tap.h

@@ -7,10 +7,13 @@
 #define TAP_H
 
 in_addr_t tap_ip4_daddr(const struct ctx *c);
+void tap_udp4_send(const struct ctx *c, in_addr_t src, in_port_t sport,
+		   in_addr_t dst, in_port_t dport,
+		   const void *in, size_t len);
+void tap_icmp4_send(const struct ctx *c, in_addr_t src, in_addr_t dst,
+		    void *in, size_t len);
 const struct in6_addr *tap_ip6_daddr(const struct ctx *c,
 				     const struct in6_addr *src);
-void tap_ip4_send(const struct ctx *c, in_addr_t src, uint8_t proto,
-		  const char *in, size_t len);
 void tap_udp6_send(const struct ctx *c,
 		   const struct in6_addr *src, in_port_t sport,
 		   const struct in6_addr *dst, in_port_t dport,