[PATCH v2 1/2] tap: break out building of udp header from tap_udp4_send function

[Jon Maloy <jmaloy@redhat.com>]

We will need to build the UDP header at other locations than in function
tap_udp4_send(), so we break that part out to a separate function.

Signed-off-by: Jon Maloy <jmaloy@redhat.com>
---
 tap.c | 32 +++++++++++++++++++++++++-------
 tap.h |  6 +++++-
 2 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/tap.c b/tap.c
index cd32a90..8087779 100644
--- a/tap.c
+++ b/tap.c
@@ -162,7 +162,7 @@ static void *tap_push_ip4h(struct iphdr *ip4h, struct in_addr src,
 }
 
 /**
- * tap_udp4_send() - Send UDP over IPv4 packet
+ * tap_push_uh4() - Build UDPv4 header with checksum
  * @c:		Execution context
  * @src:	IPv4 source address
  * @sport:	UDP source port
@@ -171,15 +171,11 @@ static void *tap_push_ip4h(struct iphdr *ip4h, struct in_addr src,
  * @in:		UDP payload contents (not including UDP header)
  * @dlen:	UDP payload length (not including UDP header)
  */
-void tap_udp4_send(const struct ctx *c, struct in_addr src, in_port_t sport,
+void *tap_push_uh4(struct udphdr *uh, struct in_addr src, in_port_t sport,
 		   struct in_addr dst, in_port_t dport,
 		   const void *in, size_t dlen)
 {
 	size_t l4len = dlen + sizeof(struct udphdr);
-	char buf[USHRT_MAX];
-	struct iphdr *ip4h = tap_push_l2h(c, buf, ETH_P_IP);
-	struct udphdr *uh = tap_push_ip4h(ip4h, src, dst, l4len, IPPROTO_UDP);
-	char *data = (char *)(uh + 1);
 	const struct iovec iov = {
 		.iov_base = (void *)in,
 		.iov_len = dlen
@@ -190,8 +186,30 @@ void tap_udp4_send(const struct ctx *c, struct in_addr src, in_port_t sport,
 	uh->dest = htons(dport);
 	uh->len = htons(l4len);
 	csum_udp4(uh, src, dst, &payload);
-	memcpy(data, in, dlen);
+	return uh + 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)
+ * @dlen:	UDP payload length (not including UDP header)
+ */
+void tap_udp4_send(const struct ctx *c, struct in_addr src, in_port_t sport,
+		   struct in_addr dst, in_port_t dport,
+		   const void *in, size_t dlen)
+{
+	size_t l4len = dlen + sizeof(struct udphdr);
+	char buf[USHRT_MAX];
+	struct iphdr *ip4h = tap_push_l2h(c, buf, ETH_P_IP);
+	struct udphdr *uh = tap_push_ip4h(ip4h, src, dst, l4len, IPPROTO_UDP);
+	char *data = tap_push_uh4(uh, src, sport, dst, dport, in, dlen);
 
+	memcpy(data, in, dlen);
 	tap_send_single(c, buf, dlen + (data - buf));
 }
 
diff --git a/tap.h b/tap.h
index dfbd8b9..3451343 100644
--- a/tap.h
+++ b/tap.h
@@ -8,6 +8,8 @@
 
 #define ETH_HDR_INIT(proto) { .h_proto = htons_constant(proto) }
 
+struct udphdr;
+
 /**
  * struct tap_hdr - tap backend specific headers
  * @vnet_len:	Frame length (for qemu socket transport)
@@ -43,7 +45,9 @@ static inline void tap_hdr_update(struct tap_hdr *thdr, size_t l2len)
 	if (thdr)
 		thdr->vnet_len = htonl(l2len);
 }
-
+void *tap_push_uh4(struct udphdr *uh, struct in_addr src, in_port_t sport,
+		   struct in_addr dst, in_port_t dport,
+		   const void *in, size_t dlen);
 void tap_udp4_send(const struct ctx *c, struct in_addr src, in_port_t sport,
 		   struct in_addr dst, in_port_t dport,
 		   const void *in, size_t dlen);
-- 
@@ -8,6 +8,8 @@
 
 #define ETH_HDR_INIT(proto) { .h_proto = htons_constant(proto) }
 
+struct udphdr;
+
 /**
  * struct tap_hdr - tap backend specific headers
  * @vnet_len:	Frame length (for qemu socket transport)
@@ -43,7 +45,9 @@ static inline void tap_hdr_update(struct tap_hdr *thdr, size_t l2len)
 	if (thdr)
 		thdr->vnet_len = htonl(l2len);
 }
-
+void *tap_push_uh4(struct udphdr *uh, struct in_addr src, in_port_t sport,
+		   struct in_addr dst, in_port_t dport,
+		   const void *in, size_t dlen);
 void tap_udp4_send(const struct ctx *c, struct in_addr src, in_port_t sport,
 		   struct in_addr dst, in_port_t dport,
 		   const void *in, size_t dlen);
-- 
2.48.1