[PATCH 1/7] tcp: pass ipv4h checksum, not a pointer to the checksum

[Laurent Vivier <lvivier@redhat.com>]

tcp_fill_headers() takes a pointer to a previously computed IPv4 header
checksum to avoid recalculating it when the payload length doesn't
change. A subsequent patch makes tcp_fill_headers() access ip4h via
with_header() which scopes it to a temporary variable, so a pointer to
ip4h->check would become dangling after the with_header() block.

Pass the checksum by value as an int instead, using -1 as the sentinel
to indicate that the checksum should be computed from scratch (replacing
the NULL pointer sentinel). As the checksum is a uint16_t, -1 cannot be
a valid checksum value in an int.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 tcp.c          |  7 +++----
 tcp_buf.c      | 10 +++++-----
 tcp_internal.h |  3 +--
 tcp_vu.c       | 12 ++++++------
 4 files changed, 15 insertions(+), 17 deletions(-)

diff --git a/tcp.c b/tcp.c
index b14586249c4e..158a5be0327e 100644
--- a/tcp.c
+++ b/tcp.c
@@ -953,8 +953,7 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn,
 			struct ethhdr *eh,
 			struct iphdr *ip4h, struct ipv6hdr *ip6h,
 			struct tcphdr *th, struct iov_tail *payload,
-			const uint16_t *ip4_check, uint32_t seq,
-			bool no_tcp_csum)
+			int ip4_check, uint32_t seq, bool no_tcp_csum)
 {
 	const struct flowside *tapside = TAPFLOW(conn);
 	size_t l4len = iov_tail_size(payload) + sizeof(*th);
@@ -974,8 +973,8 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn,
 		ip4h->saddr = src4->s_addr;
 		ip4h->daddr = dst4->s_addr;
 
-		if (ip4_check)
-			ip4h->check = *ip4_check;
+		if (ip4_check != -1)
+			ip4h->check = ip4_check;
 		else
 			ip4h->check = csum_ip4_header(l3len, IPPROTO_TCP,
 						      *src4, *dst4);
diff --git a/tcp_buf.c b/tcp_buf.c
index 41965b107567..bc0f58dd7a5e 100644
--- a/tcp_buf.c
+++ b/tcp_buf.c
@@ -172,7 +172,7 @@ static void tcp_l2_buf_pad(struct iovec *iov)
  */
 static void tcp_l2_buf_fill_headers(const struct ctx *c,
 				    struct tcp_tap_conn *conn,
-				    struct iovec *iov, const uint16_t *check,
+				    struct iovec *iov, int check,
 				    uint32_t seq, bool no_tcp_csum)
 {
 	struct iov_tail tail = IOV_TAIL(&iov[TCP_IOV_PAYLOAD], 1, 0);
@@ -233,7 +233,7 @@ int tcp_buf_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
 	if (flags & KEEPALIVE)
 		seq--;
 
-	tcp_l2_buf_fill_headers(c, conn, iov, NULL, seq, false);
+	tcp_l2_buf_fill_headers(c, conn, iov, -1, seq, false);
 
 	tcp_l2_buf_pad(iov);
 
@@ -270,7 +270,7 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
 			    ssize_t dlen, int no_csum, uint32_t seq, bool push)
 {
 	struct tcp_payload_t *payload;
-	const uint16_t *check = NULL;
+	int check = -1;
 	struct iovec *iov;
 
 	conn->seq_to_tap = seq + dlen;
@@ -279,9 +279,9 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
 	if (CONN_V4(conn)) {
 		if (no_csum) {
 			struct iovec *iov_prev = tcp_l2_iov[tcp_payload_used - 1];
-			struct iphdr *iph = iov_prev[TCP_IOV_IP].iov_base;
+			const struct iphdr *iph = iov_prev[TCP_IOV_IP].iov_base;
 
-			check = &iph->check;
+			check = iph->check;
 		}
 		iov[TCP_IOV_IP] = IOV_OF_LVALUE(tcp4_payload_ip[tcp_payload_used]);
 	} else if (CONN_V6(conn)) {
diff --git a/tcp_internal.h b/tcp_internal.h
index d9408852571f..bb7a6629839c 100644
--- a/tcp_internal.h
+++ b/tcp_internal.h
@@ -187,8 +187,7 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn,
 			struct ethhdr *eh,
 			struct iphdr *ip4h, struct ipv6hdr *ip6h,
 			struct tcphdr *th, struct iov_tail *payload,
-			const uint16_t *ip4_check, uint32_t seq,
-			bool no_tcp_csum);
+			int ip4_check, uint32_t seq, bool no_tcp_csum);
 
 int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
 			  bool force_seq, struct tcp_info_linux *tinfo);
diff --git a/tcp_vu.c b/tcp_vu.c
index 3001defb5467..a21ee3499aed 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -138,7 +138,7 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
 		seq--;
 
 	tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &payload,
-			 NULL, seq, !*c->pcap);
+			 -1, seq, !*c->pcap);
 
 	if (*c->pcap)
 		pcap_iov(&flags_elem[0].in_sg[0], 1, VNET_HLEN);
@@ -283,7 +283,7 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
  */
 static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn,
 			   struct iovec *iov, size_t iov_cnt,
-			   const uint16_t **check, bool no_tcp_csum, bool push)
+			   int *check, bool no_tcp_csum, bool push)
 {
 	const struct flowside *toside = TAPFLOW(conn);
 	bool v6 = !(inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr));
@@ -329,7 +329,7 @@ static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn,
 	tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &payload,
 			 *check, conn->seq_to_tap, no_tcp_csum);
 	if (ip4h)
-		*check = &ip4h->check;
+		*check = ip4h->check;
 }
 
 /**
@@ -350,7 +350,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 	size_t hdrlen, fillsize;
 	int v6 = CONN_V6(conn);
 	uint32_t already_sent;
-	const uint16_t *check;
+	int check;
 
 	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
 		debug("Got packet, but RX virtqueue not usable yet");
@@ -437,7 +437,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 	 */
 
 	hdrlen = tcp_vu_hdrlen(v6);
-	for (i = 0, previous_dlen = -1, check = NULL; i < head_cnt; i++) {
+	for (i = 0, previous_dlen = -1, check = -1; i < head_cnt; i++) {
 		struct iovec *iov = &elem[head[i]].in_sg[0];
 		int buf_cnt = head[i + 1] - head[i];
 		size_t frame_size = iov_size(iov, buf_cnt);
@@ -451,7 +451,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 
 		/* The IPv4 header checksum varies only with dlen */
 		if (previous_dlen != dlen)
-			check = NULL;
+			check = -1;
 		previous_dlen = dlen;
 
 		tcp_vu_prepare(c, conn, iov, buf_cnt, &check, !*c->pcap, push);
-- 
2.53.0