checksum: introduce functions to compute the header part checksum for TCP/UDP

The TCP and UDP checksums are computed using the data in the TCP/UDP
payload but also some informations in the IP header (protocol,
length, source and destination addresses).

We add two functions, proto_ipv4_header_psum() and
proto_ipv6_header_psum(), to compute the checksum of the IP
header part.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-ID: <20240303135114.1023026-8-lvivier@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

4 files changed, 90 insertions, 50 deletions

diff --git a/checksum.c b/checksum.c
index 1ac7975..f8a7b53 100644
--- a/checksum.c
+++ b/checksum.c
@@ -138,6 +138,29 @@ uint16_t csum_ip4_header(uint16_t tot_len, uint8_t protocol,
 }
 
 /**
+ * proto_ipv4_header_psum() - Calculates the partial checksum of an
+ * 			      IPv4 header for UDP or TCP
+ * @tot_len:	IPv4 Payload length (host order)
+ * @proto:	Protocol number (host order)
+ * @saddr:	Source address  (network order)
+ * @daddr:	Destination address (network order)
+ * Returns:	Partial checksum of the IPv4 header
+ */
+uint32_t proto_ipv4_header_psum(uint16_t tot_len, uint8_t protocol,
+				struct in_addr saddr, struct in_addr daddr)
+{
+	uint32_t psum = htons(protocol);
+
+	psum += (saddr.s_addr >> 16) & 0xffff;
+	psum += saddr.s_addr & 0xffff;
+	psum += (daddr.s_addr >> 16) & 0xffff;
+	psum += daddr.s_addr & 0xffff;
+	psum += htons(tot_len);
+
+	return psum;
+}
+
+/**
  * csum_udp4() - Calculate and set checksum for a UDP over IPv4 packet
  * @udp4hr:	UDP header, initialised apart from checksum
  * @saddr:	IPv4 source address
@@ -153,14 +176,10 @@ void csum_udp4(struct udphdr *udp4hr,
 	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(saddr.s_addr)
-			+ csum_fold(daddr.s_addr)
-			+ htons(len + sizeof(*udp4hr))
-			+ htons(IPPROTO_UDP);
-		/* Add in partial checksum for the UDP header alone */
-		psum += sum_16b(udp4hr, sizeof(*udp4hr));
+		uint16_t tot_len = len + sizeof(struct udphdr);
+		uint32_t psum = proto_ipv4_header_psum(tot_len, IPPROTO_UDP,
+						       saddr, daddr);
+		psum = csum_unfolded(udp4hr, sizeof(struct udphdr), psum);
 		udp4hr->check = csum(payload, len, psum);
 	}
 }
@@ -184,6 +203,27 @@ void csum_icmp4(struct icmphdr *icmp4hr, const void *payload, size_t len)
 }
 
 /**
+ * proto_ipv6_header_psum() - Calculates the partial checksum of an
+ * 			      IPv6 header for UDP or TCP
+ * @payload_len:	IPv6 payload length (host order)
+ * @proto:		Protocol number (host order)
+ * @saddr:		Source address (network order)
+ * @daddr:		Destination address (network order)
+ * Returns:	Partial checksum of the IPv6 header
+ */
+uint32_t proto_ipv6_header_psum(uint16_t payload_len, uint8_t protocol,
+				const struct in6_addr *saddr,
+				const struct in6_addr *daddr)
+{
+	uint32_t sum = htons(protocol) + htons(payload_len);
+
+	sum += sum_16b(saddr, sizeof(*saddr));
+	sum += sum_16b(daddr, sizeof(*daddr));
+
+	return sum;
+}
+
+/**
  * csum_udp6() - Calculate and set checksum for a UDP over IPv6 packet
  * @udp6hr:	UDP header, initialised apart from checksum
  * @payload:	UDP packet payload
@@ -193,14 +233,11 @@ void csum_udp6(struct udphdr *udp6hr,
 	       const struct in6_addr *saddr, const struct in6_addr *daddr,
 	       const void *payload, size_t len)
 {
-	/* Partial checksum for the pseudo-IPv6 header */
-	uint32_t psum = sum_16b(saddr, sizeof(*saddr)) +
-		        sum_16b(daddr, sizeof(*daddr)) +
-		        htons(len + sizeof(*udp6hr)) + htons(IPPROTO_UDP);
-
+	uint32_t psum = proto_ipv6_header_psum(len + sizeof(struct udphdr),
+					       IPPROTO_UDP, saddr, daddr);
 	udp6hr->check = 0;
-	/* Add in partial checksum for the UDP header alone */
-	psum += sum_16b(udp6hr, sizeof(*udp6hr));
+
+	psum = csum_unfolded(udp6hr, sizeof(struct udphdr), psum);
 	udp6hr->check = csum(payload, len, psum);
 }
 
diff --git a/checksum.h b/checksum.h
index 996f456..0f39676 100644
--- a/checksum.h
+++ b/checksum.h
@@ -15,10 +15,15 @@ uint16_t csum_fold(uint32_t sum);
 uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init);
 uint16_t csum_ip4_header(uint16_t tot_len, uint8_t protocol,
 			 struct in_addr saddr, struct in_addr daddr);
+uint32_t proto_ipv4_header_psum(uint16_t tot_len, uint8_t protocol,
+				struct in_addr saddr, struct in_addr daddr);
 void csum_udp4(struct udphdr *udp4hr,
 	       struct in_addr saddr, struct in_addr daddr,
 	       const void *payload, size_t len);
 void csum_icmp4(struct icmphdr *icmp4hr, const void *payload, size_t len);
+uint32_t proto_ipv6_header_psum(uint16_t payload_len, uint8_t protocol,
+				const struct in6_addr *saddr,
+				const struct in6_addr *daddr);
 void csum_udp6(struct udphdr *udp6hr,
 	       const struct in6_addr *saddr, const struct in6_addr *daddr,
 	       const void *payload, size_t len);
diff --git a/tcp.c b/tcp.c
index 7fb9dba..a7156ee 100644
--- a/tcp.c
+++ b/tcp.c
@@ -937,41 +937,33 @@ static void tcp_sock_set_bufsize(const struct ctx *c, int s)
 
 /**
  * tcp_update_check_tcp4() - Update TCP checksum from stored one
- * @buf:	L2 packet buffer with final IPv4 header
+ * @iph:	IPv4 header
+ * @th:		TCP header followed by TCP payload
  */
-static void tcp_update_check_tcp4(struct tcp4_l2_buf_t *buf)
+static void tcp_update_check_tcp4(const struct iphdr *iph, struct tcphdr *th)
 {
-	uint16_t tlen = ntohs(buf->iph.tot_len) - 20;
-	uint32_t sum = htons(IPPROTO_TCP);
+	uint16_t tlen = ntohs(iph->tot_len) - sizeof(struct iphdr);
+	struct in_addr saddr = { .s_addr = iph->saddr };
+	struct in_addr daddr = { .s_addr = iph->daddr };
+	uint32_t sum = proto_ipv4_header_psum(tlen, IPPROTO_TCP, saddr, daddr);
 
-	sum += (buf->iph.saddr >> 16) & 0xffff;
-	sum += buf->iph.saddr & 0xffff;
-	sum += (buf->iph.daddr >> 16) & 0xffff;
-	sum += buf->iph.daddr & 0xffff;
-	sum += htons(ntohs(buf->iph.tot_len) - 20);
-
-	buf->th.check = 0;
-	buf->th.check = csum(&buf->th, tlen, sum);
+	th->check = 0;
+	th->check = csum(th, tlen, sum);
 }
 
 /**
  * tcp_update_check_tcp6() - Calculate TCP checksum for IPv6
- * @buf:	L2 packet buffer with final IPv6 header
+ * @ip6h:	IPv6 header
+ * @th:		TCP header followed by TCP payload
  */
-static void tcp_update_check_tcp6(struct tcp6_l2_buf_t *buf)
+static void tcp_update_check_tcp6(struct ipv6hdr *ip6h, struct tcphdr *th)
 {
-	int len = ntohs(buf->ip6h.payload_len) + sizeof(struct ipv6hdr);
-
-	buf->ip6h.hop_limit = IPPROTO_TCP;
-	buf->ip6h.version = 0;
-	buf->ip6h.nexthdr = 0;
+	uint16_t payload_len = ntohs(ip6h->payload_len);
+	uint32_t sum = proto_ipv6_header_psum(payload_len, IPPROTO_TCP,
+					      &ip6h->saddr, &ip6h->daddr);
 
-	buf->th.check = 0;
-	buf->th.check = csum(&buf->ip6h, len, 0);
-
-	buf->ip6h.hop_limit = 255;
-	buf->ip6h.version = 6;
-	buf->ip6h.nexthdr = IPPROTO_TCP;
+	th->check = 0;
+	th->check = csum(th, payload_len, sum);
 }
 
 /**
@@ -1383,7 +1375,7 @@ do {									\
 
 		SET_TCP_HEADER_COMMON_V4_V6(b, conn, seq);
 
-		tcp_update_check_tcp4(b);
+		tcp_update_check_tcp4(&b->iph, &b->th);
 
 		tlen = tap_iov_len(c, &b->taph, ip_len);
 	} else {
@@ -1402,7 +1394,11 @@ do {									\
 
 		SET_TCP_HEADER_COMMON_V4_V6(b, conn, seq);
 
-		tcp_update_check_tcp6(b);
+		tcp_update_check_tcp6(&b->ip6h, &b->th);
+
+		b->ip6h.hop_limit = 255;
+		b->ip6h.version = 6;
+		b->ip6h.nexthdr = IPPROTO_TCP;
 
 		b->ip6h.flow_lbl[0] = (conn->sock >> 16) & 0xf;
 		b->ip6h.flow_lbl[1] = (conn->sock >> 8) & 0xff;
diff --git a/udp.c b/udp.c
index fb8373b..2fd6792 100644
--- a/udp.c
+++ b/udp.c
@@ -625,6 +625,7 @@ static size_t udp_update_hdr6(const struct ctx *c, int n, in_port_t dstport,
 {
 	struct udp6_l2_buf_t *b = &udp6_l2_buf[n];
 	const struct in6_addr *src, *dst;
+	uint16_t payload_len;
 	in_port_t src_port;
 	size_t ip_len;
 
@@ -634,7 +635,8 @@ static size_t udp_update_hdr6(const struct ctx *c, int n, in_port_t dstport,
 
 	ip_len = udp6_l2_mh_sock[n].msg_len + sizeof(b->ip6h) + sizeof(b->uh);
 
-	b->ip6h.payload_len = htons(udp6_l2_mh_sock[n].msg_len + sizeof(b->uh));
+	payload_len = udp6_l2_mh_sock[n].msg_len + sizeof(b->uh);
+	b->ip6h.payload_len = htons(payload_len);
 
 	if (IN6_IS_ADDR_LINKLOCAL(src)) {
 		dst = &c->ip6.addr_ll_seen;
@@ -670,17 +672,17 @@ static size_t udp_update_hdr6(const struct ctx *c, int n, in_port_t dstport,
 	}
 	b->ip6h.daddr = *dst;
 	b->ip6h.saddr = *src;
+	b->ip6h.version = 6;
+	b->ip6h.nexthdr = IPPROTO_UDP;
+	b->ip6h.hop_limit = 255;
 
 	b->uh.source = b->s_in6.sin6_port;
 	b->uh.dest = htons(dstport);
 	b->uh.len = b->ip6h.payload_len;
-
-	b->ip6h.hop_limit = IPPROTO_UDP;
-	b->ip6h.version = b->ip6h.nexthdr = b->uh.check = 0;
-	b->uh.check = csum(&b->ip6h, ip_len, 0);
-	b->ip6h.version = 6;
-	b->ip6h.nexthdr = IPPROTO_UDP;
-	b->ip6h.hop_limit = 255;
+	b->uh.check = 0;
+	b->uh.check = csum(&b->uh, payload_len,
+			   proto_ipv6_header_psum(payload_len, IPPROTO_UDP,
+						  src, dst));
 
 	return tap_iov_len(c, &b->taph, ip_len);
 }