gk: fix checksum issue

The checksum bug is because we have assumed that the two's complement
(i.e. C's operator ~) subtraction, which is currently being used, is the same as
"subtracting complements with borrow" under one's complement as required in
RFC1624. While these two operations often come up with the same result, they are
often not equal too. In addition, the two's complement subtraction is not
endianness preserving!

To solve this issue, we followed the example implmentation of RFC1624 [Eqn. 3]
in Linux kernel:
https://elixir.bootlin.com/linux/latest/source/net/ipv4/netfilter/ipt_ECN.c#L38

gk/bpf.c

@@ -151,6 +151,34 @@ pkt_ctx_to_pkt(struct gk_bpf_pkt_ctx *ctx)
 	return frame->packet->pkt;
 }
 
+/*
+ * One's complement sum.
+ *
+ * Notice that if @a and @b are little-endian, the result is also
+ * little-endian. The same is true for big-endian. In order words,
+ * this function preserves endianness.
+ *
+ * The endianness preservation is independent of the endianness of the host.
+ */
+static inline uint16_t
+onec_add(uint16_t a, uint16_t b)
+{
+	uint16_t res = a + b;
+	return res + (res < b);
+}
+
+/*
+ * The result has the same endianness of the inputs as long as
+ * all inputs have the same endianness.
+ * The endianness preservation is independent of the endianness of the host.
+ */
+static inline uint16_t
+new_ip_csum(uint16_t old_csum, uint16_t old16, uint16_t new16)
+{
+	/* According to RFC1624 [Eqn. 3]. */
+	return ~onec_add(onec_add(~old_csum, ~old16), new16);
+}
+
 static int
 update_pkt_priority(struct ipacket *packet, int priority,
 	struct gatekeeper_if *iface)
@@ -167,19 +195,19 @@ update_pkt_priority(struct ipacket *packet, int priority,
 
 	if (packet->flow.proto == RTE_ETHER_TYPE_IPV4) {
 		struct rte_ipv4_hdr *ip4hdr = packet->l3_hdr;
-		uint16_t old_val = ntohs(*(uint16_t *)ip4hdr);
+		uint16_t old_val = *(uint16_t *)ip4hdr;
 		uint16_t new_val;
 
 		mask = (1 << 2) - 1;
 
 		ip4hdr->type_of_service = (priority << 2) |
 			(ip4hdr->type_of_service & mask);
 
-		new_val = ntohs(*(uint16_t *)ip4hdr);
+		new_val = *(uint16_t *)ip4hdr;
 
 		/* According to RFC1624 [Eqn. 4]. */
-		ip4hdr->hdr_checksum = htons(ntohs(ip4hdr->hdr_checksum) -
-			~old_val - new_val);
+		ip4hdr->hdr_checksum = new_ip_csum(ip4hdr->hdr_checksum,
+			old_val, new_val);
 	} else if (likely(packet->flow.proto == RTE_ETHER_TYPE_IPV6)) {
 		struct rte_ipv6_hdr *ip6hdr = packet->l3_hdr;