inany: Helper functions for handling addresses which could be IPv4 or IPv6

struct tcp_conn stores an address which could be IPv6 or IPv4 using a
union.  We can do this without an additional tag by encoding IPv4 addresses
as IPv4-mapped IPv6 addresses.

This approach is useful wider than the specific place in tcp_conn, so
expose a new 'union inany_addr' like this from a new inany.h.  Along with
that create a number of helper functions to make working with these "inany"
addresses easier.

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

inany.h

@@ -0,0 +1,68 @@
+/* SPDX-License-Identifier: AGPL-3.0-or-later
+ * Copyright Red Hat
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ *
+ * inany.h - Types and helpers for handling addresses which could be
+ *           IPv6 or IPv4 (encoded as IPv4-mapped IPv6 addresses)
+ */
+
+#include <assert.h>
+
+/** union inany_addr - Represents either an IPv4 or IPv6 address
+ * @a6:			Address as an IPv6 address, may be IPv4-mapped
+ * @v4mapped.zero:	All zero-bits for an IPv4 address
+ * @v4mapped.one:	All one-bits for an IPv4 address
+ * @v4mapped.a4:	If @a6 is an IPv4 mapped address, the IPv4 address
+ *
+ * @v4mapped shouldn't be accessed except via helpers.
+ */
+union inany_addr {
+	struct in6_addr a6;
+	struct {
+		uint8_t zero[10];
+		uint8_t one[2];
+		struct in_addr a4;
+	} v4mapped;
+};
+
+/** inany_v4 - Extract IPv4 address, if present, from IPv[46] address
+ * @addr:	IPv4 or IPv6 address
+ *
+ * Return: IPv4 address if @addr is IPv4, NULL otherwise
+ */
+static inline const struct in_addr *inany_v4(const union inany_addr *addr)
+{
+	if (!IN6_IS_ADDR_V4MAPPED(&addr->a6))
+		return NULL;
+	return &addr->v4mapped.a4;
+}
+
+/** inany_equals - Compare two IPv[46] addresses
+ * @a, @b:	IPv[46] addresses
+ *
+ * Return: true if @a and @b are the same address
+ */
+static inline bool inany_equals(const union inany_addr *a,
+				const union inany_addr *b)
+{
+	return IN6_ARE_ADDR_EQUAL(&a->a6, &b->a6);
+}
+
+/** inany_from_af - Set IPv[46] address from IPv4 or IPv6 address
+ * @aa:		Pointer to store IPv[46] address
+ * @af:		Address family of @addr
+ * @addr:	struct in_addr (IPv4) or struct in6_addr (IPv6)
+ */
+static inline void inany_from_af(union inany_addr *aa, int af, const void *addr)
+{
+	if (af == AF_INET6) {
+		aa->a6 = *((struct in6_addr *)addr);
+	} else if (af == AF_INET) {
+		memset(&aa->v4mapped.zero, 0, sizeof(aa->v4mapped.zero));
+		memset(&aa->v4mapped.one, 0xff, sizeof(aa->v4mapped.one));
+		aa->v4mapped.a4 = *((struct in_addr *)addr);
+	} else {
+		/* Not valid to call with other address families */
+		assert(0);
+	}
+}