fuzz: add handshake act 2 target

The fuzz target uses static keys with a fuzzer-generated Act 2 packet.

tests/fuzz/fuzz-connectd-handshake-act2.c

@@ -0,0 +1,68 @@
+/* This is a fuzz test for Act 2 of the BOLT 8 handshake. We intercept io_read()
+ * to inject the fuzzer-generated Act 2 packet in the handshake.
+ *
+ * The expected sequence of events for this test is:
+ *   1. initiator calls io_write() with an Act 1 packet
+ *     - we discard the valid Act 1 packet
+ *   2. initiator calls io_read() for the Act 2 packet
+ *     - we inject the fuzzer-generated packet
+ *   3. initiator fails to validate the packet
+ */
+#include "config.h"
+#include <assert.h>
+#include <ccan/io/io.h>
+#include <ccan/mem/mem.h>
+#include <common/utils.h>
+#include <tests/fuzz/connectd_handshake.h>
+#include <tests/fuzz/libfuzz.h>
+
+/* The io_write() interceptor.
+ *
+ * This should be called exactly once, when the initiator is writing out its Act
+ * 1 packet. We check that the packet is initialized and discard it. */
+static struct io_plan *
+test_write(struct io_conn *conn, const void *data, size_t len,
+	   struct io_plan *(*next)(struct io_conn *, struct handshake *),
+	   struct handshake *h)
+{
+	++write_count;
+	assert(write_count == 1 && "too many calls to io_write()");
+
+	assert(len == ACT_ONE_SIZE);
+	memcheck(data, len);
+
+	return next(conn, h);
+}
+
+/* The io_read() interceptor.
+ *
+ * This should be called exactly once, when the initiator is reading the Act 2
+ * packet. We inject fuzzer input here. */
+static struct io_plan *test_read(struct io_conn *conn, void *data, size_t len,
+				 struct io_plan *(*next)(struct io_conn *,
+							 struct handshake *),
+				 struct handshake *h)
+{
+	++read_count;
+	assert(read_count == 1 && "too many calls to io_read()");
+
+	assert(len == ACT_TWO_SIZE);
+	assert(bytes_remaining >= len);
+	memcpy(data, bytes, len);
+	bytes += len;
+	bytes_remaining -= len;
+
+	return next(conn, h);
+}
+
+void run(const u8 *data, size_t size)
+{
+	if (size < randombytes_SEEDBYTES + ACT_TWO_SIZE)
+		return;
+
+	init_globals(data, size);
+
+	handshake(INITIATOR);
+
+	clean_tmpctx();
+}