fuzz: add a fuzz target for common/bigsize

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

tests/fuzz/fuzz-bigsize.c

@@ -0,0 +1,36 @@
+#include <assert.h>
+#include <stdint.h>
+#include <string.h>
+#include <tests/fuzz/libfuzz.h>
+
+#include <ccan/tal/tal.h>
+#include <common/bigsize.h>
+
+void init(int *argc, char ***argv)
+{
+}
+
+void run(const uint8_t *data, size_t size)
+{
+	uint8_t *wire_buff, buff[BIGSIZE_MAX_LEN];
+	const uint8_t **wire_chunks, *wire_ptr;
+	size_t wire_max;
+
+	wire_chunks = get_chunks(NULL, data, size, 8);
+	for (size_t i = 0; i < tal_count(wire_chunks); i++) {
+		wire_max = tal_count(wire_chunks[i]);
+		wire_ptr = wire_chunks[i];
+
+		bigsize_t bs = fromwire_bigsize(&wire_ptr, &wire_max);
+		if (bs != 0) {
+			/* We have a valid bigsize type, now we should not error. */
+			assert(bigsize_put(buff, bs) > 0);
+			assert(bigsize_len(bs));
+
+			wire_buff = tal_arr(NULL, uint8_t, 8);
+			towire_bigsize(&wire_buff, bs);
+			tal_free(wire_buff);
+		}
+	}
+	tal_free(wire_chunks);
+}

tests/fuzz/libfuzz.c

@@ -1,5 +1,9 @@
 #include <tests/fuzz/libfuzz.h>
 
+#include <ccan/tal/tal.h>
+#include <common/utils.h>
+#include <string.h>
+
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
 int LLVMFuzzerInitialize(int *argc, char ***argv);
 
@@ -14,3 +18,16 @@ int LLVMFuzzerInitialize(int *argc, char ***argv) {
 
 	return 0;
 }
+
+const uint8_t **get_chunks(const void *ctx, const uint8_t *data,
+			  size_t data_size, size_t chunk_size)
+{
+	size_t n_chunks = data_size / chunk_size;
+	const uint8_t **chunks = tal_arr(ctx, const uint8_t *, n_chunks);
+
+	for (size_t i = 0; i < n_chunks; i++)
+		chunks[i] = tal_dup_arr(chunks, const uint8_t,
+					data + i * chunk_size, chunk_size, 0);
+
+	return chunks;
+}