Add tests for fuzzing on scalar operation

[YafeiXie1]

Hello, I’m trying to develop the fuzz test using libfuzzer. This PR shows the tests about several properties of scalar operations. Could you give me some suggestions?

[sipa]

I don't think you need time.h.

[sipa]

Instead of including all these individual files, you can just include secp256k1.c (the main implementation file includes all the necessary code from all modules).

When you do that, you'll also need the precomputed tables (which are in separate .c files) when compiling, as the ecmult logic needs them (even when you're not actually testing those yet).

For example, you'd build with

clang++ -g -O2 -fsanitize=undefined,address,fuzzer fuzz.c precomputed_ecmult.c precomputed_ecmult_gen.c -o fuzz

(this is just temporary before we integrate it into the build system)

[sipa]

Here is some code to decide the test based on an environment variable:

typedef void (*fuzz_function)(const uint8_t* data, size_t size);

static fuzz_function selected_fuzz_function = NULL;

int LLVMFuzzerInitialize(int *argc, char ***argv) {
    const char* test_name = getenv("FUZZ");
    if (!test_name) {
        fprintf(stderr, "Select a fuzz test using the FUZZ environment variable\n");
        assert(false);
    }
    if (strcmp(test_name, "scalar_inverse") == 0) {
        selected_fuzz_test = &fuzz_scalar_inverse;
    } else if (strcmp(test_name, "scalar_negate") == 0) {
        selected_fuzz_test = &fuzz_scalar_negate;
    } else if ...
        ...
    } else {
        fprintf(stderr, "Unknown fuzz test selected using FUZZ environment variable: %s\n", test_name);
        assert(false);
    }
    return 0;
}

int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    selected_fuzz_test(data, size);
    return 0;
}

(I haven't tried to compile/run the above, there may be bugs)

You'd then invoke the tests using

FUZZ=scalar_inverse ./fuzz

[sipa]

Nit (= unimportant comment, feel free to ignore if you disagree): I find "cummutate" a bit unnatural, what about fuzz_add_commutativity for example?

[sipa]

Nit: fuzz_add_associativity ?

[sipa]

Nit: fuzz_scalar_mul_zero ?

[sipa]

You cannot use randomness in fuzz tests, because the fuzzer cannot control the randomness (coverage will be unrelated and nondeterministic, which will confuse the fuzzer's tracking of which seeds are useful). You should get the value from the fuzzer data/size here too.

[sipa]

Don't include this; you should never need randomness in fuzz tests.

[sipa]

Typo: splite -> split

[sipa]

Add a newline at the end of the file.