Modules/_xxtestfuzz/fuzzer.c

/* A fuzz test for CPython.

  The only exposed function is LLVMFuzzerTestOneInput, which is called by
  fuzzers and by the _fuzz module for smoke tests.

  To build exactly one fuzz test, as when running in oss-fuzz etc.,
  build with -D _Py_FUZZ_ONE and -D _Py_FUZZ_<test_name>. e.g. to build
  LLVMFuzzerTestOneInput to only run "fuzz_builtin_float", build this file with
      -D _Py_FUZZ_ONE -D _Py_FUZZ_fuzz_builtin_float.

  See the source code for LLVMFuzzerTestOneInput for details. */

#include <Python.h>
#include <stdlib.h>
#include <inttypes.h>

/*  Fuzz PyFloat_FromString as a proxy for float(str). */
static int fuzz_builtin_float(const char* data, size_t size) {
    PyObject* s = PyBytes_FromStringAndSize(data, size);
    if (s == NULL) return 0;
    PyObject* f = PyFloat_FromString(s);
    if (PyErr_Occurred() && PyErr_ExceptionMatches(PyExc_ValueError)) {
        PyErr_Clear();
    }

    Py_XDECREF(f);
    Py_DECREF(s);
    return 0;
}

/* Fuzz PyLong_FromUnicodeObject as a proxy for int(str). */
static int fuzz_builtin_int(const char* data, size_t size) {
    /* Pick a random valid base. (When the fuzzed function takes extra
       parameters, it's somewhat normal to hash the input to generate those
       parameters. We want to exercise all code paths, so we do so here.) */
    int base = _Py_HashBytes(data, size) % 37;
    if (base == 1) {
        // 1 is the only number between 0 and 36 that is not a valid base.
        base = 0;
    }
    if (base == -1) {
        return 0;  // An error occurred, bail early.
    }
    if (base < 0) {
        base = -base;
    }

    PyObject* s = PyUnicode_FromStringAndSize(data, size);
    if (s == NULL) {
        if (PyErr_ExceptionMatches(PyExc_UnicodeDecodeError)) {
            PyErr_Clear();
        }
        return 0;
    }
    PyObject* l = PyLong_FromUnicodeObject(s, base);
    if (l == NULL && PyErr_ExceptionMatches(PyExc_ValueError)) {
        PyErr_Clear();
    }
    PyErr_Clear();
    Py_XDECREF(l);
    Py_DECREF(s);
    return 0;
}

/* Fuzz PyUnicode_FromStringAndSize as a proxy for unicode(str). */
static int fuzz_builtin_unicode(const char* data, size_t size) {
    PyObject* s = PyUnicode_FromStringAndSize(data, size);
    if (s == NULL && PyErr_ExceptionMatches(PyExc_UnicodeDecodeError)) {
        PyErr_Clear();
    }
    Py_XDECREF(s);
    return 0;
}

/* Run fuzzer and abort on failure. */
static int _run_fuzz(const uint8_t *data, size_t size, int(*fuzzer)(const char* , size_t)) {
    int rv = fuzzer((const char*) data, size);
    if (PyErr_Occurred()) {
        /* Fuzz tests should handle expected errors for themselves.
           This is last-ditch check in case they didn't. */
        PyErr_Print();
        abort();
    }
    /* Someday the return value might mean something, propagate it. */
    return rv;
}

/* CPython generates a lot of leak warnings for whatever reason. */
int __lsan_is_turned_off(void) { return 1; }

/* Fuzz test interface.
   This returns the bitwise or of all fuzz test's return values.

   All fuzz tests must return 0, as all nonzero return codes are reserved for
   future use -- we propagate the return values for that future case.
   (And we bitwise or when running multiple tests to verify that normally we
   only return 0.) */
int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    if (!Py_IsInitialized()) {
        /* LLVMFuzzerTestOneInput is called repeatedly from the same process,
           with no separate initialization phase, sadly, so we need to
           initialize CPython ourselves on the first run. */
        Py_InitializeEx(0);
    }

    int rv = 0;

#define _Py_FUZZ_YES(test_name) (defined(_Py_FUZZ_##test_name) || !defined(_Py_FUZZ_ONE))
#if _Py_FUZZ_YES(fuzz_builtin_float)
    rv |= _run_fuzz(data, size, fuzz_builtin_float);
#endif
#if _Py_FUZZ_YES(fuzz_builtin_int)
    rv |= _run_fuzz(data, size, fuzz_builtin_int);
#endif
#if _Py_FUZZ_YES(fuzz_builtin_unicode)
    rv |= _run_fuzz(data, size, fuzz_builtin_unicode);
#endif
#undef _Py_FUZZ_YES
  return rv;
}