Could this change to PyRun_SimpleStringFlags cause an "Access violation" error?

Vvp · November 19, 2022, 12:36am

I’m currently working on my custom cpython version and recently I’ve started seeing an access violation error when using it. I think I narrowed down my issue and I would like to ask a very specific question without providing extra context.

Consider the following function (I’m using this exact commit):

github.com

python/cpython/blob/8211cf5d287acfd815b6a7f6471cdf83dcd2bb9b/Python/pythonrun.c#L479


      
              if (filename_obj == NULL) {
                  return -1;
              }
              int res = _PyRun_SimpleFileObject(fp, filename_obj, closeit, flags);
              Py_DECREF(filename_obj);
              return res;
          }
          
          

          
int
          PyRun_SimpleStringFlags(const char *command, PyCompilerFlags *flags)
          {
              PyObject *m, *d, *v;
              m = PyImport_AddModule("__main__");
              if (m == NULL)
                  return -1;
              d = PyModule_GetDict(m);
              v = PyRun_StringFlags(command, Py_file_input, d, d, flags);
              if (v == NULL) {
                  PyErr_Print();
                  return -1;

I want to know if this small change could cause an access violation if flags actually turn out to be NULL.

Vvp · November 19, 2022, 12:37am

The error in my python app that is run by this custom cpython build looks like this:

Rosuav · November 19, 2022, 1:24am

Returning 0 (NULL) from functions like this implies that you’ve set an exception. You haven’t here, so on the face of it, I would assume that it’s going to get something confused, but I’d have to test to see exactly what happens.

dstromberg · November 19, 2022, 2:06am

``

Check PyRun_StringFlags to see what it does when passed a NULL pointer.

C is certainly the kind of language where dereferencing a NULL pointer can be a bad thing.

For a taste of the sorts of things a language runtime can do, perhaps skim:
https://stromberg.dnsalias.org/~strombrg/checking-early.html