ctypes: Correctly handle NULL dlsym values

For dlsym(), a return value of NULL does not necessarily indicate
an error [1].

Therefore, to avoid using stale (or NULL) dlerror() values, we must:

 1. clear the previous error state by calling dlerror()
 2. call dlsym()
 3. call dlerror()

If the return value of dlerror() is not NULL, an error occured.

In our case, we also subjectively treat a NULL return value
from dlsym() as an error, so we format a special string for that.

[1]: https://man7.org/linux/man-pages/man3/dlsym.3.html

Signed-off-by: Georgios Alexopoulos <grgalex42@gmail.com>

Author: grgalex

Modules/_ctypes/_ctypes.c

@@ -934,6 +934,7 @@ CDataType_in_dll_impl(PyObject *type, PyTypeObject *cls, PyObject *dll,
     PyObject *obj;
     void *handle;
     void *address;
+    char *dlerr;
 
     if (PySys_Audit("ctypes.dlsym", "Os", dll, name) < 0) {
         return NULL;
@@ -967,18 +968,29 @@ CDataType_in_dll_impl(PyObject *type, PyTypeObject *cls, PyObject *dll,
         return NULL;
     }
 #else
+    dlerror();
     address = (void *)dlsym(handle, name);
-    if (!address) {
 #ifdef __CYGWIN__
-/* dlerror() isn't very helpful on cygwin */
+    if (!address) {
+        /* dlerror() isn't very helpful on cygwin */
         PyErr_Format(PyExc_ValueError,
                      "symbol '%s' not found",
                      name);
+        return NULL;
+    }
 #else
-        PyErr_SetString(PyExc_ValueError, dlerror());
-#endif
+    dlerr = dlerror();
+    if (dlerr) {
+        PyErr_SetString(PyExc_ValueError, dlerr);
+        return NULL;
+    }
+    else if (!address) {
+        PyErr_Format(PyExc_ValueError,
+                     "symbol '%s' not found",
+                     name);
         return NULL;
     }
+#endif
 #endif
     ctypes_state *st = get_module_state_by_def(Py_TYPE(type));
     return PyCData_AtAddress(st, type, address);
@@ -3706,6 +3718,7 @@ PyCFuncPtr_FromDll(PyTypeObject *type, PyObject *args, PyObject *kwds)
     PyCFuncPtrObject *self;
     void *handle;
     PyObject *paramflags = NULL;
+    char *dlerr;
 
     if (!PyArg_ParseTuple(args, "O|O", &ftuple, &paramflags))
         return NULL;
@@ -3774,19 +3787,32 @@ PyCFuncPtr_FromDll(PyTypeObject *type, PyObject *args, PyObject *kwds)
         return NULL;
     }
 #else
+    dlerror();
     address = (PPROC)dlsym(handle, name);
-    if (!address) {
 #ifdef __CYGWIN__
-/* dlerror() isn't very helpful on cygwin */
+    if (!address) {
+        /* dlerror() isn't very helpful on cygwin */
         PyErr_Format(PyExc_AttributeError,
                      "function '%s' not found",
                      name);
+        Py_DECREF(ftuple);
+        return NULL;
+    }
 #else
-        PyErr_SetString(PyExc_AttributeError, dlerror());
-#endif
+    dlerr = dlerror();
+    if (dlerr) {
+        PyErr_SetString(PyExc_AttributeError, dlerr);
+        Py_DECREF(ftuple);
+        return NULL;
+    }
+    else if (!address) {
+        PyErr_Format(PyExc_AttributeError,
+                     "function '%s' not found",
+                     name);
         Py_DECREF(ftuple);
         return NULL;
     }
+#endif
 #endif
     ctypes_state *st = get_module_state_by_def(Py_TYPE(type));
     if (!_validate_paramflags(st, type, paramflags)) {