gh-157384: Refuse inittab names in PyImport_CreateModuleFromInitfunc()

Modules created by PyImport_CreateModuleFromInitfunc() share the
extensions cache and sys.modules with built-in modules.  Passing a
spec whose name is registered in PyImport_Inittab either silently
ignored the init function (for an already loaded single-phase builtin,
or for "sys" and "builtins") or replaced the built-in module in
sys.modules (for a multi-phase builtin).  Raise ImportError instead.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HoZE3WdRWQshvinfBZ137N

Author: itamaro

Doc/c-api/import.rst

@@ -417,6 +417,11 @@ Importing Modules
    (Custom importers should do this in their
    :py:meth:`~importlib.abc.Loader.exec_module` method.)
 
+   If the spec's name is registered in :c:var:`PyImport_Inittab` (which
+   includes the :mod:`sys` and :mod:`builtins` modules), raise
+   :exc:`ImportError`; a module cannot be both a built-in module and one
+   created by this function.
+
    On error, return NULL with an exception set.
 
    .. versionadded:: 3.15

Lib/test/test_embed.py

@@ -278,6 +278,15 @@ def test_create_module_from_initfunc(self):
                          "my_test_extension.exec_slot_ran='yes'\n"
                          "<module 'embedded_ext' (static-extension)>\n"
                          "embedded_ext.executed='yes'\n"
+                         "ImportError: cannot create module 'sys' from an "
+                         "init function: a built-in module with this name "
+                         "is registered in PyImport_Inittab\n"
+                         "ImportError: cannot create module "
+                         "'create_static_module' from an init function: "
+                         "a built-in module with this name is registered "
+                         "in PyImport_Inittab\n"
+                         "sys.modules[\"create_static_module\"] "
+                         "is create_static_module=True\n"
                          )
 
     def test_inittab_submodule_multiphase(self):

Misc/NEWS.d/next/C_API/2026-09-12-18-37-23.gh-issue-157384.aQkHID.rst

@@ -0,0 +1,3 @@
+:c:func:`PyImport_CreateModuleFromInitfunc` now raises :exc:`ImportError` if
+the spec's name is registered in :c:var:`PyImport_Inittab`, instead of
+silently ignoring *initfunc* or replacing the built-in module.

Programs/_testembed.c

@@ -2425,6 +2425,13 @@ create_module(PyObject* self, PyObject* spec)
         Py_DECREF(name);
         return PyImport_CreateModuleFromInitfunc(spec, PyInit_embedded_ext);
     }
+    if (PyUnicode_EqualToUTF8(name, "sys")
+        || PyUnicode_EqualToUTF8(name, "create_static_module")) {
+        // names registered in the inittab (core module / builtin):
+        // must be refused before the init function is called
+        Py_DECREF(name);
+        return PyImport_CreateModuleFromInitfunc(spec, PyInit_embedded_ext);
+    }
     PyErr_Format(PyExc_LookupError, "static module %R not found", name);
     Py_DECREF(name);
     return NULL;
@@ -2472,6 +2479,10 @@ test_create_module_from_initfunc(void)
         L"import embedded_ext;"
         L"print(embedded_ext);"
         L"print(f'{embedded_ext.executed=}');"
+        // Names registered in the inittab are refused
+        L"try_create('sys');"
+        L"try_create('create_static_module');"
+        L"print(f'{sys.modules[\"create_static_module\"] is create_static_module=}');"
     };
     PyConfig config;
     if (PyImport_AppendInittab("create_static_module",
@@ -2502,6 +2513,14 @@ test_create_module_from_initfunc(void)
         "       create_static_module.exec_module(module)\n"
         "       module.executed = 'yes'\n"
         "sys.meta_path.append(StaticExtensionImporter)\n"
+        "def try_create(name):\n"
+        "   spec = spec_from_loader(name, StaticExtensionImporter)\n"
+        "   try:\n"
+        "       create_static_module.create_module(spec)\n"
+        "   except ImportError as exc:\n"
+        "       print(f'ImportError: {exc}')\n"
+        "   else:\n"
+        "       print(f'no ImportError for {name}!')\n"
     );
     if (result < 0) {
         fprintf(stderr, "PyRun_SimpleString() failed\n");

Python/import.c

@@ -2486,10 +2486,10 @@ is_builtin(PyObject *name)
 }
 
 static struct _inittab*
-lookup_inittab_entry(const struct _Py_ext_module_loader_info* info)
+lookup_inittab_entry(PyObject *name)
 {
     for (struct _inittab *p = INITTAB; p->name != NULL; p++) {
-        if (_PyUnicode_EqualToASCIIString(info->name, p->name)) {
+        if (_PyUnicode_EqualToASCIIString(name, p->name)) {
             return p;
         }
     }
@@ -2534,7 +2534,7 @@ create_builtin(
 
     PyModInitFunction p0 = NULL;
     if (initfunc == NULL) {
-        struct _inittab *entry = lookup_inittab_entry(&info);
+        struct _inittab *entry = lookup_inittab_entry(info.name);
         if (entry == NULL) {
             mod = NULL;
             _PyErr_SetModuleNotFoundError(name);
@@ -2601,6 +2601,19 @@ PyImport_CreateModuleFromInitfunc(
         return NULL;
     }
 
+    /* Built-in modules (including the core "sys" and "builtins" modules)
+     * share the extensions cache and sys.modules with modules created here.
+     * Refuse to create a module under a name registered in the inittab,
+     * rather than silently ignoring either initfunc or the other module. */
+    if (lookup_inittab_entry(name) != NULL) {
+        PyErr_Format(PyExc_ImportError,
+                     "cannot create module %R from an init function: "
+                     "a built-in module with this name is registered "
+                     "in PyImport_Inittab", name);
+        Py_DECREF(name);
+        return NULL;
+    }
+
     PyObject *mod = create_builtin(tstate, name, spec, initfunc);
     Py_DECREF(name);
     return mod;