[3.12] gh-116510: Fix a Crash Due to Shared Immortal Interned Strings (gh-125205)

Fix a crash caused by immortal interned strings being shared between
sub-interpreters that use basic single-phase init. In that case, the string
can be used by an interpreter that outlives the interpreter that created and
interned it. For interpreters that share obmalloc state, also share the
interned dict with the main interpreter.

This is an un-revert of gh-124646 that then addresses the Py_TRACE_REFS
failures identified by gh-124785 (i.e. backporting gh-125709 too).

(cherry picked from commit f2cb399470, AKA gh-124865)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

Doc/library/sys.rst

@@ -907,6 +907,35 @@ always available.
       It is not guaranteed to exist in all implementations of Python.
 
 
+.. function:: getobjects(limit[, type])
+
+   This function only exists if CPython was built using the
+   specialized configure option :option:`--with-trace-refs`.
+   It is intended only for debugging garbage-collection issues.
+
+   Return a list of up to *limit* dynamically allocated Python objects.
+   If *type* is given, only objects of that exact type (not subtypes)
+   are included.
+
+   Objects from the list are not safe to use.
+   Specifically, the result will include objects from all interpreters that
+   share their object allocator state (that is, ones created with
+   :c:member:`PyInterpreterConfig.use_main_obmalloc` set to 1
+   or using :c:func:`Py_NewInterpreter`, and the
+   :ref:`main interpreter <sub-interpreter-support>`).
+   Mixing objects from different interpreters may lead to crashes
+   or other unexpected behavior.
+
+   .. impl-detail::
+
+      This function should be used for specialized purposes only.
+      It is not guaranteed to exist in all implementations of Python.
+
+   .. versionchanged:: next
+
+      The result may include objects from other interpreters.
+
+
 .. function:: getprofile()
 
    .. index::

Doc/using/configure.rst

@@ -459,7 +459,7 @@ Debug options
    Effects:
 
    * Define the ``Py_TRACE_REFS`` macro.
-   * Add :func:`!sys.getobjects` function.
+   * Add :func:`sys.getobjects` function.
    * Add :envvar:`PYTHONDUMPREFS` environment variable.
 
    This build is not ABI compatible with release build (default build) or debug

Doc/whatsnew/3.12.rst

@@ -2301,3 +2301,14 @@ email
   check if the *strict* paramater is available.
   (Contributed by Thomas Dwyer and Victor Stinner for :gh:`102988` to improve
   the CVE-2023-27043 fix.)
+
+
+Notable changes in 3.12.8
+=========================
+
+sys
+---
+
+* The previously undocumented special function :func:`sys.getobjects`,
+  which only exists in specialized builds of Python, may now return objects
+  from other interpreters than the one it's called in.

Include/internal/pycore_object_state.h

@@ -24,7 +24,13 @@ struct _py_object_state {
      * together via the _ob_prev and _ob_next members of a PyObject, which
      * exist only in a Py_TRACE_REFS build.
      */
-    PyObject refchain;
+    PyObject *refchain;
+    /* In most cases, refchain points to _refchain_obj.
+     * In sub-interpreters that share objmalloc state with the main interp,
+     * refchain points to the main interpreter's _refchain_obj, and their own
+     * _refchain_obj is unused.
+     */
+    PyObject _refchain_obj;
 #endif
     int _not_used;
 };

Include/internal/pycore_runtime_init.h

@@ -132,15 +132,8 @@ extern PyTypeObject _PyExc_MemoryError;
         .context_ver = 1, \
     }
 
-#ifdef Py_TRACE_REFS
-# define _py_object_state_INIT(INTERP) \
-    { \
-        .refchain = {&INTERP.object_state.refchain, &INTERP.object_state.refchain}, \
-    }
-#else
 # define _py_object_state_INIT(INTERP) \
     { 0 }
-#endif
 
 
 // global objects

Misc/NEWS.d/next/Core_and_Builtins/2024-09-26-18-21-06.gh-issue-116510.FacUWO.rst

@@ -0,0 +1,5 @@
+Fix a crash caused by immortal interned strings being shared between
+sub-interpreters that use basic single-phase init.  In that case, the string
+can be used by an interpreter that outlives the interpreter that created and
+interned it.  For interpreters that share obmalloc state, also share the
+interned dict with the main interpreter.

Objects/object.c

@@ -159,11 +159,27 @@ _PyDebug_PrintTotalRefs(void) {
 
 #ifdef Py_TRACE_REFS
 
-#define REFCHAIN(interp) &interp->object_state.refchain
+#define REFCHAIN(interp) interp->object_state.refchain
+
+static inline int
+has_own_refchain(PyInterpreterState *interp)
+{
+    if (interp->feature_flags & Py_RTFLAGS_USE_MAIN_OBMALLOC) {
+        return (_Py_IsMainInterpreter(interp)
+            || _PyInterpreterState_Main() == NULL);
+    }
+    return 1;
+}
 
 static inline void
 init_refchain(PyInterpreterState *interp)
 {
+    if (!has_own_refchain(interp)) {
+        // Legacy subinterpreters share a refchain with the main interpreter.
+        REFCHAIN(interp) = REFCHAIN(_PyInterpreterState_Main());
+        return;
+    }
+    REFCHAIN(interp) = &interp->object_state._refchain_obj;
     PyObject *refchain = REFCHAIN(interp);
     refchain->_ob_prev = refchain;
     refchain->_ob_next = refchain;
@@ -2010,9 +2026,7 @@ void
 _PyObject_InitState(PyInterpreterState *interp)
 {
 #ifdef Py_TRACE_REFS
-    if (!_Py_IsMainInterpreter(interp)) {
-        init_refchain(interp);
-    }
+    init_refchain(interp);
 #endif
 }
 

Objects/unicodeobject.c

@@ -287,13 +287,37 @@ hashtable_unicode_compare(const void *key1, const void *key2)
     }
 }
 
+/* Return true if this interpreter should share the main interpreter's
+   intern_dict.  That's important for interpreters which load basic
+   single-phase init extension modules (m_size == -1).  There could be interned
+   immortal strings that are shared between interpreters, due to the
+   PyDict_Update(mdict, m_copy) call in import_find_extension().
+
+   It's not safe to deallocate those strings until all interpreters that
+   potentially use them are freed.  By storing them in the main interpreter, we
+   ensure they get freed after all other interpreters are freed.
+*/
+static bool
+has_shared_intern_dict(PyInterpreterState *interp)
+{
+    PyInterpreterState *main_interp = _PyInterpreterState_Main();
+    return interp != main_interp  && interp->feature_flags & Py_RTFLAGS_USE_MAIN_OBMALLOC;
+}
+
 static int
 init_interned_dict(PyInterpreterState *interp)
 {
     assert(get_interned_dict(interp) == NULL);
-    PyObject *interned = interned = PyDict_New();
-    if (interned == NULL) {
-        return -1;
+    PyObject *interned;
+    if (has_shared_intern_dict(interp)) {
+        interned = get_interned_dict(_PyInterpreterState_Main());
+        Py_INCREF(interned);
+    }
+    else {
+        interned = PyDict_New();
+        if (interned == NULL) {
+            return -1;
+        }
     }
     _Py_INTERP_CACHED_OBJECT(interp, interned_strings) = interned;
     return 0;
@@ -304,7 +328,10 @@ clear_interned_dict(PyInterpreterState *interp)
 {
     PyObject *interned = get_interned_dict(interp);
     if (interned != NULL) {
-        PyDict_Clear(interned);
+        if (!has_shared_intern_dict(interp)) {
+            // only clear if the dict belongs to this interpreter
+            PyDict_Clear(interned);
+        }
         Py_DECREF(interned);
         _Py_INTERP_CACHED_OBJECT(interp, interned_strings) = NULL;
     }
@@ -15152,6 +15179,13 @@ _PyUnicode_ClearInterned(PyInterpreterState *interp)
     }
     assert(PyDict_CheckExact(interned));
 
+    if (has_shared_intern_dict(interp)) {
+        // the dict doesn't belong to this interpreter, skip the debug
+        // checks on it and just clear the pointer to it
+        clear_interned_dict(interp);
+        return;
+    }
+
 #ifdef INTERNED_STATS
     fprintf(stderr, "releasing %zd interned strings\n",
             PyDict_GET_SIZE(interned));
@@ -15670,8 +15704,10 @@ _PyUnicode_Fini(PyInterpreterState *interp)
 {
     struct _Py_unicode_state *state = &interp->unicode;
 
-    // _PyUnicode_ClearInterned() must be called before _PyUnicode_Fini()
-    assert(get_interned_dict(interp) == NULL);
+    if (!has_shared_intern_dict(interp)) {
+        // _PyUnicode_ClearInterned() must be called before _PyUnicode_Fini()
+        assert(get_interned_dict(interp) == NULL);
+    }
 
     _PyUnicode_FiniEncodings(&state->fs_codec);
 

Python/pylifecycle.c

@@ -650,6 +650,10 @@ pycore_create_interpreter(_PyRuntimeState *runtime,
         return status;
     }
 
+    // This could be done in init_interpreter() (in pystate.c) if it
+    // didn't depend on interp->feature_flags being set already.
+    _PyObject_InitState(interp);
+
     PyThreadState *tstate = _PyThreadState_New(interp);
     if (tstate == NULL) {
         return _PyStatus_ERR("can't make first thread");
@@ -2103,6 +2107,10 @@ new_interpreter(PyThreadState **tstate_p, const PyInterpreterConfig *config)
         goto error;
     }
 
+    // This could be done in init_interpreter() (in pystate.c) if it
+    // didn't depend on interp->feature_flags being set already.
+    _PyObject_InitState(interp);
+
     status = init_interp_create_gil(tstate, config->gil);
     if (_PyStatus_EXCEPTION(status)) {
         goto error;

Python/pystate.c

@@ -686,7 +686,9 @@ init_interpreter(PyInterpreterState *interp,
                 _obmalloc_pools_INIT(interp->obmalloc.pools);
         memcpy(&interp->obmalloc.pools.used, temp, sizeof(temp));
     }
-    _PyObject_InitState(interp);
+
+    // We would call _PyObject_InitState() at this point
+    // if interp->feature_flags were alredy set.
 
     _PyEval_InitState(interp, pending_lock);
     _PyGC_InitState(&interp->gc);