gh-129069: Fix listobject.c data races due to memmove

The use of memmove and _Py_memory_repeat were not thread-safe in the
free threading build in some cases. In theory, memmove and
_Py_memory_repeat can copy byte-by-byte instead of pointer-by-pointer,
so concurrent readers could see uninitialized data or tearing.

Additionally, we should be using "release" (or stronger) ordering to be
compliant with the C11 memory model when copying objects within a list.

Author: colesbury

Objects/listobject.c

@@ -96,11 +96,7 @@ ensure_shared_on_resize(PyListObject *self)
  * of the new slots at exit is undefined heap trash; it's the caller's
  * responsibility to overwrite them with sane values.
  * The number of allocated elements may grow, shrink, or stay the same.
- * Failure is impossible if newsize <= self.allocated on entry, although
- * that partly relies on an assumption that the system realloc() never
- * fails when passed a number of bytes <= the number of bytes last
- * allocated (the C standard doesn't guarantee this, but it's hard to
- * imagine a realloc implementation where it wouldn't be true).
+ * Failure is impossible if newsize <= self.allocated on entry.
  * Note that self->ob_item may change, and even if newsize is less
  * than ob_size on entry.
  */
@@ -145,6 +141,11 @@ list_resize(PyListObject *self, Py_ssize_t newsize)
 #ifdef Py_GIL_DISABLED
     _PyListArray *array = list_allocate_array(new_allocated);
     if (array == NULL) {
+        if (newsize < allocated) {
+            // Never fail when shrinking allocations
+            Py_SET_SIZE(self, newsize);
+            return 0;
+        }
         PyErr_NoMemory();
         return -1;
     }
@@ -178,6 +179,11 @@ list_resize(PyListObject *self, Py_ssize_t newsize)
         items = NULL;
     }
     if (items == NULL) {
+        if (newsize < allocated) {
+            // Never fail when shrinking allocations
+            Py_SET_SIZE(self, newsize);
+            return 0;
+        }
         PyErr_NoMemory();
         return -1;
     }
@@ -818,8 +824,8 @@ list_repeat_lock_held(PyListObject *a, Py_ssize_t n)
             _Py_RefcntAdd(*src, n);
             *dest++ = *src++;
         }
-        // TODO: _Py_memory_repeat calls are not safe for shared lists in
-        // GIL_DISABLED builds. (See issue #129069)
+        // This list is not yet visible to other threads, so atomic repeat
+        // is not necessary even in Py_GIL_DISABLED builds.
         _Py_memory_repeat((char *)np->ob_item, sizeof(PyObject *)*output_size,
                                         sizeof(PyObject *)*input_size);
     }
@@ -882,6 +888,28 @@ list_clear_slot(PyObject *self)
     return 0;
 }
 
+// Pointer-by-pointer memmove for PyObject** arrays that is safe
+// for shared lists in Py_GIL_DISABLED builds.
+static void
+list_atomic_memmove(PyListObject *a, PyObject **dest, PyObject **src, Py_ssize_t n)
+{
+#ifndef Py_GIL_DISABLED
+    memmove(dest, src, n * sizeof(PyObject *));
+#else
+    if (dest < src) {
+        for (Py_ssize_t i = 0; i != n; i++) {
+            _Py_atomic_store_ptr_release(&dest[i], src[i]);
+        }
+    }
+    else {
+        // copy backwards to avoid overwriting src before it's read
+        for (Py_ssize_t i = n; i != 0; i--) {
+            _Py_atomic_store_ptr_release(&dest[i - 1], src[i - 1]);
+        }
+    }
+#endif
+}
+
 /* a[ilow:ihigh] = v if v != NULL.
  * del a[ilow:ihigh] if v == NULL.
  *
@@ -952,27 +980,17 @@ list_ass_slice_lock_held(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh, PyO
     }
 
     if (d < 0) { /* Delete -d items */
-        Py_ssize_t tail;
-        tail = (Py_SIZE(a) - ihigh) * sizeof(PyObject *);
-        // TODO: these memmove/memcpy calls are not safe for shared lists in
-        // GIL_DISABLED builds. (See issue #129069)
-        memmove(&item[ihigh+d], &item[ihigh], tail);
-        if (list_resize(a, Py_SIZE(a) + d) < 0) {
-            memmove(&item[ihigh], &item[ihigh+d], tail);
-            memcpy(&item[ilow], recycle, s);
-            goto Error;
-        }
+        Py_ssize_t tail = Py_SIZE(a) - ihigh;
+        list_atomic_memmove(a, &item[ihigh+d], &item[ihigh], tail);
+        (void)list_resize(a, Py_SIZE(a) + d); // NB: shrinking a list can't fail
         item = a->ob_item;
     }
     else if (d > 0) { /* Insert d items */
         k = Py_SIZE(a);
         if (list_resize(a, k+d) < 0)
             goto Error;
         item = a->ob_item;
-        // TODO: these memmove/memcpy calls are not safe for shared lists in
-        // GIL_DISABLED builds. (See issue #129069)
-        memmove(&item[ihigh+d], &item[ihigh],
-            (k - ihigh)*sizeof(PyObject *));
+        list_atomic_memmove(a, &item[ihigh+d], &item[ihigh], k - ihigh);
     }
     for (k = 0; k < n; k++, ilow++) {
         PyObject *w = vitem[k];
@@ -1056,10 +1074,25 @@ list_inplace_repeat_lock_held(PyListObject *self, Py_ssize_t n)
     for (Py_ssize_t j = 0; j < input_size; j++) {
         _Py_RefcntAdd(items[j], n-1);
     }
-    // TODO: _Py_memory_repeat calls are not safe for shared lists in
-    // GIL_DISABLED builds. (See issue #129069)
+#ifndef Py_GIL_DISABLED
     _Py_memory_repeat((char *)items, sizeof(PyObject *)*output_size,
                       sizeof(PyObject *)*input_size);
+#else
+    if (_Py_IsOwnedByCurrentThread((PyObject *)self) && !_PyObject_GC_IS_SHARED(self)) {
+        // No other threads can read this list concurrently, so atomic repeat is not necessary.
+        _Py_memory_repeat((char *)items, sizeof(PyObject *)*output_size,
+                          sizeof(PyObject *)*input_size);
+        return 0;
+    }
+
+    PyObject **src = items;
+    PyObject **dst = items + input_size;
+    Py_ssize_t remaining = output_size - input_size;
+    while (remaining > 0) {
+        _Py_atomic_store_ptr_release(dst++, *src++);
+        remaining--;
+    }
+#endif
     return 0;
 }
 
@@ -1532,7 +1565,6 @@ list_pop_impl(PyListObject *self, Py_ssize_t index)
 /*[clinic end generated code: output=6bd69dcb3f17eca8 input=c269141068ae4b8f]*/
 {
     PyObject *v;
-    int status;
 
     if (Py_SIZE(self) == 0) {
         /* Special-case most common failure cause */
@@ -1548,27 +1580,18 @@ list_pop_impl(PyListObject *self, Py_ssize_t index)
 
     PyObject **items = self->ob_item;
     v = items[index];
-    const Py_ssize_t size_after_pop = Py_SIZE(self) - 1;
-    if (size_after_pop == 0) {
+    if (Py_SIZE(self) == 1) {
         Py_INCREF(v);
         list_clear(self);
-        status = 0;
-    }
-    else {
-        if ((size_after_pop - index) > 0) {
-            memmove(&items[index], &items[index+1], (size_after_pop - index) * sizeof(PyObject *));
-        }
-        status = list_resize(self, size_after_pop);
+        return v;
     }
-    if (status >= 0) {
-        return v; // and v now owns the reference the list had
-    }
-    else {
-        // list resize failed, need to restore
-        memmove(&items[index+1], &items[index], (size_after_pop - index)* sizeof(PyObject *));
-        items[index] = v;
-        return NULL;
+    Py_ssize_t size_after_pop = Py_SIZE(self) - 1;
+    if (index < size_after_pop) {
+        list_atomic_memmove(self, &items[index], &items[index+1],
+                            size_after_pop - index);
     }
+    list_resize(self, size_after_pop);  // NB: shrinking a list can't fail
+    return v;
 }
 
 /* Reverse a slice of a list in place, from lo up to (exclusive) hi. */

Tools/tsan/suppressions_free_threading.txt

@@ -23,12 +23,6 @@ race_top:write_thread_id
 # https://gist.github.com/mpage/6962e8870606cfc960e159b407a0cb40
 thread:pthread_create
 
-# List resizing happens through different paths ending in memcpy or memmove
-# (for efficiency), which will probably need to rewritten as explicit loops
-# of ptr-sized copies to be thread-safe. (Issue #129069)
-race:list_ass_slice_lock_held
-race:list_inplace_repeat_lock_held
-
 # PyObject_Realloc internally does memcpy which isn't atomic so can race
 # with non-locking reads. See #132070
 race:PyObject_Realloc