gh-101408: PyObject_GC_Resize should calculate preheader size. (gh-101741)

1 files changed, 7 insertions, 6 deletions

diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index 1d00fc3..966c1e6 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -2361,16 +2361,17 @@ PyVarObject *
 _PyObject_GC_Resize(PyVarObject *op, Py_ssize_t nitems)
 {
     const size_t basicsize = _PyObject_VAR_SIZE(Py_TYPE(op), nitems);
+    const size_t presize = _PyType_PreHeaderSize(((PyObject *)op)->ob_type);
     _PyObject_ASSERT((PyObject *)op, !_PyObject_GC_IS_TRACKED(op));
-    if (basicsize > (size_t)PY_SSIZE_T_MAX - sizeof(PyGC_Head)) {
+    if (basicsize > (size_t)PY_SSIZE_T_MAX - presize) {
         return (PyVarObject *)PyErr_NoMemory();
     }
-
-    PyGC_Head *g = AS_GC(op);
-    g = (PyGC_Head *)PyObject_Realloc(g,  sizeof(PyGC_Head) + basicsize);
-    if (g == NULL)
+    char *mem = (char *)op - presize;
+    mem = (char *)PyObject_Realloc(mem,  presize + basicsize);
+    if (mem == NULL) {
         return (PyVarObject *)PyErr_NoMemory();
-    op = (PyVarObject *) FROM_GC(g);
+    }
+    op = (PyVarObject *) (mem + presize);
     Py_SET_SIZE(op, nitems);
     return op;
 }