gh-145792: Fix incorrect alloca allocation size in traceback.c

Lib/test/test_traceback.py

@@ -625,6 +625,18 @@ def test_signatures(self):
             str(inspect.signature(traceback.format_exception_only)),
             '(exc, /, value=<implicit>, *, show_group=False, **kwargs)')
 
+    def test_traceback_deep_recursion_alloca(self):
+
+        def recurse(n):
+            if n == 0:
+                raise RuntimeError("boom")
+            return recurse(n - 1)
+        try:
+            recurse(50)
+        except RuntimeError as exc:
+            tb = traceback.format_exception(exc)
+            assert any("RuntimeError" in line for line in tb)
+
 
 class PurePythonExceptionFormattingMixin:
     def get_exception(self, callable, slice_start=0, slice_end=-1):

Misc/NEWS.d/next/Core_and_Builtins/2026-03-11-19-09-47.gh-issue-145792.X5KUhc.rst

@@ -0,0 +1,2 @@
+Fix incorrect memory allocation in the VLA fallback macro in traceback.c
+when using alloca(), preventing potential out-of-bounds access.

Python/traceback.c

@@ -41,7 +41,7 @@
 
 #if defined(__STDC_NO_VLA__) && (__STDC_NO_VLA__ == 1)
 /* Use alloca() for VLAs. */
-#  define VLA(type, name, size) type *name = alloca(size)
+#  define VLA(type, name, size) type *name = (type *)alloca(sizeof(type) * (size))
 #elif !defined(__STDC_NO_VLA__) || (__STDC_NO_VLA__ == 0)
 /* Use actual C VLAs.*/
 #  define VLA(type, name, size) type name[size]