From f0ea126c581fe6d2803ba3a9e83d8be2c5a6961e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?B=C3=A9n=C3=A9dikt=20Tran?=
 <10796600+picnixz@users.noreply.github.com>
Date: Tue, 10 Dec 2024 16:58:17 +0100
Subject: [PATCH] gh-127563: use `dk_log2_index_bytes=3` in empty dicts
 (GH-127568)
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This fixes a UBSan failure (unaligned zero-size memcpy) in `dictobject.c`.
(cherry picked from commit 9af96f440618304e7cc609c246e1f8c8b2d7a119)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
---
 Objects/dictobject.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 4e965314945aca4..7337e290e89c102 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -458,11 +458,14 @@ estimate_log2_keysize(Py_ssize_t n)
 
 /* This immutable, empty PyDictKeysObject is used for PyDict_Clear()
  * (which cannot fail and thus can do no allocation).
+ *
+ * See https://github.com/python/cpython/pull/127568#discussion_r1868070614
+ * for the rationale of using dk_log2_index_bytes=3 instead of 0.
  */
 static PyDictKeysObject empty_keys_struct = {
         _Py_IMMORTAL_REFCNT, /* dk_refcnt */
         0, /* dk_log2_size */
-        0, /* dk_log2_index_bytes */
+        3, /* dk_log2_index_bytes */
         DICT_KEYS_UNICODE, /* dk_kind */
         1, /* dk_version */
         0, /* dk_usable (immutable) */