From: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Date: Tue, 10 Dec 2024 15:58:17 +0000 (+0100)
Subject: gh-127563: use `dk_log2_index_bytes=3` in empty dicts (GH-127568)
X-Git-Tag: v3.14.0a3~64
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=9af96f440618304e7cc609c246e1f8c8b2d7a119;p=thirdparty%2FPython%2Fcpython.git

gh-127563: use `dk_log2_index_bytes=3` in empty dicts (GH-127568)

This fixes a UBSan failure (unaligned zero-size memcpy) in `dictobject.c`.
---

diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 1c9f86438dad..05c93a3e4481 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -588,11 +588,14 @@ 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_DICT_IMMORTAL_INITIAL_REFCNT, /* dk_refcnt */
         0, /* dk_log2_size */
-        0, /* dk_log2_index_bytes */
+        3, /* dk_log2_index_bytes */
         DICT_KEYS_UNICODE, /* dk_kind */
 #ifdef Py_GIL_DISABLED
         {0}, /* dk_mutex */