From 87d7315ac57250046372b0d9ae4619ba619c8c87 Mon Sep 17 00:00:00 2001
From: Sergey B Kirpichev <skirpichev@gmail.com>
Date: Thu, 10 Oct 2024 15:42:03 +0300
Subject: [PATCH] gh-125118: don't copy arbitrary values to _Bool in the struct
 module (GH-125169)

memcopy'ing arbitrary values to _Bool variable triggers undefined
behaviour. Avoid this.
We assume that `false` is represented by all zero bytes.

Credits to Alex Gaynor.

Co-authored-by: Sam Gross <colesbury@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Petr Viktorin <encukou@gmail.com>
---
 Lib/test/test_struct.py                                      | 3 +++
 .../Library/2024-10-09-07-09-00.gh-issue-125118.J9rQ1S.rst   | 1 +
 Modules/_struct.c                                            | 5 ++---
 3 files changed, 6 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2024-10-09-07-09-00.gh-issue-125118.J9rQ1S.rst

diff --git a/Lib/test/test_struct.py b/Lib/test/test_struct.py
index e3193c7863f..04ec3ed0837 100644
--- a/Lib/test/test_struct.py
+++ b/Lib/test/test_struct.py
@@ -540,6 +540,9 @@ class StructTest(ComplexesAreIdenticalMixin, unittest.TestCase):
 
         for c in [b'\x01', b'\x7f', b'\xff', b'\x0f', b'\xf0']:
             self.assertTrue(struct.unpack('>?', c)[0])
+            self.assertTrue(struct.unpack('<?', c)[0])
+            self.assertTrue(struct.unpack('=?', c)[0])
+            self.assertTrue(struct.unpack('@?', c)[0])
 
     def test_count_overflow(self):
         hugecount = '{}b'.format(sys.maxsize+1)
diff --git a/Misc/NEWS.d/next/Library/2024-10-09-07-09-00.gh-issue-125118.J9rQ1S.rst b/Misc/NEWS.d/next/Library/2024-10-09-07-09-00.gh-issue-125118.J9rQ1S.rst
new file mode 100644
index 00000000000..5d57cdbbbc2
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-10-09-07-09-00.gh-issue-125118.J9rQ1S.rst
@@ -0,0 +1 @@
+Don't copy arbitrary values to :c:expr:`_Bool` in the :mod:`struct` module.
diff --git a/Modules/_struct.c b/Modules/_struct.c
index 4387c55b7c8..21582b945be 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -497,9 +497,8 @@ nu_ulonglong(_structmodulestate *state, const char *p, const formatdef *f)
 static PyObject *
 nu_bool(_structmodulestate *state, const char *p, const formatdef *f)
 {
-    _Bool x;
-    memcpy(&x, p, sizeof x);
-    return PyBool_FromLong(x != 0);
+    const _Bool bool_false = 0;
+    return PyBool_FromLong(memcmp(p, &bool_false, sizeof(_Bool)));
 }