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 <[email protected]>
Co-authored-by: Victor Stinner <[email protected]>
Co-authored-by: Petr Viktorin <[email protected]>

Author: 4 people

Lib/test/test_struct.py

@@ -540,6 +540,9 @@ def __bool__(self):
 
         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)

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.

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)));
 }