Revert "[3.15] gh-158583: Fix uninitialized memory read in bytes.from…

…hex() (GH-158584) (#158691)" This reverts commit 4f7af46.
pablogsal · Oct 5, 2026 · Oct 5, 2026 · Oct 5, 2026 · Oct 5, 2026 · Oct 5, 2026
commit 30df3946e19ce6a02e3b80e1662a22538b5212c7
diff --git a/Lib/test/test_bytes.py b/Lib/test/test_bytes.py
@@ -508,15 +508,6 @@ def test_fromhex(self):
                 self.type2test.fromhex(data)
             self.assertIn('at position %s' % pos, str(cm.exception))
 
-        # gh-158583: Check for out of bounds reads (uninitialized bytes).
-        # Create an array from a list to not overallocate.
-        a = array.array('B', list(b'1234  '))  # Py_ISSPACE() loop
-        self.assertEqual(self.type2test.fromhex(a), b'\x12\x34')
-
-        a = array.array('B', list(b'12345'))  # Missing second digit
-        with self.assertRaises(ValueError):
-            self.type2test.fromhex(a)
-
     def test_hex(self):
         self.assertRaises(TypeError, self.type2test.hex)
         self.assertRaises(TypeError, self.type2test.hex, 1)

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-10-03-16-14-26.gh-issue-158583.7EUMvS.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-10-03-16-14-26.gh-issue-158583.7EUMvS.rst
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
@@ -2691,35 +2691,33 @@ _PyBytes_FromHex(PyObject *string, int use_bytearray)
         if (Py_ISSPACE(*str)) {
             do {
                 str++;
-                if (str >= end) {
-                    goto done;
-                }
             } while (Py_ISSPACE(*str));
+            if (str >= end)
+                break;
         }
 
         top = _PyLong_DigitValue[*str];
         if (top >= 16) {
             invalid_char = str - start;
             goto error;
         }
-
         str++;
-        if (str >= end) {
-            invalid_char = -1;
-            goto error;
-        }
 
         bot = _PyLong_DigitValue[*str];
         if (bot >= 16) {
-            invalid_char = str - start;
+            /* Check if we had a second digit */
+            if (str >= end){
+                invalid_char = -1;
+            } else {
+                invalid_char = str - start;
+            }
             goto error;
         }
         str++;
 
         *buf++ = (unsigned char)((top << 4) + bot);
     }
 
-  done:
     if (view.obj != NULL) {
        PyBuffer_Release(&view);
     }