gh-71810: Fix _PyLong_AsByteArray() undefined behavior (#138873)

vstinner · web-flow · commit 8b5ce31c2b44 · 2025-09-14T11:25:53.000+02:00
Don't read p[-1] when p is an empty string: when n==0.
diff --git a/Objects/longobject.c b/Objects/longobject.c
@@ -1214,8 +1214,14 @@ _PyLong_AsByteArray(PyLongObject* v,
            just above didn't get to ensure there's a sign bit, and the
            loop below wouldn't add one either.  Make sure a sign bit
            exists. */
-        unsigned char msb = *(p - pincr);
-        int sign_bit_set = msb >= 0x80;
+        int sign_bit_set;
+        if (n > 0) {
+            unsigned char msb = *(p - pincr);
+            sign_bit_set = msb >= 0x80;
+        }
+        else {
+            sign_bit_set = 0;
+        }
         assert(accumbits == 0);
         if (sign_bit_set == do_twos_comp)
             return 0;
