Avoid integer overflow when decoding bytes/charptr (GH-3535)

Fixes GH-3534.

Cython/Utility/StringTools.c

@@ -518,9 +518,9 @@ static CYTHON_INLINE PyObject* __Pyx_decode_c_string(
         if (stop < 0)
             stop += length;
     }
-    length = stop - start;
-    if (unlikely(length <= 0))
+    if (unlikely(stop <= start))
         return PyUnicode_FromUnicode(NULL, 0);
+    length = stop - start;
     cstring += start;
     if (decode_func) {
         return decode_func(cstring, length, errors);
@@ -554,9 +554,9 @@ static CYTHON_INLINE PyObject* __Pyx_decode_c_bytes(
     }
     if (stop > length)
         stop = length;
-    length = stop - start;
-    if (unlikely(length <= 0))
+    if (unlikely(stop <= start))
         return PyUnicode_FromUnicode(NULL, 0);
+    length = stop - start;
     cstring += start;
     if (decode_func) {
         return decode_func(cstring, length, errors);

tests/run/bytesmethods.pyx

 cimport cython
 
+cdef extern from *:
+    cdef Py_ssize_t PY_SSIZE_T_MIN
+    cdef Py_ssize_t PY_SSIZE_T_MAX
+
+SSIZE_T_MAX = PY_SSIZE_T_MAX
+SSIZE_T_MIN = PY_SSIZE_T_MIN
+
+
 b_a = b'a'
 b_b = b'b'
 
@@ -114,6 +122,14 @@ def bytes_decode(bytes s, start=None, stop=None):
     <BLANKLINE>
     >>> print(bytes_decode(s, -300, -500))
     <BLANKLINE>
+    >>> print(bytes_decode(s, SSIZE_T_MIN, SSIZE_T_MIN))
+    <BLANKLINE>
+    >>> print(bytes_decode(s, SSIZE_T_MIN, SSIZE_T_MAX))
+    abaab
+    >>> print(bytes_decode(s, SSIZE_T_MAX, SSIZE_T_MIN))
+    <BLANKLINE>
+    >>> print(bytes_decode(s, SSIZE_T_MAX, SSIZE_T_MAX))
+    <BLANKLINE>
 
     >>> s[:'test']                       # doctest: +ELLIPSIS
     Traceback (most recent call last):

tests/run/charptr_decode.pyx

 
 cimport cython
 
+cdef extern from *:
+    cdef Py_ssize_t PY_SSIZE_T_MIN
+    cdef Py_ssize_t PY_SSIZE_T_MAX
+
+
 ############################################################
 # tests for char* slicing
 
@@ -118,6 +123,19 @@ def slice_charptr_dynamic_bounds_non_name():
             (cstring+1)[:].decode('UTF-8'),
             (cstring+1)[return1():return5()].decode('UTF-8'))
 
+@cython.test_assert_path_exists("//PythonCapiCallNode")
+@cython.test_fail_if_path_exists("//AttributeNode")
+def slice_charptr_decode_large_bounds():
+    """
+    >>> print(str(slice_charptr_decode_large_bounds()).replace("u'", "'"))
+    ('abcABCqtp', '', '', '')
+    """
+    return (cstring[PY_SSIZE_T_MIN:9].decode('UTF-8'),
+            cstring[PY_SSIZE_T_MAX:PY_SSIZE_T_MIN].decode('UTF-8'),
+            cstring[PY_SSIZE_T_MIN:PY_SSIZE_T_MIN].decode('UTF-8'),
+            cstring[PY_SSIZE_T_MAX:PY_SSIZE_T_MAX].decode('UTF-8'))
+
+
 cdef return1(): return 1
 cdef return3(): return 3
 cdef return4(): return 4