Issue #16743: Fix mmap overflow check on 32 bit Windows

2 files changed, 18 insertions, 11 deletions

diff --git a/Lib/test/test_mmap.py b/Lib/test/test_mmap.py
index 28b52a9..4407c5c 100644
--- a/Lib/test/test_mmap.py
+++ b/Lib/test/test_mmap.py
@@ -693,6 +693,13 @@ class LargeMmapTests(unittest.TestCase):
 
     def test_large_filesize(self):
         with self._make_test_file(0x17FFFFFFF, b" ") as f:
+            if sys.maxsize < 0x180000000:
+                # On 32 bit platforms the file is larger than sys.maxsize so
+                # mapping the whole file should fail -- Issue #16743
+                with self.assertRaises(OverflowError):
+                    mmap.mmap(f.fileno(), 0x180000000, access=mmap.ACCESS_READ)
+                with self.assertRaises(ValueError):
+                    mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
             with mmap.mmap(f.fileno(), 0x10000, access=mmap.ACCESS_READ) as m:
                 self.assertEqual(m.size(), 0x180000000)
 
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 33c143e..352dd52 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -1140,7 +1140,6 @@ new_mmap_object(PyTypeObject *type, PyObject *args, PyObject *kwdict)
 #  endif
     if (fd != -1 && fstat(fd, &st) == 0 && S_ISREG(st.st_mode)) {
         if (map_size == 0) {
-            off_t calc_size;
             if (st.st_size == 0) {
                 PyErr_SetString(PyExc_ValueError,
                                 "cannot mmap an empty file");
@@ -1151,13 +1150,12 @@ new_mmap_object(PyTypeObject *type, PyObject *args, PyObject *kwdict)
                                 "mmap offset is greater than file size");
                 return NULL;
             }
-            calc_size = st.st_size - offset;
-            map_size = calc_size;
-            if (map_size != calc_size) {
+            if (st.st_size - offset > PY_SSIZE_T_MAX) {
                 PyErr_SetString(PyExc_ValueError,
                                  "mmap length is too large");
-                 return NULL;
-             }
+                return NULL;
+            }
+            map_size = (Py_ssize_t) (st.st_size - offset);
         } else if (offset + (size_t)map_size > st.st_size) {
             PyErr_SetString(PyExc_ValueError,
                             "mmap length is greater than file size");
@@ -1354,11 +1352,13 @@ new_mmap_object(PyTypeObject *type, PyObject *args, PyObject *kwdict)
                 Py_DECREF(m_obj);
                 return NULL;
             }
-            if (offset - size > PY_SSIZE_T_MAX)
-                /* Map area too large to fit in memory */
-                m_obj->size = (Py_ssize_t) -1;
-            else
-                m_obj->size = (Py_ssize_t) (size - offset);
+            if (size - offset > PY_SSIZE_T_MAX) {
+                PyErr_SetString(PyExc_ValueError,
+                                "mmap length is too large");
+                Py_DECREF(m_obj);
+                return NULL;
+            }
+            m_obj->size = (Py_ssize_t) (size - offset);
         } else {
             m_obj->size = map_size;
             size = offset + map_size;