Index: Lib/test/test_mmap.py
===================================================================
--- Lib/test/test_mmap.py	(revision 70247)
+++ Lib/test/test_mmap.py	(working copy)
@@ -338,6 +338,28 @@
         mf.close()
         f.close()
 
+        # more excessive test
+        data = "0123456789"
+        for dest in range(len(data)):
+            for src in range(len(data)):
+                for count in range(len(data) - max(dest, src)):
+                    expected = list(data)
+                    expected[dest:dest+count] = expected[src:src+count]
+                    expected = "".join(expected)
+                    m = mmap.mmap(-1, len(data))
+                    m[:] = data
+                    m.move(dest, src, count)
+                    self.assertEqual(m[:], expected)
+                    m.close()
+
+        # should not crash
+        m = mmap.mmap(-1, 1)
+        try:
+            m.move(1, 1, -1)
+        except ValueError:
+            pass
+        m.close()
+
     def test_anonymous(self):
         # anonymous mmap.mmap(-1, PAGE)
         m = mmap.mmap(-1, PAGESIZE)
Index: Modules/mmapmodule.c
===================================================================
--- Modules/mmapmodule.c	(revision 70247)
+++ Modules/mmapmodule.c	(working copy)
@@ -612,10 +612,8 @@
 		return NULL;
 	} else {
 		/* bounds check the values */
-		if (/* end of source after end of data?? */
-			((src+count) > self->size)
-			/* dest will fit? */
-			|| (dest+count > self->size)) {
+		unsigned long pos = src > dest ? src : dest;
+		if (self->size >= pos && count > self->size - pos) {
 			PyErr_SetString(PyExc_ValueError,
 					"source or destination out of range");
 			return NULL;