diff -r c8212fca8747 Modules/_io/fileio.c
--- a/Modules/_io/fileio.c	Wed Jun 05 02:07:46 2013 +0200
+++ b/Modules/_io/fileio.c	Thu Jun 06 22:06:13 2013 +0200
@@ -833,27 +833,39 @@ portable_lseek(int fd, PyObject *posobj,
 #else
         pos = PyLong_AsLong(posobj);
 #endif
-        if (PyErr_Occurred())
-            return NULL;
+        if (PyErr_Occurred()) {
+            if (!PyErr_ExceptionMatches(PyExc_OverflowError))
+                return NULL;
+            PyErr_Clear();
+#if defined(HAVE_LARGEFILE_SUPPORT)
+            pos = PyLong_AsUnsignedLongLong(posobj);
+#else
+            pos = PyLong_AsUnsignedLong(posobj);
+#endif
+            if (PyErr_Occurred())
+                return NULL;
+        }
     }
 
-    if (_PyVerify_fd(fd)) {
-        Py_BEGIN_ALLOW_THREADS
+    if (!_PyVerify_fd(fd)) {
+        return PyErr_SetFromErrno(PyExc_IOError);
+    }
+
+    errno = 0;
+    Py_BEGIN_ALLOW_THREADS
 #if defined(MS_WIN64) || defined(MS_WINDOWS)
-        res = _lseeki64(fd, pos, whence);
+    res = _lseeki64(fd, pos, whence);
 #else
-        res = lseek(fd, pos, whence);
+    res = lseek(fd, pos, whence);
 #endif
-        Py_END_ALLOW_THREADS
-    } else
-        res = -1;
-    if (res < 0)
+    Py_END_ALLOW_THREADS
+    if (res == (off_t)-1 && errno)
         return PyErr_SetFromErrno(PyExc_IOError);
 
 #if defined(HAVE_LARGEFILE_SUPPORT)
-    return PyLong_FromLongLong(res);
+    return PyLong_FromUnsignedLongLong(res);
 #else
-    return PyLong_FromLong(res);
+    return PyLong_FromUnsignedLong(res);
 #endif
 }
 
diff -r c8212fca8747 Modules/posixmodule.c
--- a/Modules/posixmodule.c	Wed Jun 05 02:07:46 2013 +0200
+++ b/Modules/posixmodule.c	Thu Jun 06 22:06:13 2013 +0200
@@ -7438,6 +7438,7 @@ posix_lseek(PyObject *self, PyObject *ar
     off_t pos, res;
 #endif
     PyObject *posobj;
+
     if (!PyArg_ParseTuple(args, "iOi:lseek", &fd, &posobj, &how))
         return NULL;
 #ifdef SEEK_SET
@@ -7449,16 +7450,28 @@ posix_lseek(PyObject *self, PyObject *ar
     }
 #endif /* SEEK_END */
 
-#if !defined(HAVE_LARGEFILE_SUPPORT)
+#if defined(HAVE_LARGEFILE_SUPPORT)
+    pos = PyLong_AsLongLong(posobj);
+#else
     pos = PyLong_AsLong(posobj);
-#else
-    pos = PyLong_AsLongLong(posobj);
-#endif
-    if (PyErr_Occurred())
-        return NULL;
+#endif
+    if (PyErr_Occurred()) {
+        if (!PyErr_ExceptionMatches(PyExc_OverflowError))
+            return NULL;
+        PyErr_Clear();
+#if defined(HAVE_LARGEFILE_SUPPORT)
+        pos = PyLong_AsUnsignedLongLong(posobj);
+#else
+        pos = PyLong_AsUnsignedLong(posobj);
+#endif
+        if (PyErr_Occurred())
+            return NULL;
+    }
 
     if (!_PyVerify_fd(fd))
         return posix_error();
+
+    errno = 0;
     Py_BEGIN_ALLOW_THREADS
 #if defined(MS_WIN64) || defined(MS_WINDOWS)
     res = _lseeki64(fd, pos, how);
@@ -7466,13 +7479,14 @@ posix_lseek(PyObject *self, PyObject *ar
     res = lseek(fd, pos, how);
 #endif
     Py_END_ALLOW_THREADS
-    if (res < 0)
+
+    if (res == (off_t)-1 && errno)
         return posix_error();
 
 #if !defined(HAVE_LARGEFILE_SUPPORT)
-    return PyLong_FromLong(res);
-#else
-    return PyLong_FromLongLong(res);
+    return PyLong_FromUnsignedLong(res);
+#else
+    return PyLong_FromUnsignedLongLong(res);
 #endif
 }