[3.11] bpo-35191: Fix unexpected integer truncation in socket.setblocking() (GH-10415)

On platforms with 64-bit long, socket.setblocking(x) treated all x
which lower 32 bits are zero as False due to integer truncation.

Reported by ubsan.

Author: Alexey Izbyshev

Lib/test/test_socket.py

@@ -4705,10 +4705,10 @@ def testSetBlocking_overflow(self):
             self.skipTest('needs UINT_MAX < ULONG_MAX')
 
         self.serv.setblocking(False)
-        self.assertEqual(self.serv.gettimeout(), 0.0)
+        self.assert_sock_timeout(self.serv, 0.0)
 
         self.serv.setblocking(_testcapi.UINT_MAX + 1)
-        self.assertIsNone(self.serv.gettimeout())
+        self.assert_sock_timeout(self.serv, None)
 
     _testSetBlocking_overflow = support.cpython_only(_testSetBlocking)
 

Misc/NEWS.d/next/Library/2018-11-08-18-44-04.bpo-35191.s29bWu.rst

@@ -0,0 +1,2 @@
+Fix unexpected integer truncation in :meth:`socket.setblocking` which caused
+it to interpret multiples of ``2**32`` as ``False``.

Modules/socketmodule.c

@@ -2815,12 +2815,14 @@ For IP sockets, the address info is a pair (hostaddr, port).");
 static PyObject *
 sock_setblocking(PySocketSockObject *s, PyObject *arg)
 {
-    long block;
+    long value;
+    int block;
 
-    block = PyLong_AsLong(arg);
-    if (block == -1 && PyErr_Occurred())
+    value = PyLong_AsLong(arg);
+    if (value == -1 && PyErr_Occurred())
         return NULL;
 
+    block = (value != 0);
     s->sock_timeout = _PyTime_FromSeconds(block ? -1 : 0);
     if (internal_setblocking(s, block) == -1) {
         return NULL;