From: Benjamin Peterson <benjamin@python.org>
Date: Tue, 5 Sep 2017 05:23:42 +0000 (-0700)
Subject: bpo-31347: _PyObject_FastCall_Prepend: do not call memcpy if args might not be null... 
X-Git-Tag: v3.7.0a1~155
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=a3070d530c70477273cacbc61660b318582fff44;p=python

bpo-31347: _PyObject_FastCall_Prepend: do not call memcpy if args might not be null (#3329)

Passing NULL as the second argument to to memcpy is undefined behavior even if the size is 0.
---

diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-09-04-16-35-06.bpo-31347.KDuf2w.rst b/Misc/NEWS.d/next/Core and Builtins/2017-09-04-16-35-06.bpo-31347.KDuf2w.rst
new file mode 100644
index 0000000000..52a6168e63
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2017-09-04-16-35-06.bpo-31347.KDuf2w.rst	
@@ -0,0 +1 @@
+Fix possible undefined behavior in _PyObject_FastCall_Prepend.
diff --git a/Objects/call.c b/Objects/call.c
index 4294a9beb0..92464327fb 100644
--- a/Objects/call.c
+++ b/Objects/call.c
@@ -854,9 +854,9 @@ _PyObject_FastCall_Prepend(PyObject *callable,
 
     /* use borrowed references */
     args2[0] = obj;
-    memcpy(&args2[1],
-           args,
-           (nargs - 1)* sizeof(PyObject *));
+    if (nargs > 1) {
+        memcpy(&args2[1], args, (nargs - 1) * sizeof(PyObject *));
+    }
 
     result = _PyObject_FastCall(callable, args2, nargs);
     if (args2 != small_stack) {