The length of a path is not always correctly handled in posixmodule.c, the worst case is posix__getvolumepathname() which pass a length two times smaller than the buffer (GetVolumePathNameW() expects a number of WCHAR characters, not a number of bytes).

MAX_PATH includes the final null character, so no need to write MAX_PATH+1, especially when the function supports path longer than MAX_PATH-1 wide characters by allocating a larget buffer.

http://msdn.microsoft.com/en-us/library/aa365247%28VS.85%29.aspx
"
Maximum Path Length Limitation

In the Windows API (with some exceptions discussed in the following paragraphs), the maximum length for a path is MAX_PATH, which is defined as 260 characters. A local path is structured in the following order: drive letter, colon, backslash, name components separated by backslashes, and a terminating null character. For example, the maximum path on drive D is "D:\some 256-character path string<NUL>" where "<NUL>" represents the invisible terminating null character for the current system codepage. (The characters < > are used here for visual clarity and cannot be part of a valid path string.)"

Attached patch fixes usage of MAX_PATH in posixmodule.c.

posix__getfullpathname():

-    char outbuf[MAX_PATH*2];
+    char outbuf[MAX_PATH];
...
-        wchar_t woutbuf[MAX_PATH*2], *woutbufp = woutbuf;
+        wchar_t woutbuf[MAX_PATH], *woutbufp = woutbuf;

I don't know what "*2" was used here, probably a mistake with sizeof(wchar_t)=2? (try to allocate bytes, where the length is a number of wide characters)


posix__getvolumepathname() is new in Python 3.4, so not need to backport its fix.


path_converter():

     length = PyBytes_GET_SIZE(bytes);
 #ifdef MS_WINDOWS
-    if (length > MAX_PATH) {
+    if (length > MAX_PATH-1) {
         FORMAT_EXCEPTION(PyExc_ValueError, "%s too long for Windows");
         Py_DECREF(bytes);
         return 0;

I don't know if this fix should be backported to Python 3.3.

Oh, I forgot to mention that my initial concern was the following warning:

  ..\Modules\posixmodule.c(4057): warning C4267: 'function' : conversion from 'size_t' to 'DWORD', possible loss of data [C:\buildbot.python.org\3.x.kloth-win64\build\PCbuild\pythoncore.vcxproj]

bufsize (Py_ssize_t) is passed to GetVolumePathNameW() which expected a DWORD. An OverflowError must be raised if bufsize is larger than DWORD_MAX (the constant is defined in Modules/_winapi.c, not in posixmodule.c).

But I prefer to fix the warning after this issue.

Fine with your volumepathname patch. The core of the code was contributed and I didn't check it too carefully as it clearly worked. (For some definition of "worked").

New changeset b3eb42a657a3 by Victor Stinner in branch 'default':
Issue #19636: Fix posix__getvolumepathname(), raise an OverflowError if
http://hg.python.org/cpython/rev/b3eb42a657a3

New changeset 46aecfc5e374 by Victor Stinner in branch 'default':
Issue #19636: Fix usage of MAX_PATH in posixmodule.c
http://hg.python.org/cpython/rev/46aecfc5e374