Bug 1003471: Python 1.5.2 security vulnerability still present in 2.3.4

That's the title of the report, but the hole was probably plugged since
Python 2.0.  See corresponding checkin to PC/getpathp.c:  a crucial
precondition for joinpath() was neither documented nor verified, and there
are so many callers with so many conditional paths that no "eyeball
analysis" is satisfactory.  Now Python dies with a fatal error if the
precondition isn't satisfied, instead of allowing a buffer overrun.

NOT TESTED!  The Windows version of the patch was, but not this one.  I
don't feel like waiting for someone to notice the patch I attached to the
bug report.  If it doesn't compile, sorry, but fix it <wink>.  If it
does compile, it's "obviously correct".

Modules/getpath.c

@@ -190,10 +190,14 @@ isdir(char *filename)                   /* Is directory */
 }
 
 
-/* joinpath requires that any buffer argument passed to it has at
-   least MAXPATHLEN + 1 bytes allocated.  If this requirement is met,
-   it guarantees that it will never overflow the buffer.  If stuff
-   is too long, buffer will contain a truncated copy of stuff.
+/* Add a path component, by appending stuff to buffer.
+   buffer must have at least MAXPATHLEN + 1 bytes allocated, and contain a
+   NUL-terminated string with no more than MAXPATHLEN characters (not counting
+   the trailing NUL).  It's a fatal error if it contains a string longer than
+   that (callers must be careful!).  If these requirements are met, it's
+   guaranteed that buffer will still be a NUL-terminated string with no more
+   than MAXPATHLEN characters at exit.  If stuff is too long, only as much of
+   stuff as fits will be appended.
 */
 static void
 joinpath(char *buffer, char *stuff)
@@ -206,6 +210,8 @@ joinpath(char *buffer, char *stuff)
         if (n > 0 && buffer[n-1] != SEP && n < MAXPATHLEN)
             buffer[n++] = SEP;
     }
+    if (n > MAXPATHLEN)
+    	Py_FatalError("buffer overflow in getpath.c's joinpath()");
     k = strlen(stuff);
     if (n + k > MAXPATHLEN)
         k = MAXPATHLEN - n;