bug#865: 23.0.60; The directory is unsafe today

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Date: Wed, 03 Sep 2008 14:05:43 -0400
> Cc: emacs-pretest-bug@gnu.org, 865@emacsbugs.donarmstrong.com
> 
> > If you really want to make sure no one else can write you have to set up
> > the access rights for that. On w32 that means you should modify the ACL
> > list. The current code is far from that.
> 
> The current code "does just that" using file-modes.
> Now, of course, depending on how file-modes are mapped to&from ACLs,
> this may DTRT or not.

The problem is not the mapping of ACLs into file modes, the problem is
the opposite mapping.  The current Emacs code assumes Posix semantics,
where a file created like this:

  (letf (((default-file-modes) ?\700)) (make-directory dir t))

will not be accessible by anyone else but the user who created it.
But on Windows, this does not do what it does on Posix, because the
Posix rwx model does not map well into ACLs.  If we want this to work
on platforms that support ACLs (including some varieties of GNU/Linux
systems), we need to use a more sophisticated abstraction.