Subject: bug#41218: 26.3; Windows: file-writable-p returns t for a file owned by the Administrators group but not writable by the user, a member of the Administrators group

From: "Michael Hoffman" <[email-masked]>
Date: Wed, 13 May 2020 12:05:51 -0400

Thanks for looking into this.

On Wed, May 13, 2020 at 10:25 AM Eli Zaretskii wrote:

> And the fact that one normally needs to jump through hoops (elevating your
> session, using tools that make the file explicitly owned by a privileged
> group, etc.) to create such a situation on Windows is one more reason not
> to bother about this too much. IOW, the situation is rare.
>

The situation arises for me when viewing files within `%ProgramFiles%`
which are often set by installers or similar systems to be owned by
Administrators when installed for all users.

Specifically, it most often comes up when I am using Emacs, as installed by
Chocolatey. Chocolatey installs Emacs to be owned by the Administrators
group.

When I view the definition of a function that is part of the Lisp code that
comes with Emacs `describe-function`, Emacs visits the installed `.el`
file, which I cannot write to. Emacs thinks I can write to it though, which
causes two annoyances:

1. It is easy to make accidental changes to a file that is not writable
(and then I cannot save the file).
2. Flycheck immediately gives me an error message because it tries to open
a file in a place it shouldn't. For example:
Error while checking syntax automatically: (file-error "Opening output
file" "Permission denied" "c:/Program
Files/Emacs/emacs-26.2/share/emacs/26.2/lisp/flycheck_help-fns.el")

Whether installed via Chocolatey or some other manner, I think having the
site Emacs owned by Administrators without direct write access without
elevation is a good practice and keeps one from accidentally changing
things.

> Of course, if someone can explain how to perform this test in a way that
> takes ACLs into account and would be reliable, we could consider
> implementing it (assuming it isn't too expensive, since such a test will
> have to be performed each time a user saves a buffer to its file).
>

The `AuthzAccessCheck()` function from the Authz API might be able to do
this:

https://docs.microsoft.com/en-us/windows/win32/api/authz/nf-authz-authzaccesscheck
https://docs.microsoft.com/en-us/windows/win32/secauthz/how-dacls-control-access-to-an-object

Michael