bug#12632: file permissions checking mishandled when setuid

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 23 Oct 2012 12:27:21 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: rgm@gnu.org, 12632@debbugs.gnu.org
> 
> Rather than try to fix these problems, which are on a platform
> I'm not familiar with, I'll change the proposed code to
> do the following.  This should be safe since it is what the
> trunk currently does now (modulo some refactoring) when on
> DOS or Windows platforms.  And it can be tuned for DOS and
> Windows later, as needed.
> 
> /* If FILE is a searchable directory or a symlink to a
>    searchable directory, return true.  Otherwise return
>    false and set errno to an error number.  */
> bool
> file_accessible_directory_p (char const *file)
> {
> #ifdef DOS_NT
>   /* File names may have drive prefixes and "/" is not the only
>      separator, so the POSIXish approach doesn't work in general.
>      Use a straightforward approach instead.  */
>   return file_directory_p (file) && check_executable (file);

Just file_directory_p is enough, the other test adds no useful
information that isn't already gathered by the first test.  (The
executable bit of a directory is invented on DOS_NT platforms, it
doesn't exist on disk.)