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Date: Tue, 11 Aug 2009 09:00:46 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
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Subject: Re: Perl bug (was Re: [1.7] cygwin allows writing to readonly 	files)
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On Aug 11 04:49, Reini Urban wrote:
> 2009/8/11 Reini Urban:
> > 2009/8/10 Corinna Vinschen:
> >> On Aug 10 20:11, Alexey Borzenkov wrote:
> >>> Anyway, it means there is a bug in perl, because on Linux:
> >>>
> >>> root AT kitsu:~# touch test.txt
> >>> root AT kitsu:~# chmod 0444 test.txt
> >>> root AT kitsu:~# perl -e 'print "writable\n" if -w "test.txt"'
> >>> writable
> >>>
> >>> On Cygwin 1.7 perl thinks that the file is not writable.
> >>
> >> Indeed.  Checking with strace I found that the test is the same on Linux
> >> and Cygwin.  In both cases perl uses stat(), and the returned permissions
> >> are the same (0444).  Further experimenting shows that perl has a
> >> hardcoded uid == 0 test which must obviously fail on Cygwin.  If I change
> >> the user's uid to 0, the string "writable" is printed by the above command.
> >>
> >> That's a bug in perl.  There are other OSes out there which have
> >> root-like permissions for non-0 uids.  Perl should use the access()
> >> function to check for read/write/execute permissions, which always
> >> returns the correct result independent of the uid of the current user.
> >
> > Thanks.
> > I'll carry it along to p5p, but it will definitely not appear in
> > upstream 5.10.1
> > because this gate is already closed.
> > Even a horrible performance problem with
> > File::Spec::Cygwin::case_tolerant was not fixed.
> >
> > But I work on a fix to be included in blead and in my cygwin package.
> 
> Bug confirmed too early. It's actually defined and described this way.
> access() is not used for performance reasons. See perldoc perlfunc -X
> 
> If you are using ACLs, there is a pragma called C<filetest> that may
> produce more accurate results than the bare stat() mode bits.
> When under the C<use filetest 'access'> the above-mentioned filetests
> will test whether the permission can (not) be granted using the
> access() family of system calls.  Also note that the C<-x> and C<-X> may
> under this pragma return true even if there are no execute permission
> bits set (nor any extra execute permission ACLs).  This strangeness is
> due to the underlying system calls' definitions. Note also that, due to
> the implementation of C<use filetest 'access'>, the C<_> special
> filehandle won't cache the results of the file tests when this pragma is
> in effect.  Read the documentation for the C<filetest> pragma for more
> information.

As far as I'm concerned this is still a bug.  It will result in
different behaviour of the same script on different platforms for no
apparent reason.

> $ ./perl -e 'print "writable\n" if -w "test.txt"'
> 
> $ ./perl -e 'use filetest "access"; print "writable\n" if -w "test.txt"'
> writable
> 
> I can turn on access checks easily for CYGWIN but cygwin perl is already
> slow enough, so I will not do that.
> 
> Changing the uid==0 check to check the Administrators gid is more promising.
> i.e.
> --- doio.c.orig 2009-04-18 19:17:04.000000000 +0200
> +++ doio.c      2009-08-11 04:46:09.343750000 +0200
> @@ -1918,7 +1918,11 @@
>       return (mode & statbufp->st_mode) ? TRUE : FALSE;
> 
>  #else /* ! DOSISH */
> +# ifndef __CYGWIN__
>      if ((effective ? PL_euid : PL_uid) == 0) { /* root is special */
> +# else
> +    if ((effective ? PL_egid : PL_gid) == 544) { /* member of
> Administrators? */
> +# endif
>         if (mode == S_IXUSR) {
>             if (statbufp->st_mode & 0111 || S_ISDIR(statbufp->st_mode))
>                 return TRUE;
> 
> but this didn't help me, because Administrators is not my first group.
> So I call this a known limitation for all ACL aware filesystems.

That might be a good workaround nevertheless.  You should just test the
list of supplementary groups as well, along these lines:

  # ifndef __CYGWIN__
     if ((effective ? PL_euid : PL_uid) == 0) { /* root is special */
  # else
     int is_admin = false;
     gid_t gid = effective ? PL_egid : PL_gid;
     int cnt, i;
     gid_t *glist;

     cnt = getgroups (0, NULL);
     if (cnt > 0)
       {
	 glist = (gid_t *) malloc (cnt * sizeof (gid_t));
	 cnt = getgroups (cnt, glist);
	 for (i = 0; i < cnt; ++i)
	   if (gid == glist[i])
	     {
	       is_admin = true;
	       break;
	     }
	 free (glist);
       }
     if (is_admin)
  # endif


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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