Subject: Perl bug (was Re: [1.7] cygwin allows writing to readonly files)
From: Corinna Vinschen <[email-masked]>
To: [email-masked]
Cc: Reini Urban <[email-masked]>
Date: Mon, 10 Aug 2009 18:40:57 +0200
List-Id: <cygwin.cygwin.com>

On Aug 10 20:11, Alexey Borzenkov wrote:
> On Mon, Aug 10, 2009 at 5:25 PM, Corinna
> Vinschen<[email-masked]> wrote:
> > That's a bug in your testsuite.  I assume you're running the tests as
> > administrator, right?  Administrators have the right to write to all
> > files, even R/O files, according to POSIX rules.  Your test would fail
> > on Linux as well, if you're running it as root.
> 
> Well, it's not my testsuite, but yes, I'm running under administrator
> account. But it makes me wonder, how does it work? Do you change ACLs
> temporarily?

No.  It's a "user privileges" thingy.  See
http://msdn.microsoft.com/en-us/library/aa379306%28VS.85%29.aspx

The SE_BACKUP_NAME and SE_RESTORE_NAME privileges are in the
administrator's user token, but they are not enabled by default.  Cygwin
just enables them at startup time, if they are available in the user
token.  Therefore, a Cygwin process has the usual POSIX-like permissions
for admin users.  It's no magic which isn't available to any other
native Win32 application.

> Anyway, it means there is a bug in perl, because on Linux:
> 
> root@kitsu:~# touch test.txt
> root@kitsu:~# chmod 0444 test.txt
> root@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.

Corinna

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

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