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Date: Tue, 11 Aug 2009 12:44:36 +0200
Message-ID: <6910a60908110344n56b7a7f9j9ead9ceeae6a1608@mail.gmail.com>
Subject: Re: Perl bug (was Re: [1.7] cygwin allows writing to readonly files)
From: Reini Urban <rurban AT x-ray DOT at>
To: cygwin AT cygwin DOT com
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2009/8/11 Corinna Vinschen:
> 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. =A0Checking with strace I found that the test is the same on =
Linux
>> >> and Cygwin. =A0In both cases perl uses stat(), and the returned permi=
ssions
>> >> are the same (0444). =A0Further experimenting shows that perl has a
>> >> hardcoded uid =3D=3D 0 test which must obviously fail on Cygwin. =A0I=
f I change
>> >> the user's uid to 0, the string "writable" is printed by the above co=
mmand.
>> >>
>> >> That's a bug in perl. =A0There are other OSes out there which have
>> >> root-like permissions for non-0 uids. =A0Perl 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. =A0Also note that the C<-x> and C<-X> m=
ay
>> under this pragma return true even if there are no execute permission
>> bits set (nor any extra execute permission ACLs). =A0This 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. =A0Read the documentation for the C<filetest> pragma for more
>> information.
>
> As far as I'm concerned this is still a bug. =A0It 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=3D=3D0 check to check the Administrators gid is more pr=
omising.
>> i.e.
>> --- doio.c.orig 2009-04-18 19:17:04.000000000 +0200
>> +++ doio.c =A0 =A0 =A02009-08-11 04:46:09.343750000 +0200
>> @@ -1918,7 +1918,11 @@
>> =A0 =A0 =A0 return (mode & statbufp->st_mode) ? TRUE : FALSE;
>>
>> =A0#else /* ! DOSISH */
>> +# ifndef __CYGWIN__
>> =A0 =A0 =A0if ((effective ? PL_euid : PL_uid) =3D=3D 0) { /* root is spe=
cial */
>> +# else
>> + =A0 =A0if ((effective ? PL_egid : PL_gid) =3D=3D 544) { /* member of
>> Administrators? */
>> +# endif
>> =A0 =A0 =A0 =A0 if (mode =3D=3D S_IXUSR) {
>> =A0 =A0 =A0 =A0 =A0 =A0 if (statbufp->st_mode & 0111 || S_ISDIR(statbufp=
->st_mode))
>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 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. =A0You should just test the
> list of supplementary groups as well, along these lines:

We already have an ingroup() check in this Perl_cando() function, so
there is no
need to write it again. But it is disabled in perl core for this code path.
See http://perl5.git.perl.org/perl.git/blob/HEAD:/doio.c#l1929
It would be:
    if (ingroup(544,effective))
	return TRUE;		/* Administrators read and write anything */
but this is simply not true, as under unix. Windows Administrators
fall under the same ACL restrictions as normal users. So only using
access() is reliable.

I rather suggest to add the filetest access hint to the failing
ExtUtils::MakeMaker
module, and Module::Install and Module::Build also.
--=20
Reini Urban
http://phpwiki.org/           http://murbreak.at/

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