Unable to unset setguid bit

[Lars Noodén]

In Precise, it is possible to set the guid bit for a directory but not to unset it.

$ chmod g=rwxs Y
foo@precise:/tmp/X$ ls -l
total 4
drwxrwsr-x 2 foo adm 4096 Feb 5 18:48 Y

foo@precise:/tmp/X$ chmod g= Y
foo@precise:/tmp/X$ ls -l
total 4
drwx--Sr-x 2 maintenance adm 4096 Feb 5 18:48 Y
foo@precise:/tmp/X$ chmod g=rwx Y
maintenance@precise:/tmp/X$ ls -l
total 4
drwxrwsr-x 2 foo adm 4096 Feb 5 18:48 Y

foo@precise:/tmp/X$ chmod 0775 Y
foo@precise:/tmp/X$ ls -l
total 4
drwxrwsr-x 2 foo adm 4096 Feb 5 18:48 Y

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: bash 4.2-1ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-14.23-generic-pae 3.2.3
Uname: Linux 3.2.0-14-generic-pae i686
ApportVersion: 1.91-0ubuntu1
Architecture: i386
Date: Sun Feb 5 18:52:29 2012
InstallationMedia: Ubuntu-Server 12.04 LTS "Precise Pangolin" - Alpha i386 (20111129)
ProcEnviron:
 LANGUAGE=en_US:en
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: bash
UpgradeStatus: No upgrade log present (probably fresh install)

[Andrew McCarthy (andrewmccarthy)]

Thanks for taking the time to report this. The behaviour you describe may seem surprising (and it was to me, too!), but is actually what the man page describes:

       chmod preserves a directory's set-user-ID and set-group-ID bits unless
       you explicitly specify otherwise. You can set or clear the bits with
       symbolic modes like u+s and g-s, and you can set (but not clear) the
       bits with a numeric mode.

So, you can clear the setgid bit on the directory in your example with "chmod g-s Y", but not with "chmod 0775 Y". Since this is the defined behaviour that others may have come to expect for both interactive use and in long-established scripts, it's unlikely to be changed now.