I’ve recently come across a situation where the inherent design of POSIX ACLs has left me scratching my head for a solution to the problem of setting up a “project” or “group share” directory on Linux. The problem is as follows: We have several different projects or groups that desire a directory where any and every file created, copied or moved to said directory will become owned by a particular group and have group read/write permissions set automatically.
Most of the problem is solved through age-old UNIX techniques. For group ownership, all we need to do is setup the top-level directory to be owned by the “project” or “group share” group and setgid the directory:
$ mkdir project1 $ chown .projgroup project1 $ chmod g+s project1
This effectively forces every file created, moved or copied into the “project1” directory to be owned by group “projgroup”. So far, so good. The difficulties begin when we attempt to use default ACLs to enforce the permissions of any files created, moved or copied into the directory.
The POSIX ACL standard defines “default” ACLs which can be applied to a directory, which are in turn inherited by newly created/copied/moved child files and directories. While the default ACLs are inherited properly, the ACL mask when applied to files copied into the group share directory WITHOUT previous group write set prevents the files from being group writable!
$ getfacl project1 # file: project1 # owner: root # group: projgroup user::rwx group::rwx other::r-- default:user::rw- default:group::rw- default:group:projgroup:rw- default:mask::rwx default:other::r--
So far so good, right?
$ ls -alh test -rw-r--r-- 1 user1 user 0 Apr 23 15:10 test $ cp test project1 $ ls -alh project1/test -rw-r--r--+ 1 user1 projgroup 0 Apr 23 15:10 project1/test
What the… ?!?! No group write? Noooooo!
$ getfacl project1 # file: project1/test # owner: user1 # group: projgroup user::rw- group::rw- #effective:r-- group:projgroup:rw- #effective:r-- mask::r-- other::r--
And so we have the great POSIX ACL mask problem, which is by design in fact. Still looking for a complete solution that doesn’t involve global trying to force a specific umask on every account… It would be nice if I could ensure that every file had group write set before it was copied into the group share directory but alas, I cannot. Telling users to manually check and change permissions is also a pain. Cron jobs to change group write recursively is also ugly. Please, someone provide me with the solution.