From version 2.6 onward, Linux supports ACLs. ACLs extend the traditional UNIX file permissions model, allowing file permissions to be controlled on a per-user and per-group basis.
The final versions (Draft 17) of the draft POSIX.1e and POSIX.2c standards are available online at http://wt.tuxomania.net/publications/posix.1e/.
The acl(5) manual page gives an overview of ACLs and some guidance on the portability of the various ACL library functions implemented on Linux.
Details of the Linux implementation of ACLs and extended attributes can be found in [Grünbacher, 2003]. Andreas Grünbacher maintains a web site containing information about ACLs at http://acl.bestbits.at/.