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$FreeBSD: release/9.1.0/sys/ufs/ufs/README.acls 105456 2002-10-19 16:09:16Z rwatson $ UFS Access Control Lists Copyright The UFS Access Control Lists implementation is copyright Robert Watson, and is made available under a Berkeley-style license. About UFS Access Control Lists (ACLs) Access control lists allow the association of fine-grained discretionary access control information with files and directories, extending the base UNIX permission model in a (mostly) compatible way. This implementation largely follows the POSIX.1e model, and relies on the availability of extended attributes to store extended components of the ACL, while maintaining the base permission information in the inode. Using UFS Access Control Lists (ACLs) Support for UFS access control lists may be enabled by adding: options UFS_ACL to your kernel configuration. As ACLs rely on the availability of extended attributes, your file systems must have support for extended attributes. For UFS2, this is supported natively, so no further configuration is necessary. For UFS1, you must also enable the optional extended attributes support documented in README.extattr. A summary of the instructions and ACL-specific information follows. To enable support for ACLs on a file system, the 'acls' mount flag must be set for the file system. This may be set using the tunefs '-a' flag: tunefs -a enable /dev/md0a Or by using the mount-time flag: mount -o acls /dev/md0a /mnt The flag may also be set in /etc/fstab. Note that mounting a file system previously configured for ACLs without ACL-support will result in incorrect application of discretionary protections. Likewise, mounting an ACL-enabled file system without kernel support for ACLs will result in incorrect application of discretionary protections. If the kernel is not configured for ACL support, a warning will be printed by the kernel at mount-time. For reliability purposes, it is recommended that the superblock flag be used instead of the mount-time flag, as this will avoid re-mount isses with the root file system. For reliability and performance reasons, the use of ACLs on UFS1 is discouraged; UFS2 extended attributes provide a more reliable storage mechanism for ACLs. Currently, support for ACLs on UFS1 requires the use of UFS1 EAs, which may be enabled by adding: options UFS_EXTATTR to your kernel configuration file and rebuilding. Because of filesystem mount atomicity requirements, it is also recommended that: options UFS_EXTATTR_AUTOSTART be added to the kernel so as to support the atomic enabling of the required extended attributes with the filesystem mount operation. To enable ACLs, two extended attributes must be available in the EXTATTR_NAMESPACE_SYSTEM namespace: "posix1e.acl_access", which holds the access ACL, and "posix1e.acl_default" which holds the default ACL for directories. If you're using UFS1 Extended Attributes, the following commands may be used to create the necessary EA backing files for ACLs in the filesystem root of each filesystem. In these examples, the root filesystem is used; see README.extattr for more details. mkdir -p /.attribute/system cd /.attribute/system extattrctl initattr -p / 388 posix1e.acl_access extattrctl initattr -p / 388 posix1e.acl_default On the next mount of the root filesystem, the attributes will be automatically started, and ACLs will be enabled.