GNU Linux/Permissions/POSIX ACLs
- 1 Overview
- 2 Requirements for using ACLs
- 3 Enabling ACL support
- 4 ACL Types
- 5 ACL Entries
- 6 Mask
POSIX Access Control Lists (ACLs) allow different permissions for different users or groups to be assigned to files or directories, independent of the original owner or the owning group. This means, in addition to the file owner, the file group, and others, additional users and groups can be granted or denied access by using POSIX ACLs. Said another way, the permissions defined by ACLs are a superset of the permissions specified by the 9-bit UNIX permission model. Read, write and execute permissions are the only capabilities that the ACL system deals with. Additional features such as
setuid and sticky bits are handled exclusively through the traditional mode bits.
Requirements for using ACLs
- Supported by the filesystem used to store content
- The filesystem is mounted with the
- The appropriate package containing the command-line
getfacltools is installed
- the package is named
- the package is named
- Current version of file utils (
mv, etc) with support for working with ACLs
Enabling ACL support
Confirm Kernel has support built-in
Here we're looking in the @/boot/config-2.6.32-73-server@ kernel config file on an Ubuntu 10.04 LTS server to verify that the kernel was built with ACL support for the filesystem(s) that we're using. In our case we're only using Ext4, but as you can see below this kernel includes support for the other filesystems listed in the conf as well.
$ grep _ACL /boot/config-$(uname -r)
CONFIG_EXT2_FS_POSIX_ACL=y CONFIG_EXT3_FS_POSIX_ACL=y CONFIG_EXT4_FS_POSIX_ACL=y CONFIG_REISERFS_FS_POSIX_ACL=y CONFIG_JFS_POSIX_ACL=y CONFIG_FS_POSIX_ACL=y CONFIG_XFS_POSIX_ACL=y CONFIG_OCFS2_FS_POSIX_ACL=y CONFIG_BTRFS_FS_POSIX_ACL=y CONFIG_GENERIC_ACL=y CONFIG_TMPFS_POSIX_ACL=y CONFIG_NFS_V3_ACL=y CONFIG_NFSD_V2_ACL=y CONFIG_NFSD_V3_ACL=y CONFIG_NFS_ACL_SUPPORT=m
Confirm filesystem is mounted with proper support
First, you can use
tune2fs to check the default mount options for the filesystem in question:
sudo tune2fs -l /dev/sdaX | grep acl
Default mount options: user_xattr acl
If it's not there, it would need to be added as a mount option for any filesystem that supports ACLs. Also, you should double-check
/proc/mounts to confirm that ACL support hasn't been overridden.
$ grep noacl /proc/mounts
acl mount option
If if it is not already enabled, you can set
acl as a default mount option by using the
tune2fs -o option partition command:
sudo tune2fs -o acl /dev/sdXY
Using the default mount options instead of an entry in
/etc/fstab is very useful for external drives as those partitions will be mounted with the
acl option enabled on other Linux machines. In that case there is no need to edit
/etc/fstab on every machine.
acl is specified as default mount option when creating an ext2/3/4 filesystem. This is configured in
Every object can be thought of as having associated with it an ACL that governs the discretionary access to that object; this ACL is referred to as an
access ACL. In addition, a directory may have an associated ACL that governs the initial access ACL for objects created within that directory; this ACL is referred to as a
access ACLsto grant permission to a specific file or directory
default ACLsto set permissions at the directory level for all files in the directory
Directories can have a
default ACL, which is a special kind of ACL defining the access permissions that objects in the directory inherit when they are created. A
default ACL affects both subdirectories and files.
The following are the ways in which the permissions of a directory's
default ACLs are passed to the files and subdirectories in it:
- A subdirectory inherits the
default ACLsof the parent directory both as its
default ACLand as an
- If a file inside that directory does not have an ACL (already), it inherits the permissions of the default ACLs of the directory as its
All system calls that create file system objects use a mode parameter that defines the access permissions for the newly created file system object. If the parent directory does not have a
default ACL, the permission bits as defined by the umask are subtracted from the permissions as passed by the mode parameter, with the result being assigned to the new object. If a
default ACL exists for the parent directory, the permission bits assigned to the new object correspond to the overlapping portion of the permissions of the mode parameter and those that are defined in the
default ACL. The umask is disregarded in this case.
access ACLset for an individual file can override the
- You cannot set a
default ACLon a regular file (it is intended for directories only to set inheritance)
- Setting the
default ACLon the top-level folder does NOT set the actual permissions for that directory, only those created beneath it.
- You'll need to explicitly set the ACL entries at the top-most level.
|Type||Man Page Name||Text Form||Description|
||This entry denotes access rights for the file owner.|
||These entries denote access rights for users identified by the entry's qualifier.|
||This entry denotes access rights for the file group.|
||These entries denote access rights for groups identified by the entry's qualifier.|
||This entry denotes the maximum access rights that can be granted by entries of type ACL_USER, ACL_GROUP_OBJ, or ACL_GROUP.|
||This entry denotes access rights for processes that do not match any other entry in the ACL.|
When an access check is performed, the ACL_USER_OBJ (owner) and ACL_USER (named user) entries are tested against the effective user ID. The effective group ID, as well as all supplementary group IDs are tested against the ACL_GROUP_OBJ (owning group) and ACL_GROUP (named group) entries.
Correspondence Between Acl Entries And File Permission Bits
There is a correspondence between the file owner, group, and other permissions and specific ACL entries:
- The owner permissions correspond to the permissions of the ACL_USER_OBJ (owner,
- If the ACL has an ACL_MASK entry, the group permissions correspond to the permissions of the ACL_MASK entry. Otherwise, if the ACL has no ACL_MASK entry, the group permissions correspond to the permissions of the ACL_GROUP_OBJ (owning group, group::rwx) entry.
- The other permissions correspond to the permissions of the ACL_OTHER_OBJ (other,
The file owner, group, and other permissions always match the permissions of the corresponding ACL entry. Modification of the file permission bits results in the modification of the associated ACL entries, and modification of these ACL entries results in the modification of the file permission bits.
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