Provided by: attr_2.4.47-1ubuntu1_amd64 
NAME
attr - Extended attributes
DESCRIPTION
Extended attributes are name:value pairs associated permanently with files and directories, similar to
the environment strings associated with a process. An attribute may be defined or undefined. If it is
defined, its value may be empty or non-empty.
Extended attributes are extensions to the normal attributes which are associated with all inodes in the
system (i.e. the stat(2) data). They are often used to provide additional functionality to a filesystem
- for example, additional security features such as Access Control Lists (ACLs) may be implemented using
extended attributes.
Users with search access to a file or directory may retrieve a list of attribute names defined for that
file or directory.
Extended attributes are accessed as atomic objects. Reading retrieves the whole value of an attribute
and stores it in a buffer. Writing replaces any previous value with the new value.
Space consumed for extended attributes is counted towards the disk quotas of the file owner and file
group.
Currently, support for extended attributes is implemented on Linux by the ext2, ext3, ext4, XFS, JFS and
reiserfs filesystems.
EXTENDED ATTRIBUTE NAMESPACES
Attribute names are zero-terminated strings. The attribute name is always specified in the fully
qualified namespace.attribute form, eg. user.mime_type, trusted.md5sum, system.posix_acl_access, or
security.selinux.
The namespace mechanism is used to define different classes of extended attributes. These different
classes exist for several reasons, e.g. the permissions and capabilities required for manipulating
extended attributes of one namespace may differ to another.
Currently the security, system, trusted, and user extended attribute classes are defined as described
below. Additional classes may be added in the future.
Extended security attributes
The security attribute namespace is used by kernel security modules, such as Security Enhanced Linux.
Read and write access permissions to security attributes depend on the policy implemented for each
security attribute by the security module. When no security module is loaded, all processes have read
access to extended security attributes, and write access is limited to processes that have the
CAP_SYS_ADMIN capability.
Extended system attributes
Extended system attributes are used by the kernel to store system objects such as Access Control Lists
and Capabilities. Read and write access permissions to system attributes depend on the policy
implemented for each system attribute implemented by filesystems in the kernel.
Trusted extended attributes
Trusted extended attributes are visible and accessible only to processes that have the CAP_SYS_ADMIN
capability (the super user usually has this capability). Attributes in this class are used to implement
mechanisms in user space (i.e., outside the kernel) which keep information in extended attributes to
which ordinary processes should not have access.
Extended user attributes
Extended user attributes may be assigned to files and directories for storing arbitrary additional
information such as the mime type, character set or encoding of a file. The access permissions for user
attributes are defined by the file permission bits.
The file permission bits of regular files and directories are interpreted differently from the file
permission bits of special files and symbolic links. For regular files and directories the file
permission bits define access to the file's contents, while for device special files they define access
to the device described by the special file. The file permissions of symbolic links are not used in
access checks. These differences would allow users to consume filesystem resources in a way not
controllable by disk quotas for group or world writable special files and directories.
For this reason, extended user attributes are only allowed for regular files and directories, and access
to extended user attributes is restricted to the owner and to users with appropriate capabilities for
directories with the sticky bit set (see the chmod(1) manual page for an explanation of Sticky
Directories).
FILESYSTEM DIFFERENCES
The kernel and the filesystem may place limits on the maximum number and size of extended attributes that
can be associated with a file. Some file systems, such as ext2/3 and reiserfs, require the filesystem to
be mounted with the user_xattr mount option in order for extended user attributes to be used.
In the current ext2, ext3 and ext4 filesystem implementations, each extended attribute must fit on a
single filesystem block (1024, 2048 or 4096 bytes, depending on the block size specified when the
filesystem was created).
In the XFS and reiserfs filesystem implementations, there is no practical limit on the number or size of
extended attributes associated with a file, and the algorithms used to store extended attribute
information on disk are scalable.
In the JFS filesystem implementation, names can be up to 255 bytes and values up to 65,535 bytes.
ADDITIONAL NOTES
Since the filesystems on which extended attributes are stored might also be used on architectures with a
different byte order and machine word size, care should be taken to store attribute values in an
architecture independent format.
AUTHORS
Andreas Gruenbacher, <a.gruenbacher@bestbits.at> and the SGI XFS development team, <linux-
xfs@oss.sgi.com>.
SEE ALSO
getfattr(1), setfattr(1).
ATTR(5)