attr - Extended attributes
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
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
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
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).
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
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.
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.
Andreas Gruenbacher, <firstname.lastname@example.org> and the SGI XFS
development team, <email@example.com>.