xattr - 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 use listxattr
retrieve a list of attribute names defined for that file or directory.
Extended attributes are accessed as atomic objects. Reading (getxattr
retrieves the whole value of an attribute and stores it in a buffer. Writing
(2)) replaces any previous value with the new value.
Space consumed for extended attributes may be counted towards the disk quotas of
the file owner and file group.
Attribute names are null-terminated strings. The attribute name is always
specified in the fully qualified namespace.attribute
form, for example,
The namespace mechanism is used to define different classes of extended
attributes. These different classes exist for several reasons; for example,
the permissions and capabilities required for manipulating extended attributes
of one namespace may differ to another.
Currently, the security
, and user
extended attribute classes are defined as described below. Additional classes
may be added in the future.
The security attribute namespace is used by kernel security modules, such as
Security Enhanced Linux, and also to implement file capabilities (see
(7)). 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
System extended attributes are used by the kernel to store system objects such
as Access Control Lists. 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 are visible and accessible only to processes that
have the CAP_SYS_ADMIN
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
User extended 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: read permission is required to retrieve the
attribute value, and writer permission is required to change it.
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, user extended attributes are allowed only for regular files and
directories, and access to user extended 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 the sticky
The kernel and the filesystem may place limits on the maximum number and size of
extended attributes that can be associated with a file. The VFS imposes
limitations that an attribute names is limited to 255 bytes and an attribute
value is limited to 64 kB. The list of attribute names that can be
returned is also limited to 64 kB (see BUGS in listxattr
Some filesystems, such as Reiserfs (and, historically, ext2 and ext3), require
the filesystem to be mounted with the user_xattr
mount option in order
for user extended attributes to be used.
In the current ext2, ext3, and ext4 filesystem implementations, the total bytes
used by the names and values of all of a file's extended attributes must fit
in a single filesystem block (1024, 2048 or 4096 bytes, depending on the block
size specified when the filesystem was created).
In the Btrfs, XFS, and Reiserfs filesystem implementations, there is no
practical limit on the number of extended attributes associated with a file,
and the algorithms used to store extended attribute information on disk are
In the JFS, XFS, and Reiserfs filesystem implementations, the limit on bytes
used in an EA value is the ceiling imposed by the VFS.
In the Btrfs filesystem implementation, the total bytes used for the name,
value, and implementation overhead bytes is limited to the filesystem
value (16 kB by default).
Extended attributes are not specified in POSIX.1, but some other systems (e.g.,
the BSDs and Solaris) provide a similar feature.
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
This page was formerly named attr