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.
Attribute names are zero-terminated strings. The attribute name is always
specified in the fully qualified namespace.attribute
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
, 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. 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 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 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 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
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
mount option in order for extended user attributes to be
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.
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
Andreas Gruenbacher, <email@example.com
> and the SGI XFS
development team, <firstname.lastname@example.org