attr - Extended attributes
Extended attributes are name:value pairs associated permanently with files and directo-
ries, 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
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 and
XFS filesystem patches, which can be downloaded from http://acl.bestbits.at/ and
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, or sys-
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 user, trusted, and system extended attribute classes are defined as
described below. Additional classes may be added in the future.
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 direc-
tories the file permission bits define access to the file's contents, while for special
files they define access to the device described by the special file. The file permis-
sions 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 disallowed for symbolic links and special
files, 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).
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 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 in the kernel.
The kernel and the filesystem may place limits on the maximum number and size of extended
attributes that can be associated with a file.
In the current ext2 and ext3 filesystem implementations, all extended attributes must fit
on a single filesystem block (1024, 2048 or 4096 bytes, depending on the block size speci-
fied when the filesystem was created). This limit may be removed in a future version.
In the XFS filesystem implementation, 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 scalable (stored either inline in the inode, as an
extent, or in a B+ tree).
Since the filesystems on which extended attributes are stored might also be used on archi-
tectures with a different byte order and machine word size, care should be taken to store
attribute values in an architecture independent format.
Andreas Gruenbacher, <email@example.com> and the SGI XFS development team,