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ATTR(5)			      File Formats Manual		       ATTR(5)

       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 re-
       places any previous value with the new value.

       Space consumed for extended attributes is counted towards the disk quo-
       tas 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 al-
       ways specified in the fully  qualified  namespace.attribute  form,  eg.
       user.mime_type,	 trusted.md5sum,   system.posix_acl_access,  or	 secu-

       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 at-
       tributes	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	 capa-

   Extended system attributes
       Extended	 system	 attributes are	used by	the kernel to store system ob-
       jects such as Access Control Lists and Capabilities.   Read  and	 write
       access  permissions  to	system	attributes depend on the policy	imple-
       mented for each system attribute	implemented by filesystems in the ker-

   Trusted extended attributes
       Trusted	extended  attributes  are  visible and accessible only to pro-
       cesses 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 informa-
       tion in extended	attributes to which ordinary processes should not have

   Extended user attributes
       Extended	user attributes	may be assigned	to files and  directories  for
       storing arbitrary additional information	such as	the mime type, charac-
       ter set or encoding of a	file. The  access  permissions	for  user  at-
       tributes	are defined by the file	permission bits.

       The  file  permission  bits of regular files and	directories are	inter-
       preted 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  spe-
       cial files and directories.

       For  this reason, extended user attributes are only allowed for regular
       files and directories, and access to extended user  attributes  is  re-
       stricted	 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 ex-
       tended 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 practi-
       cal  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	archi-
       tecture independent format.

       Andreas Gruenbacher, <>	and the	SGI XFS	devel-
       opment team, <>.

       getfattr(1), setfattr(1).



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