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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
       replaces 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
       always 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
       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	 capa-

   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 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
       attributes 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
       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
       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, <a.gruenbacher@bestbits.at> and the SGI XFS devel-
       opment team, <linux-xfs@oss.sgi.com>.

       getfattr(1), setfattr(1).