XATTR(7)		   Linux Programmer's Manual		      XATTR(7)



NAME
       xattr - Extended attributes

DESCRIPTION
       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  filesys-
       tem--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(2) to
       retrieve a list of attribute names defined for that file or directory.

       Extended	 attributes  are  accessed  as atomic objects.	Reading (getx-
       attr(2)) retrieves the whole value of an attribute and stores it	 in  a
       buffer.	Writing (setxattr(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.

   Extended attribute namespaces
       Attribute  names	 are  null-terminated  strings.	 The attribute name is
       always specified in the fully qualified namespace.attribute  form,  for
       example,	 user.mime_type,  trusted.md5sum,  system.posix_acl_access, or
       security.selinux.

       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, 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, and also to implement  file  capabili-
       ties (see capabilities(7)).  Read and write access permissions to secu-
       rity 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-
       bility.

   Extended system attributes
       Extended system attributes are used  by	the  kernel  to	 store	system
       objects	such  as  Access Control Lists.	 Read and write access permis-
       sions to system attributes depend on the policy	implemented  for  each
       system attribute implemented by filesystems in the kernel.

   Trusted extended attributes
       Trusted	extended  attributes  are  visible and accessible only to pro-
       cesses 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 access.

   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: 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	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 allowed only 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 the sticky bit).

   Filesystem differences
       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(2)).

       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 extended user 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  files  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 scalable.

       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 nodesize value (16 kB by default).

CONFORMING TO
       Extended	 attributes  are not specified in POSIX.1, but some other sys-
       tems (e.g., the BSDs and Solaris) provide a similar feature.

NOTES
       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.

       This page was formerly named attr(5).

SEE ALSO
       getfattr(1),  setfattr(1),  getxattr(2),	 listxattr(2), removexattr(2),
       setxattr(2), acl(5), capabilities(7)

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2015-05-01			      XATTR(7)