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       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 use listxattr(2) to
       retrieve a list of attribute names defined for that file or directory.

       Extended   attributes   are   accessed   as  atomic  objects.   Reading
       (getxattr(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

       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
       capabilities  (see capabilities(7)).  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

   Extended system attributes
       Extended system attributes are used  by  the  kernel  to  store  system
       objects   such   as  Access  Control  Lists.   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
       Trusted   extended  attributes  are  visible  and  accessible  only  to
       processes 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,
       character  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
       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 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

       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).


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


       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 format.

       This page was formerly named attr(5).


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


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