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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 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
       quotas 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

       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

   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 by filesystems in the

   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 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  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 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
       practical 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
       architecture independent format.


       Andreas   Gruenbacher,  <>  and  the  SGI  XFS
       development team, <>.


       getfattr(1), setfattr(1).