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       Extended attributes are name:value pairs associated perma­
       nently with files and directories, similar to the environ­
       ment  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 Con­
       trol  Lists  (ACLs)  may  be  implemented  using  extended

       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.  Read­
       ing 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  and  XFS  filesystem  patches,
       which  can  be downloaded from http://acl.bestbits.at/ and
       http://oss.sgi.com/projects/xfs/ respectively.


       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, or system.posix_acl_access.

       The  namespace  mechanism  is  used  to  define  different
       classes of extended attributes.  These  different  classes
       exist  for several reasons, e.g. the permissions and capa­
       bilities required for manipulating extended attributes  of
       one namespace may differ to another.

       Currently the user, trusted, and system extended attribute
       classes are defined as described below. Additional classes
       may be added in the future.

   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

       For  this  reason, extended user attributes are disallowed
       for symbolic  links  and  special  files,  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).

   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 system attributes
       Extended system attributes are used by the kernel to store
       system  objects such as Access Control Lists and Capabili­
       ties.   Read  and  write  access  permissions  to   system
       attributes  depend on the policy implemented for each sys­
       tem attribute implemented in the kernel.


       The kernel and the filesystem may place limits on the max­
       imum  number  and  size of extended attributes that can be
       associated with a file.

       In the current ext2 and ext3  filesystem  implementations,
       all  extended  attributes  must fit on a single filesystem
       block (1024, 2048 or 4096 bytes, depending  on  the  block
       size  specified  when  the  filesystem  was created). This
       limit may be removed in a future version.

       In the XFS filesystem implementation, there is no  practi­
       cal  limit on the number of extended attributes associated
       with a file, and the algorithms  used  to  store  extended
       attribute  information on disk are scalable (stored either
       inline in the inode, as an extent, or in a B+ tree).


       Since the filesystems on  which  extended  attributes  are
       stored  might also be used on architectures with a differ­
       ent byte order and machine word size, care should be taken
       to  store  attribute values in an architecture independent


       Andreas Gruenbacher, <a.gruenbacher@computer.org> and  the
       SGI XFS development team, <linux-xfs@oss.sgi.com>.




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