Provided by: attr_2.4.47-1ubuntu1_amd64 bug


       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

       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 security.selinux.

       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 capability.

   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 kernel.

   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


       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, <linux->.


       getfattr(1), setfattr(1).