ATTR(5)                                                                                                                  ATTR(5)



NAME
       attr - 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 filesystem - for example,  additional  secu-
       rity 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 direc-
       tory.

       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.

EXTENDED ATTRIBUTE NAMESPACES
       Attribute names are zero-terminated strings.  The attribute name is  always  specified  in  the  fully  qualified  names-
       pace.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  imple-
       mented 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 per-
       missions 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).

FILESYSTEM DIFFERENCES
       The  kernel and the filesystem may place limits on the maximum number and size of extended attributes that can be associ-
       ated 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.

ADDITIONAL 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 architecture independent format.

AUTHORS
       Andreas Gruenbacher, <a.gruenbacherATbestbits.at> and the SGI XFS development team, <linux-xfsAToss.com>.

SEE ALSO
       getfattr(1), setfattr(1).



                                                                                                                         ATTR(5)