NAME

       xattr - 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 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 security.selinux.

       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 capability.

   System extended attributes
       System extended 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.

   User extended attributes
       User  extended  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,  user  extended  attributes  are  allowed  only  for  regular files and
       directories, and access to user extended 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-imposed limits on attribute names
       and values are 255 bytes and 64 kB, respectively.  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  user  extended
       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  file's  extended  attributes  must  fit  in  a  single
       filesystem block (1024, 2048 or 4096 bytes, depending on the block size specified when the
       filesystem was created).

       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).

STANDARDS

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

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.

       This page was formerly named attr(5).

SEE ALSO

       attr(1),   getfattr(1),   setfattr(1),   getxattr(2),    ioctl_iflags(2),    listxattr(2),
       removexattr(2), setxattr(2), acl(5), capabilities(7), selinux(8)