XATTR(7)						     Linux Programmer's Manual							  XATTR(7)

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  direc-
       tory.

       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 capabili-
       ties (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.

   Extended system attributes
       Extended system 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.

   Extended user attributes
       Extended user attributes may be assigned to files and directories for storing arbitrary additional information such as the mime type, char-
       acter 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,  extended  user attributes are allowed only 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 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 imposes limitations that an attribute names is limited to 255 bytes and an attribute value  is  limited	to  64 kB.   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 extended user 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).

CONFORMING TO

       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

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

Linux								    2018-04-30								  XATTR(7)