attr(5) — Linux manual page


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

NAME         top

       xattr - Extended attributes

DESCRIPTION         top

       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

       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

       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

       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 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 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

       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         top

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

NOTES         top

       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         top

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

COLOPHON         top

       This page is part of release 5.07 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-06-09                         XATTR(7)

Pages that refer to this page: attr(1)chacl(1)getfattr(1)setfattr(1)attr(5)ext2(5)ext3(5)ext4(5)tmpfiles.d(5)xattr(7)