xattr(7) — Linux manual page

NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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 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 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 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         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), selinux(8)

COLOPHON         top

       This page is part of release 5.10 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
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-06-09                       XATTR(7)

Pages that refer to this page: systemd-nspawn(1)capget(2)getxattr(2)ioctl_iflags(2)listxattr(2)removexattr(2)setxattr(2)selabel_get_digests_all_partial_matches(3)selinux_restorecon(3)systemd.exec(5)tmpfs(5)capabilities(7)mount(8)