xattr

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 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
       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)

COLOPHON
       This page is part of release 5.13 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)