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
       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 5.04 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                             2019-08-02                          XATTR(7)