xattr(7) - Linux manual page (original) (raw)

xattr(7) Miscellaneous Information 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)](../man2/stat.2.html)
   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)](../man2/listxattr.2.html) to retrieve a list of attribute names defined for
   that file or directory.

   Extended attributes are accessed as atomic objects.  Reading
   ([getxattr(2)](../man2/getxattr.2.html)) retrieves the whole value of an attribute and stores
   it in a buffer.  Writing ([setxattr(2)](../man2/setxattr.2.html)) 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.mimetype, trusted.md5sum, system.posixaclaccess, 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)](../man1/chmod.1.html) 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-imposed limits on attribute names and values are 255 bytes and 64 kB, respectively. 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).

STANDARDS 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).

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Linux man-pages 6.10 2024-06-13 xattr(7)