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XATTR(7) Linux Programmer's Manual XATTR(7)
NAME
xattr - Extended attributes
DESCRIPTION
Extended attributes are name:value pairs associated permanently with files and directo-
ries, 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 En-
hanced 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 pro-
cesses 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 pol-
icy 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 per-
mission 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 di-
rectories 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 al-
low 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 directo-
ries, 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 attri-
bute 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 listx-
attr(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 at-
tributes 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 filesys-
tem 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 im-
plementation overhead bytes is limited to the filesystem nodesize value (16 kB by de-
fault).
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 archi-
tectures 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 5.05 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)
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