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.
Extended system attributes
Extended system 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.
Extended user attributes
Extended user 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, extended user attributes are allowed only for regular
files and directories, and access to extended user 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 extended user 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), attr(1), ioctl_iflags(2),
listxattr(2), removexattr(2), setxattr(2), acl(5), capabilities(7),
selinux(8)
COLOPHON
This page is part of release 4.15 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 2017-09-15 XATTR(7)
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