расширенные атрибуты (Extended attributes)
Имя (Name)
xattr - Extended attributes
Описание (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).
Стандарты (Conforming to)
Extended attributes are not specified in POSIX.1, but some other
systems (e.g., the BSDs and Solaris) provide a similar feature.
Примечание (Note)
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).
Смотри также (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)
