attr
ATTR(5) ATTR(5)
NAME
attr - 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 filesys-
tem - 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 retrieve a list of
attribute names defined for that file or directory.
Extended attributes are accessed as atomic objects. Reading retrieves
the whole value of an attribute and stores it in a buffer. Writing
replaces any previous value with the new value.
Space consumed for extended attributes is counted towards the disk
quotas of the file owner and file group.
Currently, support for extended attributes is implemented on Linux by
the ext2, ext3 and XFS filesystem patches, which can be downloaded
from http://acl.bestbits.at/ and http://oss.sgi.com/projects/xfs/
respectively.
EXTENDED ATTRIBUTE NAMESPACES
Attribute names are zero-terminated strings. The attribute name is
always specified in the fully qualified namespace.attribute form, eg.
user.mime_type, trusted.md5sum, system.posix_acl_access, or secu-
rity.selinux.
The namespace mechanism is used to define different classes of
extended attributes. These different classes exist for several rea-
sons, e.g. 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. 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 capa-
bility.
Extended system attributes
Extended system attributes are used by the kernel to store system
objects such as Access Control Lists and Capabilities. Read and write
access permissions to system attributes depend on the policy imple-
mented for each system attribute implemented by filesystems in the
kernel.
Trusted extended attributes
Trusted extended attributes are visible and accessible only to pro-
cesses that have the CAP_SYS_ADMIN capability (the super user usually
has this capability). Attributes in this class are used to implement
mechanisms in user space (i.e., outside the kernel) which keep infor-
mation 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, char-
acter set or encoding of a file. The access permissions for user
attributes are defined by the file permission bits.
The file permission bits of regular files and directories are inter-
preted 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 spe-
cial files and directories.
For this reason, extended user attributes are only allowed 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 Sticky Directories).
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.
In the current ext2 and ext3 filesystem implementations, all extended
attributes must fit on a single filesystem block (1024, 2048 or 4096
bytes, depending on the block size specified when the filesystem was
created). This limit may be removed in a future version.
In the XFS filesystem implementation, 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 (stored either inline in the inode, as an extent, or in a B+
tree).
ADDITIONAL 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 archi-
tecture independent format.
AUTHORS
Andreas Gruenbacher, <a.gruenbacher@computer.org> and the SGI XFS
development team, <linux-xfs@oss.sgi.com>.
SEE ALSO
getfattr(1), setfattr(1).
ATTR(5)
