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
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 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
EXTENDED ATTRIBUTE NAMESPACES
Attribute names are zero-terminated strings.
The attribute name is always specified in the fully qualified
R user.mime_type ,R trusted.md5sum ,R system.posix_acl_access ,
R security.selinux .
The namespace mechanism is used to define different classes of extended
These different classes exist for several reasons, e.g. the permissions
and capabilities required for manipulating extended attributes of one
namespace may differ to another.
R security ,R system ,R trusted ,
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 capability.
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 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 (the super user usually has this
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.
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 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
manual page for an explanation of Sticky Directories).
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).
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
and the SGI XFS development team,