ATTR(5) File Formats Manual ATTR(5)
attr - Extended attributes
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 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, or system.posix_acl_access.
The namespace mechanism is used to define different classes of extended attributes. 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.
Currently the user, trusted, and system extended attribute classes are defined as described below. Additional classes may be added in the
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 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 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 disallowed for symbolic links and special files, 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).
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 capability). Attributes in this class are used to implement mechanisms in user space (i.e., outside the kernel) which keep informa-
tion in extended attributes to which ordinary processes should not have access.
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 in the kernel.
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+
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.
Andreas Gruenbacher, <firstname.lastname@example.org> and the SGI XFS development team, <email@example.com>.