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 files 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).