Extended attribute

Extended file attributes are file system features that enable users to associate computer files with metadata not interpreted by the filesystem, whereas regular attributes have a purpose strictly defined by the filesystem (such as permissions or records of creation and modification times). Unlike forks, which can usually be as large as the maximum file size, extended attributes are usually limited in size to a value significantly smaller than the maximum file size. Typical uses include storing the author of a document, the character encoding of a plain-text document, or a checksum, cryptographic hash or digital certificate, and discretionary access control information.

In AIX, the JFS2 v2 filesystem supports extended attributes, which are accessible using the getea command. The getea,setea,listea,statea, and removeea APIs support fetching, setting, listing, getting information about, and removing extended attributes.

In FreeBSD 5.0 and later, the UFS1 and UFS2 filesystems support extended attributes, using the extattr_ family of system calls. Any regular file may have a list of extended attributes. Each attribute consists of a name and the associated data. The name must be a null-terminated string, and exists in a namespace identified by a small-integer namespace identifier. Currently, two namespaces exist: user and system. The user namespace has no restrictions with regard to naming or contents. The system namespace is primarily used by the kernel for access control lists and mandatory access control.

In Linux, the ext2, ext3, ext4, JFS, Squashfs, Yaffs2, ReiserFS, XFS, Btrfs, OrangeFS, Lustre, OCFS2 1.6 and F2FS filesystems support extended attributes (abbreviated xattr) when enabled in the kernel configuration. Any regular file or directory may have extended attributes consisting of a name and associated data. The name must be a null-terminated string prefixed by a namespace identifier and a dot character. Currently, four namespaces exist: user, trusted, security and system. The user namespace has no restrictions with regard to naming or contents. The system namespace is primarily used by the kernel for access control lists. The security namespace is used by SELinux, for example.

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