|XATTR(7)||Linux Programmer's Manual||XATTR(7)|
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 use listxattr(2) to retrieve a list of attribute names defined for that file or directory.
Extended attributes are accessed as atomic objects. Reading (getxattr(2)) retrieves the whole value of an attribute and stores it in a buffer. Writing (setxattr(2)) replaces any previous value with the new value.
Space consumed for extended attributes may be counted towards the disk quotas of the file owner and file group.
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.
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).
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 file's 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).
This page was formerly named attr(5).