Extended Attributes

How

Extended attributes are name-value pairs.

Each file can have multiple EAs.
Name is a null-terminated, case-sensitive string, up to 255 characters.
Value can be up to 64kB (VFS limit).
Some file systems impose limits on total data in EAs that are associated with a file.
e.g., one disk block for ext2 and ext3.

EA names are of the form namespace.name.

namespace is one of the following:

user: can be manipulated by unprivileged userland processes, subject to file permissions;
- retrieving EA value requires read permission; changing EA value requires write permission.
- Some file systems (e.g., ext2, etx3, Reiserfs) require mount -o user_xattr.
trusted: can be manipulated by privileged (CAP_SYS_ADMIN) userland processes;
extended: used by kernel to associate system objects with a file.
- Only current use is to implement Access Control Lists (ACLs).
security: provided to support storage of file security labels for operating system security modules (devised to support SELinux).