man > xattr(7)

xattr(7)                          Miscellaneous Information Manual                          xattr(7)

NAME
       xattr - Extended attributes

DESCRIPTION
       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 in‐
       odes in the system (i.e., the stat(2) data).  They are often used to provide additional func‐
       tionality 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.

   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,  sys‐
       tem.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  En‐
       hanced  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.

   System extended attributes
       System extended attributes are used by the kernel to store system objects such as Access Con‐
       trol Lists.  Read and write access permissions to system attributes depend on the policy  im‐
       plemented 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 ordi‐
       nary processes should not have access.

   User extended attributes
       User extended attributes may be assigned to files and directories for storing arbitrary addi‐
       tional  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 directo‐
       ries 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 con‐
       sume filesystem resources in a way not  controllable  by  disk  quotas  for  group  or  world
       writable special files and directories.

       For this reason, user extended attributes are allowed only for regular files and directories,
       and access to user extended attributes is restricted to the owner and to users with appropri‐
       ate 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 at‐
       tributes  that  can be associated with a file.  The VFS-imposed limits on attribute names and
       values are 255 bytes and 64 kB, respectively.  The list of attribute names that  can  be  re‐
       turned 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 user extended 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 cre‐
       ated).

       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  imple‐
       mentation overhead bytes is limited to the filesystem nodesize value (16 kB by default).

STANDARDS
       Extended  attributes are not specified in POSIX.1, but some other systems (e.g., the BSDs and
       Solaris) provide a similar feature.

NOTES
       Since the filesystems on which extended attributes are stored might also be used on architec‐
       tures with a different byte order and machine word size, care should be taken  to  store  at‐
       tribute values in an architecture-independent format.

       This page was formerly named attr(5).

SEE ALSO
       attr(1),  getfattr(1),  setfattr(1),  getxattr(2),  ioctl_iflags(2),  listxattr(2),  removex‐
       attr(2), setxattr(2), acl(5), capabilities(7), selinux(8)

Linux man-pages 6.7                          2023-10-31                                     xattr(7)