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XATTR(7) Linux Programmer's 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  inodes  in  the system (i.e., the stat(2) data).
     They are often used to provide additional functionality to  a  filesys-
     tem--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  (getx-
     attr(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,  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 capabili-
     ties (see capabilities(7)).  Read and write access permissions to secu-
     rity  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  capa-
     bility.

 Extended system attributes
     Extended  system  attributes  are  used  by  the kernel to store system
     objects such as Access Control Lists.  Read and  write  access  permis-
     sions  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  pro-
     cesses  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, charac-
     ter 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 inter-
     preted 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  spe-
     cial 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 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).

CONFORMING TO

     Extended attributes are not specified in POSIX.1, but some  other  sys-
     tems (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 architectures with a different byte order  and  machine
     word  size, care should be taken to store attribute values in an archi-
     tecture-independent format.

     This page was formerly named attr(5).