attr

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 al-
       ways specified in the fully qualified namespace.attribute form, for ex-
       ample,  user.mime_type, trusted.md5sum, system.posix_acl_access, or se-
       curity.selinux.

       The namespace mechanism is used to define different classes of extended
       attributes.  These different classes exist for several reasons; for ex-
       ample, the permissions and capabilities required for  manipulating  ex-
       tended 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  at-
       tribute 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 ob-
       jects such as Access Control Lists.  Read and write access  permissions
       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.

   User extended attributes
       User extended 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  at-
       tributes  are  defined  by the file permission bits: read permission is
       required to retrieve the attribute value, and writer permission is  re-
       quired 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, user extended attributes are allowed only for regular
       files and directories, and access to user extended  attributes  is  re-
       stricted  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  attri-
       bute  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 user extended attributes to be used.

       In the current ext2, ext3, and ext4 filesystem implementations, the to-
       tal  bytes used by the names and values of all of a file's extended at-
       tributes 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).

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

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2019-08-02                          XATTR(7)