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ATTR(5)                                                                ATTR(5)



NAME
       attr - 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 filesystem -  for  exam-
       ple,  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 retrieve a list of attribute names defined for that file or
       directory.

       Extended  attributes  are  accessed  as  atomic objects.  Reading retrieves the whole value of an attribute and
       stores it in a buffer.  Writing replaces any previous value with the new value.

       Space consumed for extended attributes is counted towards the disk quotas of the file owner and file group.

       Currently, support for extended attributes is implemented on Linux by the ext2, ext3, ext4, XFS, JFS and  reis-
       erfs filesystems.

EXTENDED ATTRIBUTE NAMESPACES
       Attribute  names  are  zero-terminated  strings.  The attribute name is always specified in the fully qualified
       namespace.attribute form, eg.  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, e.g. 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.  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.

   Extended system attributes
       Extended system attributes are used by the kernel to store system objects such  as  Access  Control  Lists  and
       Capabilities.  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 processes that have the CAP_SYS_ADMIN capability
       (the  super  user  usually  has this 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, character set or encoding of a file. The access  permissions  for  user  attributes  are
       defined by the file permission bits.

       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 only allowed 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 Sticky Directories).

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.   Some  file  systems, such as ext2/3 and reiserfs, 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, each extended attribute must  fit  on  a  single
       filesystem  block (1024, 2048 or 4096 bytes, depending on the block size specified when the filesystem was cre-
       ated).

       In the XFS and reiserfs filesystem implementations, there is no practical  limit  on  the  number  or  size  of
       extended  attributes associated with a file, and the algorithms used to store extended attribute information on
       disk are scalable.

       In the JFS filesystem implementation, names can be up to 255 bytes and values up to 65,535 bytes.

ADDITIONAL NOTES
       Since the filesystems on which extended attributes are stored might also be used on architectures with  a  dif-
       ferent  byte  order  and  machine  word size, care should be taken to store attribute values in an architecture
       independent format.

AUTHORS
       Andreas Gruenbacher, <a.gruenbacherATbestbits.at> and the SGI XFS development team, <linux-xfsAToss.com>.

SEE ALSO
       getfattr(1), setfattr(1).



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