FreeBSD/Linux Kernel Cross Reference
sys/ufs/ufs/README.extattr

     $FreeBSD: releng/5.2/sys/ufs/ufs/README.extattr 105417 2002-10-18 21:11:36Z rwatson $
     
       UFS Extended Attributes Copyright
     
     The UFS Extended Attributes implementation is copyright Robert Watson, and
     is made available under a Berkeley-style license.
     
       About UFS Extended Attributes
     
    Extended attributes allow the association of additional arbitrary
    meta-data with files and directories.  Extended attributes are defined in
    the form name=value, where name is an nul-terminated string in the style
    of a filename, and value is a binary blob of zero or more bytes. The UFS
    extended attribute service layers support for extended attributes onto a
    backing file, in the style of the quota implementation, meaning that it
    requires no underlying format changes in the filesystem.  This design
    choice exchanges simplicity, usability and easy deployment for
    performance.  When defined, extended attribute names exist in a series of
    disjoint namespaces: currently, two namespaces are defined:
    EXTATTR_NAMESPACE_SYSTEM and EXTATTR_NAMESPACE_USER.  The primary
    distinction lies in the protection model: USER EAs are protected using the
    normal inode protections, whereas SYSTEM EAs require privilege to access
    or modify.
    
      Using UFS Extended Attributes
    
    Support for UFS extended attributes is natively available in UFS2, and
    requires no special configuration.  For reliability, administrative,
    and performance reasons, if you plan to use extended attributes, it
    is recommended that you use UFS2 in preference to UFS1.
    
    Support for UFS extended attributes may be enabled for UFS1 by adding:
    
            options UFS_EXTATTR
    
    to your kernel configuration file.  This allows UFS-based filesystems to
    support extended attributes, but requires manual administration of EAs
    using the extattrctl tool, including the starting of EA support for each
    filesystem, and the enabling of individual attributes for the file
    system.  The extattrctl utility may be used to initialize backing files
    before first use, to start and stop EA service on a filesystem, and to
    enable and disable named attributes.  The command lines for extattrctl
    take the following forms:
    
      extattrctl start [path]
      extattrctl stop [path]
      extattrctl initattr [-f] [-p path] [attrsize] [attrfile]
      extattrctl enable [path] [attrnamespace] [attrname] [attrfile]
      extattrctl disable [path] [attrnamespace] [attrname]
    
    In each case, [path] is used to indicate the mounted filesystem on which
    to perform the operation.  [attrnamespace] refers to the namespace in
    which the attribute is being manipulated, and may be "system" or "user".  
    The [attrname] is the attribute name to use for the operation. The
    [attrfile] argument specifies the attribute backing file to use. When
    using the "initattr" function to initialize a backing file, the maximum
    size of attribute data must be defined in bytes using the [attrsize]
    field.  Optionally, the [-p path] argument may be used to indicate to
    extattrctl that it should pre-allocate space for EA data, rather than
    creating a sparse backing file.  This prevents attribute operations from
    failing in low disk-space conditions (which can be important when EAs are
    used for security purposes), but pre-allocation will consume space
    proportional to the product of the defined maximum attribute size and
    number of attributes on the specified filesystem.
    
    Manual configuration increases administrative overhead, but also
    introduces the possibility of race conditions during filesystem mount, if
    EAs are used to support other features, as starting the EAs manually is
    not atomic with the mount operation.  To address this problem, an
    additional kernel option may be defined to auto-start EAs on a UFS file
    system based on special directories at mount-time:
    
            options UFS_EXTATTR_AUTOSTART
    
    If this option is defined, UFS will search for a ".attribute"
    sub-directory of the filesystem root during the mount operation.  If it
    is found, EA support will be started for the filesystem.  UFS will then
    search for "system" and "user" sub-directories of the ".attribute"
    directory for any potential backing files, and enable an EA for each valid
    backing file with the name of the backing file as the attribute name.  
    For example, by creating the following tree, the two EAs,
    posix1e.acl_access and posix1e.acl_default will be enabled in the system
    namespace of the root filesystem, reserving space for attribute data:
    
      mkdir -p /.attribute/system
      cd /.attribute/system
      extattrctl initattr -p / 388 posix1e.acl_access
      extattrctl initattr -p / 388 posix1e.acl_default
    
    On the next mount of the root filesystem, the attributes will be
    automatically started.

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