ACL(5)			    BSD File Formats Manual			ACL(5)

NAME
     acl -- Access Control Lists

DESCRIPTION
     This manual page describes POSIX Access Control Lists, which are used to
     define more fine-grained discretionary access rights for files and direc-
     tories.

ACL TYPES
     Every object can be thought of as having associated with it an ACL that
     governs the discretionary access to that object; this ACL is referred to
     as an access ACL. In addition, a directory may have an associated ACL
     that governs the initial access ACL for objects created within that
     directory; this ACL is referred to as a default ACL.

ACL ENTRIES
     An ACL consists of a set of ACL entries. An ACL entry specifies the
     access permissions on the associated object for an individual user or a
     group of users as a combination of read, write and search/execute permis-
     sions.

     An ACL entry contains an entry tag type, an optional entry tag qualifier,
     and a set of permissions.	We use the term qualifier to denote the entry
     tag qualifier of an ACL entry.

     The qualifier denotes the identifier of a user or a group, for entries
     with tag types of ACL_USER or ACL_GROUP, respectively. Entries with tag
     types other than ACL_USER or ACL_GROUP have no defined qualifiers.

     The following entry tag types are defined:

	   ACL_USER_OBJ	   The ACL_USER_OBJ entry denotes access rights for
			   the file owner.

	   ACL_USER	   ACL_USER entries denote access rights for users
			   identified by the entry's qualifier.

	   ACL_GROUP_OBJ   The ACL_GROUP_OBJ entry denotes access rights for
			   the file group.

	   ACL_GROUP	   ACL_GROUP entries denote access rights for groups
			   identified by the entry's qualifier.

	   ACL_MASK	   The ACL_MASK entry denotes the maximum access
			   rights that can be granted by entries of type
			   ACL_USER, ACL_GROUP_OBJ, or ACL_GROUP.

	   ACL_OTHER	   The ACL_OTHER entry denotes access rights for pro-
			   cesses that do not match any other entry in the
			   ACL.

     When an access check is performed, the ACL_USER_OBJ and ACL_USER entries
     are tested against the effective user ID. The effective group ID, as well
     as all supplementary group IDs are tested against the ACL_GROUP_OBJ and
     ACL_GROUP entries.

VALID ACLs
     A valid ACL contains exactly one entry with each of the ACL_USER_OBJ,
     ACL_GROUP_OBJ, and ACL_OTHER tag types. Entries with ACL_USER and
     ACL_GROUP tag types may appear zero or more times in an ACL. An ACL that
     contains entries of ACL_USER or ACL_GROUP tag types must contain exactly
     one entry of the ACL_MASK tag type. If an ACL contains no entries of
     ACL_USER or ACL_GROUP tag types, the ACL_MASK entry is optional.

     All user ID qualifiers must be unique among all entries of ACL_USER tag
     type, and all group IDs must be unique among all entries of ACL_GROUP tag
     type.

       The acl_get_file() function returns an ACL with zero ACL entries as the
     default ACL of a directory, if the directory is not associated with a
     default ACL. The acl_set_file() function also accepts an ACL with zero
     ACL entries as a valid default ACL for directories, denoting that the
     directory shall not be associated with a default ACL. This is equivalent
     to using the acl_delete_def_file() function.

CORRESPONDENCE BETWEEN ACL ENTRIES AND FILE PERMISSION BITS
     The permissions defined by ACLs are a superset of the permissions speci-
     fied by the file permission bits.

     There is a correspondence between the file owner, group, and other per-
     missions and specific ACL entries: the owner permissions correspond to
     the permissions of the ACL_USER_OBJ entry. If the ACL has an ACL_MASK
     entry, the group permissions correspond to the permissions of the
     ACL_MASK entry.  Otherwise, if the ACL has no ACL_MASK entry, the group
     permissions correspond to the permissions of the ACL_GROUP_OBJ entry.
     The other permissions correspond to the permissions of the ACL_OTHER_OBJ
     entry.

     The file owner, group, and other permissions always match the permissions
     of the corresponding ACL entry. Modification of the file permission bits
     results in the modification of the associated ACL entries, and modifica-
     tion of these ACL entries results in the modification of the file permis-
     sion bits.

OBJECT CREATION AND DEFAULT ACLs
     The access ACL of a file object is initialized when the object is created
     with any of the creat(), mkdir(), mknod(), mkfifo(), or open() functions.
     If a default ACL is associated with a directory, the mode parameter to
     the functions creating file objects and the default ACL of the directory
     are used to determine the ACL of the new object:

     1.	  The new object inherits the default ACL of the containing directory
	  as its access ACL.

     2.	  The access ACL entries corresponding to the file permission bits are
	  modified so that they contain no permissions that are not contained
	  in the permissions specified by the mode parameter.

     If no default ACL is associated with a directory, the mode parameter to
     the functions creating file objects and the file creation mask (see
     umask(2)) are used to determine the ACL of the new object:

     1.	  The new object is assigned an access ACL containing entries of tag
	  types ACL_USER_OBJ, ACL_GROUP_OBJ, and ACL_OTHER. The permissions of
	  these entries are set to the permissions specified by the file cre-
	  ation mask.

     2.	  The access ACL entries corresponding to the file permission bits are
	  modified so that they contain no permissions that are not contained
	  in the permissions specified by the mode parameter.

ACCESS CHECK ALGORITHM
     A process may request read, write, or execute/search access to a file
     object protected by an ACL. The access check algorithm determines whether
     access to the object will be granted.

     1.	  If the effective user ID of the process matches the user ID of the
	  file object owner, then

	      if the ACL_USER_OBJ entry contains  the  requested  permissions,
	      access is granted,

	      else access is denied.

     2.	  else if the effective user ID of the process matches the qualifier
	  of any entry of type ACL_USER, then

	      if the matching ACL_USER entry and the  ACL_MASK	entry  contain
	      the requested permissions, access is granted,

	      else access is denied.

     3.	  else if the effective group ID or any of the supplementary group IDs
	  of the process match the file group or the qualifier of any entry of
	  type ACL_GROUP, then

	      if the ACL contains an ACL_MASK entry, then

		  if  the ACL_MASK entry and any of the matching ACL_GROUP_OBJ
		  or ACL_GROUP	entries	 contain  the  requested  permissions,
		  access is granted,

		  else access is denied.

	      else  (note  that	 there	can be no ACL_GROUP entries without an
	      ACL_MASK entry)

		  if the ACL_GROUP_OBJ entry contains  the  requested  permis-
		  sions, access is granted,

		  else access is denied.

	 4.   else if the ACL_OTHER entry contains the requested permissions,
	      access is granted.

	 5.   else access is denied.

ACL TEXT FORMS
     A long and a short text form for representing ACLs is defined. In both
     forms, ACL entries are represented as three colon separated fields: an
     ACL entry tag type, an ACL entry qualifier, and the discretionary access
     permissions. The first field contains one of the following entry tag type
     keywords:

	   user	   A user ACL entry specifies the access granted to either the
		   file owner (entry tag type ACL_USER_OBJ) or a specified
		   user (entry tag type ACL_USER).

	   group   A group ACL entry specifies the access granted to either
		   the file group (entry tag type ACL_GROUP_OBJ) or a speci-
		   fied group (entry tag type ACL_GROUP).

	   mask	   A mask ACL entry specifies the maximum access which can be
		   granted by any ACL entry except the user entry for the file
		   owner and the other entry (entry tag type ACL_MASK).

	   other   An other ACL entry specifies the access granted to any
		   process that does not match any user or group ACL entries
		   (entry tag type ACL_OTHER).

     The second field contains the user or group identifier of the user or
     group associated with the ACL entry for entries of entry tag type
     ACL_USER or ACL_GROUP, and is empty for all other entries. A user identi-
     fier can be a user name or a user ID number in decimal form. A group
     identifier can be a group name or a group ID number in decimal form.

     The third field contains the discretionary access permissions. The read,
     write and search/execute permissions are represented by the r, w, and x
     characters, in this order. Each of these characters is replaced by the -
     character to denote that a permission is absent in the ACL entry.	When
     converting from the text form to the internal representation, permissions
     that are absent need not be specified.

     White space is permitted at the beginning and end of each ACL entry, and
     immediately before and after a field separator (the colon character).

   LONG TEXT FORM
     The long text form contains one ACL entry per line. In addition, a number
     sign (#) may start a comment that extends until the end of the line. If
     an ACL_USER, ACL_GROUP_OBJ or ACL_GROUP ACL entry contains permissions
     that are not also contained in the ACL_MASK entry, the entry is followed
     by a number sign, the string "effective:", and the effective access per-
     missions defined by that entry. This is an example of the long text form:

	   user::rw-
	   user:lisa:rw-	 #effective:r--
	   group::r--
	   group:toolies:rw-	 #effective:r--
	   mask::r--
	   other::r--

   SHORT TEXT FORM
     The short text form is a sequence of ACL entries separated by commas, and
     is used for input. Comments are not supported. Entry tag type keywords
     may either appear in their full unabbreviated form, or in their single
     letter abbreviated form. The abbreviation for user is u, the abbreviation
     for group is g, the abbreviation for mask is m, and the abbreviation for
     other is o.  The permissions may contain at most one each of the follow-
     ing characters in any order: r, w, x.  These are examples of the short
     text form:

	   u::rw-,u:lisa:rw-,g::r--,g:toolies:rw-,m::r--,o::r--
	   g:toolies:rw,u:lisa:rw,u::wr,g::r,o::r,m::r

RATIONALE
     IEEE 1003.1e draft 17 defines Access Control Lists that include entries
     of tag type ACL_MASK, and defines a mapping between file permission bits
     that is not constant. The standard working group defined this relatively
     complex interface in order to ensure that applications that are compliant
     with IEEE 1003.1 ("POSIX.1") will still function as expected on systems
     with ACLs. The IEEE 1003.1e draft 17 contains the rationale for choosing
     this interface in section B.23.

CHANGES TO THE FILE UTILITIES
     On a system that supports ACLs, the file utilities ls(1), cp(1), and
     mv(1) change their behavior in the following way:

     o	 For files that have a default ACL or an access ACL that contains more
	 than the three required ACL entries, the ls(1) utility in the long
	 form produced by ls -l displays a plus sign (+) after the permission
	 string.

     o	 If the -p flag is specified, the cp(1) utility also preserves ACLs.
	 If this is not possible, a warning is produced.

     o	   The mv(1) utility always preserves ACLs. If this is not possible, a
	 warning is produced.

     The effect of the chmod(1) utility, and of the chmod(2) system call, on
     the access ACL is described in CORRESPONDENCE BETWEEN ACL ENTRIES AND
     FILE PERMISSION BITS.

STANDARDS
     The IEEE 1003.1e draft 17 ("POSIX.1e") document describes several secu-
     rity extensions to the IEEE 1003.1 standard. While the work on 1003.1e
     has been abandoned, many UNIX style systems implement parts of POSIX.1e
     draft 17, or of earlier drafts.

     Linux Access Control Lists implement the full set of functions and utili-
     ties defined for Access Control Lists in POSIX.1e, and several exten-
     sions.  The implementation is fully compliant with POSIX.1e draft 17;
     extensions are marked as such.  The Access Control List manipulation
     functions are defined in the ACL library (libacl, -lacl). The POSIX com-
     pliant interfaces are declared in the <sys/acl.h> header.	Linux-specific
     extensions to these functions are declared in the <acl/libacl.h> header.

SEE ALSO
     chmod(1), creat(2), getfacl(1), ls(1), mkdir(2), mkfifo(2), mknod(2),
     open(2), setfacl(1), stat(2), umask(1)

   POSIX 1003.1e DRAFT 17
     http://www.guug.de/~winni/posix.1e/download.html

   POSIX 1003.1e FUNCTIONS BY CATEGORY
     ACL storage management
	  acl_dup(3), acl_free(3), acl_init(3)

     ACL entry manipulation
	  acl_copy_entry(3), acl_create_entry(3), acl_delete_entry(3),
	  acl_get_entry(3), acl_valid(3)

	  acl_add_perm(3), acl_calc_mask(3), acl_clear_perms(3),
	  acl_delete_perm(3), acl_get_permset(3), acl_set_permset(3)

	  acl_get_qualifier(3), acl_get_tag_type(3), acl_set_qualifier(3),
	  acl_set_tag_type(3)

     ACL manipulation on an object
	  acl_delete_def_file(3), acl_get_fd(3), acl_get_file(3),
	  acl_set_fd(3), acl_set_file(3)

     ACL format translation
	  acl_copy_entry(3), acl_copy_ext(3), acl_from_text(3),
	  acl_to_text(3), acl_size(3)

   POSIX 1003.1e FUNCTIONS BY AVAILABILITY
     The first group of functions is supported on most systems with POSIX-like
     access control lists, while the second group is supported on fewer sys-
     tems.  For applications that will be ported the second group is best
     avoided.

     acl_delete_def_file(3), acl_dup(3), acl_free(3), acl_from_text(3),
     acl_get_fd(3), acl_get_file(3), acl_init(3), acl_set_fd(3),
     acl_set_file(3), acl_to_text(3), acl_valid(3)

     acl_add_perm(3), acl_calc_mask(3), acl_clear_perms(3), acl_copy_entry(3),
     acl_copy_ext(3), acl_copy_int(3), acl_create_entry(3),
     acl_delete_entry(3), acl_delete_perm(3), acl_get_entry(3),
     acl_get_permset(3), acl_get_qualifier(3), acl_get_tag_type(3),
     acl_set_permset(3), acl_set_qualifier(3), acl_set_tag_type(3),
     acl_size(3)

   LINUX EXTENSIONS
     These non-portable extensions are available on Linux systems.

     acl_check(3), acl_cmp(3), acl_entries(3), acl_equiv_mode(3),
     acl_error(3), acl_extended_fd(3), acl_extended_file(3),
     acl_extended_file_nofollow(3), acl_from_mode(3), acl_get_perm(3),
     acl_to_any_text(3)

AUTHOR
     Andreas Gruenbacher, <a.gruenbacher@bestbits.at>

Linux ACL			March 23, 2002			     Linux ACL