NAME | DESCRIPTION | ACL TYPES | ACL ENTRIES | VALID ACLs | CORRESPONDENCE BETWEEN ACL ENTRIES AND FILE PERMISSION BITS | OBJECT CREATION AND DEFAULT ACLs | ACCESS CHECK ALGORITHM | ACL TEXT FORMS | RATIONALE | CHANGES TO THE FILE UTILITIES | STANDARDS | SEE ALSO | AUTHOR | COLOPHON

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

NAME         top

     acl — Access Control Lists

DESCRIPTION         top

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

ACL TYPES         top

     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         top

     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
     permissions.

     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
                           processes 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         top

     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         top

     The permissions defined by ACLs are a superset of the permissions
     specified by the file permission bits.

     There is a correspondence between the file owner, group, and other
     permissions 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 modification of these ACL entries results in the
     modification of the file permission bits.

OBJECT CREATION AND DEFAULT ACLs         top

     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 direc‐
          tory 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 con‐
          tained 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
          creation mask.

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

ACCESS CHECK ALGORITHM         top

     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 permis‐
                sions, 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 con‐
                tain 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 per‐
                      missions, 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         top

     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 speci‐
                   fied 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 iden‐
     tifier 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 representa‐
     tion, 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 charac‐
     ter).

   LONG TEXT FORM
     The long text form contains one ACL entry per line. In addition, a num‐
     ber 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 permissions 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 key‐
     words 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 following 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         top

     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         top

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

     ·   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 per‐
         mission string.

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

     ·     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         top

     The IEEE 1003.1e draft 17 (“POSIX.1e”) document describes several
     security 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
     utilities defined for Access Control Lists in POSIX.1e, and several
     extensions.  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 compliant 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         top

     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
     systems.  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         top

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

COLOPHON         top

     This page is part of the acl (manipulating access control lists)
     project.  Information about the project can be found at
     http://savannah.nongnu.org/projects/acl.  If you have a bug report for
     this manual page, see http://savannah.nongnu.org/bugs/?group=acl.  This
     page was obtained from the project's upstream Git repository
     git://git.savannah.nongnu.org/acl.git on 2017-07-05.  If you discover
     any rendering problems in this HTML version of the page, or you believe
     there is a better or more up-to-date source for the page, or you have
     corrections or improvements to the information in this COLOPHON (which
     is not part of the original manual page), send a mail to
     man-pages@man7.org

Linux ACL                      March 23, 2002                      Linux ACL

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