ACL(5) - man - phpman

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 directories.

ACL TYPES
     Every object can be thought of as having associated with it an ACL that governs the discre‐
     tionary access to that object; this ACL is referred to as an access ACL. In addition, a direc‐
     tory 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 permissions.

     An ACL entry contains an entry tag type, an optional entry tag qualifier, and a set of permis‐
     sions.  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 en‐
                           try'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
     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 di‐
     rectory, 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 specified by the file permis‐
     sion 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 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 associ‐
     ated ACL entries, and modification of these ACL entries results in the modification of the file
     permission 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 pa‐
          rameter.

     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 contained in the permissions specified by the mode pa‐
          rameter.

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 permis‐
                sions, 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 en‐
                      tries 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  permissions,  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 specified group (entry tag type ACL_GROUP).

           mask    A mask ACL entry specifies the maximum access which can be granted by any ACL en‐
                   try 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 en‐
     tries. A user identifier can be a user name or a user ID number in decimal form. A group iden‐
     tifier 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/exe‐
     cute 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 en‐
     try.  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 fol‐
     lowed 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 keywords may either appear in their full unabbrevi‐
     ated form, or in their single letter abbreviated form. The abbreviation for user is u, the ab‐
     breviation 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
     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 behav‐
     ior in the following way:

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

     ••   If the -p flag is specified, the cp(1) utility also preserves ACLs.  If this is not possi‐
         ble, 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 de‐
     scribed in CORRESPONDENCE BETWEEN ACL ENTRIES AND FILE PERMISSION BITS.

STANDARDS
     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 im‐
     plement 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 func‐
     tions are defined in the ACL library (libacl, -lacl). The POSIX compliant interfaces are de‐
     clared 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), mount(8), open(2),
     setfacl(1), stat(2), umask(1)

   POSIX 1003.1e DRAFT 17
     http://wt.tuxomania.net/publications/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
     Andreas Gruenbacher, <andreas.gruenbacher AT gmail.com>

Linux ACL                       March 23, 2002                       Linux ACL