ACL(5) 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 di-
rectories.
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 di-
rectory; 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 ac-
cess permissions on the associated object for an individual user or a
group of users as a combination of read, write and search/execute per-
missions.
An ACL entry contains an entry tag type, an optional entry tag quali-
fier, 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
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 en-
try.
The file owner, group, and other permissions always match the permis-
sions of the corresponding ACL entry. Modification of the file permis-
sion 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
The access ACL of a file object is initialized when the object is cre-
ated with any of the creat(), mkdir(), mknod(), mkfifo(), or open()
functions. If a default ACL is associated with a directory, the mode pa-
rameter 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 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
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 en-
try 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 re-
quested 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, ac-
cess 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 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 representation, permis-
sions 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 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 permis-
sions 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 ab-
breviation for group is g, the abbreviation for mask is m, and the ab-
breviation 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 compli-
ant with IEEE 1003.1 (“POSIX.1”) will still function as expected on sys-
tems 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:
• 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
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 util-
ities 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-spe-
cific extensions to these functions are declared in the <acl/libacl.h>
header.
NOTES
DENIED PERMISSIONS AND LINUX USER NAMESPACES
While ACLs can be used to deny processes permissions based on the groups
they are in, this is considered bad practice. Privileged helpers such
as newuidmap(1) can give unprivileged processes access to the
setgroups(2) system call, which allows them to drop supplementary group
membership and render restrictions based on that membership ineffective.
For further details, see user_namespaces(7).
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
https://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@gmail.com>
Linux ACL March 23, 2002 ACL(5)
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