acl(5) acl(5)
NAME [Toc] [Back]
acl - introduction to HFS access control lists
DESCRIPTION [Toc] [Back]
Access control lists are a key enforcement mechanism of discretionary
access control (see Definitions below), for specifying access to files
by users and groups more selectively than traditional HP-UX mechanisms
allow.
HP-UX already enables non-privileged users or processes, such as file
owners, to allow or deny other users access to files and other objects
on a ``need to know'' basis, as determined by their user and/or group
identity (see passwd(4) and group(4)). This level of control is
accomplished by setting or manipulating a file's permission bits to
grant or restrict access by owner, group, and others (see chmod(2)).
ACLs offer a greater degree of selectivity than permission bits. ACLs
allow the file owner or superuser to permit or deny access to a list
of users, groups, or combinations thereof.
ACLs are supported as a superset of the UNIX operating system
discretionary access control (DAC) mechanism for files, but not for
other objects such as inter-process communication (IPC) objects.
This manual page describes ACLs as implemented on HFS file systems
only. See aclv(5) for a description of ACLs in JFS file systems.
Definitions [Toc] [Back]
Because control of access to data is a key concern of computer
security, we provide the following definitions, based on those of the
Department of Defense Trusted Computer System Evaluation Criteria, to
explain further both the concepts of access control and its relevance
to HP-UX security features:
access ``A specific type of interaction between a subject
and an object that results in the flow of
information from one to the other.'' Subjects
include ``persons, processes, or devices that
cause information to flow among objects or change
the system state.'' Objects include files
(ordinary files, directories, special files,
FIFOs, etc.) and inter-process communication (IPC)
features (shared memory, message queues,
semaphores, sockets).
access control list (ACL)
An access control list is a set of (user.group,
mode) entries associated with a file that specify
permissions for all possible user-ID/group-ID
combinations.
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access control list (ACL) entry
An entry in an ACL that specifies access rights
for one user and group ID combination.
change permission
The right to alter DAC information (permission
bits or ACL entries). Change permission is
granted to object (file) owners and to privileged
users.
discretionary access control (DAC)
``A means of restricting access to objects based
on the identity of subjects and/or groups to which
they belong. The controls are discretionary in
the sense that a subject with a certain access
permission is capable of passing that permission
(perhaps indirectly) to any other subject.''
mode Three bits in each ACL entry which represent read,
write, and execute/search permissions. These bits
may exist in addition to the 16 mode bits
associated with every file in the file system (see
glossary(9)).
privilege The ability to ignore access restrictions and
change restrictions imposed by security policy and
implemented in an access control mechanism. In
HP-UX, superusers and members of certain groups
(see privgrp(4)) are the only privileged users.
restrictive versus permissive
An individual ACL entry is considered restrictive
or permissive, depending on context. Restrictive
entries deny a user and/or group access that would
otherwise be granted by less-specific base or
optional ACL entries (see below). Permissive
entries grant a user and/or group access that
would otherwise be denied by less-specific base or
optional ACL entries.
Access Control List Entries [Toc] [Back]
An access control list (ACL) consists of sets of (user.group, mode)
entries associated with a file that specify permissions. Each entry
specifies for one user-ID/group-ID combination a set of access
permissions, including read, write, and execute/search.
To help understand the relationship between access control lists and
traditional file permissions, consider the following file and its
permissions:
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-rwxr-xr-- james admin datafile
The file owner is user james.
The file's group is admin.
The name of the file is datafile.
The file owner permissions are rwx.
The file group permissions are r-x.
The file other permissions are r--.
In an ACL, user and group IDs can be represented by names or numbers,
found in /etc/passwd. The following special symbols can also be used:
% Symbol representing no specific user or group.
@ Symbol representing the current file owner or group.
Base ACL Entries [Toc] [Back]
When a file is created, three base access control list entries are
mapped from the file's access permission bits to match a file's owner
and group and its traditional permission bits. Base ACL entries can
be changed by the chmod(2) and setacl(2) system calls.
(uid.%,mode) Base ACL entry for the file's owner
(%.gid,mode) Base ACL entry for the file's group
(%.%,mode) Base entry for other users
(Except where noted, examples are represented in short form notation.
See ACL Notation, below.)
Optional ACL entries [Toc] [Back]
Optional access control list entries contain additional access control
information, which the user can set with the setacl(2) system call to
further allow or deny file access. Up to thirteen additional
user/group combinations can be specified.
For example, the following optional access control list entries can be
associated with our file:
(mary.admin, rwx) Grant read, write, and execute access to user
mary in group admin.
(george.%, ---) Deny any access to user george in no specific
group.
ACL Notation [Toc] [Back]
Supported library calls and commands that manage ACLs recognize three
different symbolic representations:
operator form For input of entire ACLs and modifications to existing
ACLs, in a syntax similar to that used by chmod(1).
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short form Easier to read, intended primarily for output.
chacl(1) accepts this form as input so that it can
interpret output from lsacl(1).
long form A multi-line format useful for greater clarity, and
supported only for output.
For our example file, the base ACL entries could be represented in the
three notations as follows:
operator form james.% = rwx, %.admin = rx, %.% = r
short form (james.%,rwx) (%.admin,r-x) (%.%,r--)
long form rwx james.%
r-x %.admin
r-- %.%
In addition to basic ACL usage, some library calls and commands
understand and use a variation of operator and short forms. See the
section below on ACL Patterns.
ACL Uniqueness [Toc] [Back]
Entries are unique in each ACL. There can only be one (u.g, mode)
entry for any pair of u and g values; one (u.%, mode) entry for a
given value of u; one (%.g, mode) entry for a given value of g; and
one (%.%, mode) entry for each file. For example, an ACL can have a
(23.14, mode) entry and a (23.%, mode) entry, but not two (23.14,
mode) entries or two (23.%, mode) entries.
Access Check Algorithm [Toc] [Back]
ACL entries can be categorized by four levels of specificity. In
access checking, ACLs are compared to the effective user and group IDs
in this order:
(u.g, rwx) specific user, specific group
(u.%, rwx) specific user, no specific group
(%.g, rwx) no specific user, specific group
(%.%, rwx) no specific user, no specific group
Once an entry for the combination of a process effective user ID and
effective group ID (or any supplementary group ID) is matched, no
further (that is, less specific) entries are checked. More specific
entries that match take precedence over any less specific ones that
also match.
If a process has more than one group ID (that is, a non-null
supplementary groups list), more than one (u.g, mode) or (%.g, mode)
entry might apply for that process. If so, the access modes in all
matching entries (of the same level of specificity, u.g or %.g) are
OR'd together. Access is granted if the resulting mode bits allow it.
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Since entries are unique, the order of entries in each entry type is
insignificant.
Because the traditional UNIX permission bits are mapped into base ACL
entries, they are included in access checks.
If a request is made for more than one type of access, such as opening
a file for both reading and writing, access is granted only if the
process is allowed all requested types of access. Note that access
can be granted if the process has two groups in its groups list, one
of which is only allowed read access, and the other of which is only
allowed write access. In other words, even if the requested access is
not granted by any one entry, it may be granted by a combination of
entries due to the process belonging to several groups.
Operator Form of ACLs (input only) [Toc] [Back]
user. group operator mode [ operator mode ]... , ...
Multiple entries are separated by commas, as in chmod(1). Each entry
consists of a user identifier and group identifier followed by one or
more operators and mode characters, as in the mode syntax accepted by
chmod(1).
The entire ACL must be a single argument, and thus should be quoted to
the shell if it contains whitespace or special characters. Whitespace
is ignored except within names. A null ACL is legitimate, and means
either ``no access'' or ``no changes'', depending on context.
Each user or group ID may be represented by:
name Valid user or group name.
number Valid numeric ID value.
% ``No specific user or group,'' as appropriate.
@ ``Current file owner or group,'' as appropriate; useful
for referring to a file's u.% and %.g base ACL entries.
An operator is always required in each entry. Operators are:
= Set all bits in the entry to the given mode value.
+ Set the indicated mode bits in the entry.
- Clear the indicated mode bits in the entry.
The mode is represented by an octal value of 0 through 7; or any
combination of r, w, and x can be given in any order (see EXAMPLES
below). A null mode denies access if the operator is =, or represents
``no change'' if the operator is + or -.
Multiple entries and multiple operator-mode parts in an entry are
applied in the order specified. Conflicts do not result in error; the
last specified entry or operator takes effect. Entries need not
appear in any particular order.
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Note that chmod(1) allows only u, g, o, or a to refer symbolically to
the file owner, group, other, or all users, respectively. Since ACLs
work with arbitrary user and group identifiers, @ is provided as a
convenience.
The exact syntax is:
acl ::= [entry[,entry]...]
entry ::= id . id op mode [op mode]...
id ::= name | number | % | @
op ::= = | + | -
mode ::= 0..7 | [char[char]...]
char ::= r | w | x
Short Form of ACLs (input and output) [Toc] [Back]
(user . group, mode) ...
Short form differs from operator form in several ways:
+ Entries are surrounded by parentheses rather than being separated
by commas.
+ Each entry specifies the mode, including all mode bits. It is
not possible to change the mode value with + and - operators.
However, the comma functions like the = operator in operator
form.
+ For clarity, hyphens represent unset permission bits in the
output of the mode field and are allowed in input. This
resembles the mode output style used by ls(1).
Multiple entries are concatenated. For consistency with operator
form, a dot (.) is used to separate user and group IDs.
On output, no whitespace is printed except in names (if any). ID
numbers are printed if no matching names are known. Either ID can be
printed as % for ``no specific user or group.'' The mode is
represented as <r|-><w|-><x|->, that is, it always has three
characters, padded with hyphens for unset mode bits. If the ACL is
read from the system, entries are ordered by specificity, then by
numeric values of ID parts.
On input, the entire ACL must be a single argument, and thus should be
quoted to the shell if it contains whitespace or special characters.
Whitespace is ignored except within names. A null ACL is legitimate,
and means either ``no access'' or ``no changes'', depending on
context.
User and group IDs are represented as in operator form.
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The mode is represented by an octal value of 0 through 7; or any
combination of r, w, x and - (ignored) can be given in any order (see
EXAMPLES below). A null mode denies access.
Redundancy does not result in error; the last entry for any userID/group-ID
combination takes effect. Entries need not appear in any
particular order.
The exact syntax is:
acl ::= [entry[entry]...]
entry ::= (id.id,mode)
id ::= name | number | % | @
mode ::= 0..7 | [char[char]...]
char ::= r | w | x | -
Long Form of ACLs (output only) [Toc] [Back]
mode user . group
Each entry occupies a single line of output. The mode appears first
in a fixed-width field, using hyphens (for unset mode bits) for easy
vertical scanning. Each user and group ID is shown as a name if
known, a number if unknown, or % for ``no specific user or group.''
Entries are ordered from most to least specific, then by numeric
values of ID parts.
Note that every ACL printed has at least three entries, the base ACL
entries (that is, uid.%, %.gid, and %.%).
The exact syntax is:
acl ::= entry[<newline>entry]...
entry ::= mode<space>id.id
mode ::= <r|-><w|-><x|->
id ::= name | number | %
ACL Patterns [Toc] [Back]
Some library calls and commands recognize and use ACL patterns instead
of exact ACLs to allow operations on all entries that match the
patterns. ACL syntax is extended in the following ways:
wildcard user and group IDs
A user or group name of * (wildcard) matches the user
or group ID in any entry, including % (no specific user
or group).
mode bits on, off, or ignored
For operator-form input, the operators =, +, and - are
applied as follows:
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= entry mode value matches this mode value exactly
+ these bits turned on in entry mode value
- these bits turned off in entry mode value
When only + and - operators are used, commands ignore
the values of unspecified mode bits.
Short-form patterns treat the mode identically to the =
operator in operator form.
wildcard mode values
A mode of * (wildcard) in operator or short form input
(for example, ``ajs.%=*'' or ``(ajs.%,*)'') matches any
mode value, provided no other mode value is given in a
operator-form entry. Also, the mode part of an entry
can be omitted altogether for the same effect.
entries not combined
Entries with matching user and group ID values are not
combined. Each entry specified is applied separately
by commands that accept patterns.
ACL Operations Supported [Toc] [Back]
The system calls setac
allow setting or getting the
entire ACL for a file in the form of an array of acl_entry structures.
To check access rights to a file, see access(2) and getaccess(2).
Various library calls are provided to manage ACLs:
acltostr(3C) Convert acl_entry arrays to printable strings.
strtoacl(3C) Parse and convert ACL strings to acl_entry arrays.
strtoaclpatt(3C)
Parse and convert ACL pattern strings to acl_entry_patt
arrays.
setaclentry(3C)
fsetaclentry Add, modify, or delete a single ACL entry in one file's
ACL.
cpacl(3C)
fcpacl Copy an ACL and file miscellaneous mode bits (see
chmod(2)) from one file to another, transfer ownership
if needed (see below), and handle remote files
correctly.
chownacl(3C) Change the file owner and/or group represented in an
ACL, that is, transfer ownership (see below).
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The following commands are available to manage ACLs and permissions:
chacl(1) Add, modify, or delete individual entries or all
optional entries in ACLs on one or more files, remove
all access to files, or incorporate ACLs into
permission bits.
lsacl(1) List ACLs on files.
chmod(1) Change permission bits and other file miscellaneous
mode bits.
ls(1) In long form, list permission bits and other file
attributes.
find(1) Find files according to their attributes, including
ACLs.
getaccess(1) List access rights to file(s).
ACL Interaction with stat(2), chmod(2), and chown(2)
stat The st_mode field summarizes the caller's access rights to the
file. It differs from file permission bits only if the file
has one or more optional entries applicable to the caller.
The st_basemode field provides the file's actual permission
bits. The st_acl field indicates the presence of optional ACL
entries in the file's ACL.
The st_mode field contains a user-dependent summary, so that
programs ignorant of ACLs that use stat(2) and chmod(2) are
more likely to produce expected results, and so that stat(2)
provides reasonable information about remote files over NFS.
The st_basemode and st_acl fields are useful only for local
files.
chmod For conformance with IEEE Standard POSIX 1003.1-1988, chmod(2)
deletes any optional entries in a file's ACL. Unfortunately,
since chmod(2) is used to set file miscellaneous mode bits as
well as permission bits, extra effort is required in some
cases to preserve a file's ACL.
chown If the new owner and/or group of a file does not already have
an optional (u.%, mode) and/or (%.g, mode) entry in the file's
ACL, it inherits the old owner's and/or group's file access
permission bits and base ACL entry:
(id1,mode1) -> (id2,mode1)
This is the traditional behavior. However, if the new owner
and/or group of a file already has an optional (u.%, mode)
and/or (%.g, mode) entry in the file's ACL, the ACL does not
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change:
(id1, mode1) -> (id1, mode1)
(id2, mode2) -> (id2, mode2)
Existing access information in the ACL is preserved. However,
because the old optional ACL entry becomes the new base ACL
entry and vice versa, the file's access permission bits
change.
Transferring ownership of ACLs by chown(2) allows a file to be
transferred to a different user or group, or copied by a
different user or group than the owner (using cpacl(3C) or
chownacl(3C)), and later returned to the original owner or
group without net changes to its ACL. The extra complexity is
necessary because:
+ ACLs are a backward-compatible superset of permission
bits (which are coupled to file owner and group IDs), not
a replacement for them.
+ it enables users and programs that deal with ACLs to do
so simply, rather than with a combination of permission
bits and ACL entries. Also, the access check algorithm
is simpler and more symmetrical; permission bits do not
``eclipse'' or ``mask'' ACL entries.
EXAMPLES [Toc] [Back]
Operator Form
The following sets the %.% entry to restrict ``other'' users to only
reading the file.
chacl '%.% = r' myfile
The following allows user ``bill'' in any group to write the file,
assuming that no restrictive entry is more specific than the bill.%
entry (for example, a bill.adm entry that denies writing).
chacl 'bill.% +w' myfile
The following ACL specification contains two entries. The first one
deletes write and adds read capability to the entry for user 12, group
4. The second entry denies access for any unspecified user in any
unspecified group.
chacl '12.4-w+r, %.% =' myfile
The following pair of entries sets the u.% entry for the file's owner
to allow both read and execute and results in adding write and execute
capabilities for ``other'' users (the ``%.%'' entry). Note that a
mode character is purposely repeated for illustration purposes.
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chacl '@.% = 5, %.% + xwx' myfile
Short Form [Toc] [Back]
Here is a typical ACL as it might be printed. It allows user jpc to
read or execute the file while in group adm; it denies user ajs access
to the file while in group trux; it allows user jpc in any group
(except adm) to only read the file; any other user in group bin may
read or execute the file; and any other user may only read the file.
(jpc.adm,r-x)(ajs.trux,---)(jpc.%,r--)(%.bin,r-x)(%.%,r--)
The following allows ``other'' users to only read the file.
chacl '(%.%,r)' myfile
The following sets write-only access for user bill in any group.
chacl '(bill.%,-w-)' myfile
The following sets the entry for user 12 in group 4 to allow read and
write.
chacl '(12.4,wr)' myfile
The following sets the base ACL entry for the file's owner to allow
both read and execute, and sets write and execute capabilities for
``other'' users (the ``%.%'' entry).
chacl '(@.%, 5) (%.%, xwx)' myfile
Long Form [Toc] [Back]
Here is the same ACL as in an earlier example, printed in long form.
r-x jpc.adm
--- ajs.trux
r-- jpc.%
r-x %.bin
r-- %.%
ACL Patterns [Toc] [Back]
The following command locates files whose ACLs contain an entry that
allows read access and denies write access to some user/group
combination.
find / -acl '*.*+r-w' -print
The following matches entries for any user in group bin and for user
tammy in any group, regardless of the entries' mode values. Matching
optional ACL entries are deleted and mode values in matching base ACL
entries are set to zero:
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chacl -d '%.bin, tammy.*=*' myfile
The following matches all entries, deleting optional entries and
setting mode values of base ACL entries to zero:
chacl -d '(*.*,*)' myfile
HEADERS [Toc] [Back]
Header <sys/acl.h>
The <sys/acl.h> header file defines the following constants to govern
the numbers of entries per ACL:
NACLENTRIES maximum number of entries per ACL,
including base entries
NBASEENTRIES number of base entries
NOPTENTRIES number of optional entries
The ACL entry structure structacl_entry is also defined, and includes
the following members:
aclid_t uid; /* user ID */
aclid_t gid; /* group ID */
aclmode_t mode; /* see <unistd.h> */
The <sys/acl.h> header also defines the types aclid_t and aclmode_t.
Non-specific user and group ID values:
ACL_NSUSER non-specific user ID
ACL_NSGROUP non-specific group ID
A special nentries value ACL_DELOPT is used with setacl(2) to delete
optional entries.
Header <sys/getaccess.h>
The <sys/getaccess.h> header defines constants for use with
getaccess(2).
Special parameter values for uid:
UID_EUID use effective user ID
UID_RUID use real user ID
UID_SUID use saved user ID
Special parameter values for ngroups:
NGROUPS_EGID process's effective gid
NGROUPS_RGID process's real gid
NGROUPS_SGID process's saved gid
NGROUPS_SUPP process's supplementary groups only
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NGROUPS_EGID_SUPP process's eff gid plus supp groups
NGROUPS_RGID_SUPP process's real gid plus supp groups
NGROUPS_SGID_SUPP process's saved gid plus supp groups
Header <acllib.h>
The <acllib.h> header file defines several constants for use with ACL
support library calls.
Symbolic forms of ACLs for acltostr():
FORM_SHORT [Toc] [Back]
FORM_LONG
Magic values for various calls:
ACL_FILEOWNER file's owner ID
ACL_FILEGROUP file's group ID
ACL_ANYUSER wildcard user ID
ACL_ANYGROUP wildcard group ID
MODE_DEL delete one ACL entry
Mask for valid mode bits in ACL entries:
MODEMASK (R_OK | W_OK | X_OK)
The <acllib.h> header also defines the struct acl_entry_patt ACL
pattern entry structure, which includes the following members:
aclid_t uid; /* user ID */
aclid_t gid; /* group ID */
aclmode_t onmode; /* mode bits that must be on */
aclmode_t offmode; /* mode bits that must be off */
WARNINGS [Toc] [Back]
ACLs are intended for use on ordinary files and directories. Optional
ACL entries are not recommended on files that are manipulated by
certain system utilities, such as terminal special files and LP
scheduler control files. These utilities might delete optional
entries, including those whose intent is restrictive, without warning
as a consequence of calling chmod(2), thereby increasing access
unexpectedly.
Most, but not all, supported utilities are able to handle ACLs
correctly. However, only the fbackup(1M) and frecover(1M) file
archive utilities handle access control lists properly. When using
programs (such as archive programs ar(1), cpio(1), ftio(1), tar(1),
and dump(1M)) unable to handle ACLs on files with optional ACL
entries, note the Access Control List information included on their
respective reference pages, to avoid loss of data.
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If a user name is defined in the /etc/passwd file or a group name is
defined in the /etc/group file as % or @, or for patterns, *, ACL
syntax cannot reference that name as itself because the symbols have
other meanings. However, such users or groups can still be referenced
by their ID numbers. User and/or group names must not include the
following characters:
. Do not use in user names.
+ Do not use in group names.
- Do not use in group names.
= Do not use for operator form input of group names.
, Do not use for short form or for operator form patterns.
) Do not use for short form patterns.
It is possible to specify an ACL pattern using the @ (file owner or
group) or * (wildcard) symbols so that it cannot match certain files,
perhaps depending on their ownership, by giving two entries, one with
specific values and the other using @ or *, which are equivalent for a
file but contain different mode values. For example:
find / -acl '(ajs.%,r)(@.%,rw)' -print
cannot match a file owned by ajs.
DEPENDENCIES [Toc] [Back]
NFS NFS does not support ACLs on remote files. Individual manual
entries specify the behavior of various system calls, library
calls, and commands under these circumstances. Be careful when
transferring a file with optional entries over a network or when
manipulating a remote file because optional entries may be
silently deleted.
AUTHOR [Toc] [Back]
The access control list design described here was developed by HP.
FILES [Toc] [Back]
<sys/acl.h> Header file that supports setacl(2) and
getacl(2).
<sys/getaccess.h> Header file that supports getaccess(2).
<acllib.h> Header file that supports ACL library
calls.
/etc/passwd Defines user names and user and group ID
values.
/etc/group Defines group names.
SEE ALSO [Toc] [Back]
chacl(1), chmod(1), cp(1), find(1), getaccess(1), ln(1), ls(1),
lsacl(1), mv(1), rm(1), fbackup(1M), frecover(1M), fsck(1M), fsdb(1M)
access(2), chmod(2), chown(2), creat(2), getaccess(2), getacl(2),
mknod(2), open(2), setacl(2), stat(2), acltostr(3C), chownacl(3C),
cpacl(3C), setaclentry(3C), strtoacl(3C), group(4), passwd(4),
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privgrp(4), aclv(5).
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