acl(5)

NAME

acl — introduction to HFS access control lists (ACLs)

DESCRIPTION

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 nonprivileged 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 interprocess communication (IPC) objects.

This manpage describes ACLs as implemented on HFS file systems only. See aclv(5) for a description of ACLs in JFS file systems.

Definitions

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, and so forth) and interprocess communication (IPC) features (shared memory, message queues, semaphores, sockets).
access control list (ACL): An access control list is a set of user, group, and mode entries associated with a file that specify permissions for all possible user ID annd group ID combinations.
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

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:

-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

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

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

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).
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 multiline 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

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

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 nonnull 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 ORed together. Access is granted if the resulting mode bits allow it. 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)

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.

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)

(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.

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 user-ID/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)

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

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:

=: 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

The system calls setacl(2) and getacl(2) 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(3C): Add, modify, or delete a single ACL entry in one file's ACL.
cpacl(3C), fcpacl(3C): 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).

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, chmod, and chown

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 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

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.

chacl '@.% = 5, %.% + xwx' myfile 

Short Form

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

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

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:

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

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: Nonspecific user ID
ACL_NSGROUP: Nonspecific 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
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 
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

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.

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

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

The access control list design described here was developed by HP.

FILES

<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.

acl(5)

Technical documentation

» Table of Contents

» Index

NAME

DESCRIPTION

Definitions

Access Control List Entries

Base ACL Entries

Optional ACL entries

ACL Notation

ACL Uniqueness

Access Check Algorithm

Operator Form of ACLs (input only)

Short Form of ACLs (input and output)

Long Form of ACLs (output only)

ACL Patterns

ACL Operations Supported

ACL Interaction with stat, chmod, and chown

EXAMPLES

Operator Form

Short Form

Long Form

ACL Patterns

HEADERS

Header <sys/acl.h>

Header <sys/getaccess.h>

Header <acllib.h>

WARNINGS

DEPENDENCIES

NFS

AUTHOR

FILES

SEE ALSO