ACL(5) Standards, Environments, and Macros ACL(5)

acl - Access Control Lists

Access control lists (ACLs) are discretionary access control mechanisms that grant and deny access to files and directories. Two different ACL models are supported in this release: POSIX-draft ACLs and NFSv4 ACLs.

The older, POSIX-draft model is supported by the UFS file system. This model is based on a withdrawn ACL POSIX specification that was never standardized. It was subsequently withdrawn by the POSIX committee.

The other model is based on the standards of the NFSv4 working group and is an approved standard from the Internet Engineering Task Force (IETF). The ZFS file system uses the NFSv4 model, and provides richer semantics and finer grained permission capabilities than the POSIX-draft model.

POSIX-draft ACLs provide an alternative security mechanism to basic UNIX file permissions. Their purpose is to further restrict access to files and directories or to extend permissions to a particular user. ACLs can be used to change the permissions for the standard owner, group and other class bits of a file's mode. ACLs can give additional users and groups access to the file. A directory can also have a special kind of ACL called a default ACL, which defines ACL entries to be inherited by descendents of the directory. POSIX-draft ACLs have an ACL entry called mask. The mask defines the maximum permissions that can be granted to additional user and group entries. Whenever a file is created or its mode is changed by chmod(1) or chmod(2), the mask is recomputed. It is recomputed to be the group permission defined in the mode passed to chmod(2).

The POSIX-draft ACL model uses the standard rwx model of traditional UNIX permissions.

An ACL is represented as follows:


acl_entry[,acl_entry]...

Each acl_entry contains one ACL entry. An ACL entry is represented by two or three colon-separated(:) fields.

user:[uid]:perms

If uid blank, it represents the file owner.

group:[gid]:perms

If gid is blank, it represents the owning group.

other:perms

Represents the file other class.

mask:perms

Defines the MAX permission to hand out.

For example to give user joe read and write permissions, the ACL entry is specified as:


user:joe:rw-

The NFSv4 ACL model is based loosely on the Windows NT ACL model. NFSv4 ACLs provide a much richer ACL model than POSIX-draft ACLs.

The major differences between NFSv4 and POSIX-draft ACLs are as follows:

o
NFSv4 ACLs provide finer grained permissions than the rwx model.
o
NFSv4 ACLs allow for both ALLOW and DENY entries.
o
NFSv4 ACLs provide a rich set of inheritance semantics. POSIX ACLs also have inheritance, but with the NFSv4 model you can control the following inheritance features:
o
Whether inheritance cascades to both files and directories or only to files or directories.
o
In the case of directories, you can indicate whether inheritance is applied to the directory itself, to just one level of subdirectories, or cascades to all subdirectories of the directory.
o
NFSv4 ACLs provide a mechanism for hooking into a system's audit trail. Currently, illumos does not support this mechanism.
o
NFSv4 ACLs enable administrators to specify the order in which ACL entries are checked. With POSIX-draft ACLs the file system reorders ACL entries into a well defined, strict access, checking order.

POSIX-draft ACL semantics can be achieved with NFSv4 ACLs. However, only some NFSv4 ACLs can be translated to equivalent POSIX-draft ACLs.

Permissions can be specified in three different chmod ACL formats: verbose, compact, or positional. The verbose format uses words to indicate that the permissions are separated with a forward slash (/) character. Compact format uses the permission letters and positional format uses the permission letters or the hyphen (-) to identify no permissions.

The permissions for verbose mode and their abbreviated form in parentheses for compact and positional mode are described as follows:

read_data (r)

Permission to read the data of the file

list_directory (r)

Permission to list the contents of a directory.

write_data (w)

Permission to modify a file's data anywhere in the file's offset range. This includes the ability to grow the file or write to any arbitrary offset.

add_file (w)

Permission to add a new file to a directory.

append_data (p)

The ability to modify the file's data, but only starting at EOF. Currently, this permission is not supported.

add_subdirectory (p)

Permission to create a subdirectory to a directory.

read_xattr (R)

The ability to read the extended attributes of a file or do a lookup in the extended attributes directory.

write_xattr (W)

The ability to create extended attributes or write to the extended attributes directory.

execute (x)

Permission to execute a file.

read_attributes (a)

The ability to read basic attributes (non-ACLs) of a file. Basic attributes are considered to be the stat level attributes. Allowing this access mask bit means that the entity can execute ls(1) and stat(2).

write_attributes (A)

Permission to change the times associated with a file or directory to an arbitrary value.

delete (d)

Permission to delete the file.

delete_child (D)

Permission to delete a file within a directory.

read_acl (c)

Permission to read the ACL.

write_acl (C)

Permission to write the ACL or the ability to execute chmod(1) or setfacl(1).

write_owner (o)

Permission to change the owner or the ability to execute chown(1) or chgrp(1).

synchronize (s)

Permission to access a file locally at the server with synchronous reads and writes. Currently, this permission is not supported.

The following inheritance flags are supported by NFSv4 ACLs:

file_inherit (f)

Inherit to all newly created files in a directory.

dir_inherit (d)

Inherit to all newly created directories in a directory.

inherit_only (i)

Placed on a directory, but does not apply to the directory itself, only to newly created files and directories. This flag requires file_inherit and/or dir_inherit to indicate what to inherit.

no_propagate (n)

Placed on directories and indicates that ACL entries should only be inherited one level of the tree. This flag requires file_inherit and/or dir_inherit to indicate what to inherit.

successful_access (S)

Indicates whether an alarm or audit record should be initiated upon successful accesses. Used with audit/alarm ACE types.

failed_access (F)

Indicates whether an alarm or audit record should be initiated when access fails. Used with audit/alarm ACE types.

inherited (I)

ACE was inherited.

-

No permission granted.

An NFSv4 ACL is expressed using the following syntax:


acl_entry[,acl_entry]...
    owner@:<perms>[:inheritance flags]:<allow|deny>
    group@:<perms>[:inheritance flags]:<allow|deny>
    everyone@:<perms>[:inheritance flags]:<allow|deny>
    user:<username>:<perms>[:inheritance flags]:<allow|deny>
    usersid:<sid string>:<perms>[:inheritance flags]:<allow|deny>
    group:<groupname>:<perms>[:inheritance flags]:<allow|deny>
    groupsid:<sid string>:<perms>[:inheritance flags]:<allow|deny>
    sid:<sid string>:<perms>[:inheritance flags]:<allow|deny>

owner@

File owner

group@

Group owner

user

Permissions for a specific user

group

Permissions for a specific group

Permission and inheritance flags are separated by a / character.

ACL specification examples:


user:fred:read_data/write_data/read_attributes:file_inherit:allow
owner@:read_data:allow,group@:read_data:allow,user:tom:read_data:deny

Using the compact ACL format, permissions are specified by using 14 unique letters to indicate permissions.

Using the positional ACL format, permissions are specified as positional arguments similar to the ls -V format. The hyphen (-), which indicates that no permission is granted at that position, can be omitted and only the required letters have to be specified.

The letters above are listed in the order they would be specified in positional notation.

With these letters you can specify permissions in the following equivalent ways.


user:fred:rw------R------:file_inherit:allow

Or you can remove the - and scrunch it together.


user:fred:rwR:file_inherit:allow

The inheritance flags can also be specified in a more compact manner, as follows:


user:fred:rwR:f:allow
user:fred:rwR:f------:allow

Several utilities support the manipulation of ACLs. The following utilities accommodate both ACL models:

chmod

The chmod utility has been enhanced to allow for the setting and deleting of ACLs. This is achieved by extending the symbolic-mode argument to support ACL manipulation. See chmod(1) for details.

compress

When a file is compressed any ACL associated with the original file is preserved with the compressed file.

cp

By default, cp ignores ACLs, unless the -p option is specified. When -p is specified the owner and group id, permission modes, modification and access times, ACLs, and extended attributes if applicable are preserved.

cpio

ACLs are preserved when the -P option is specified.

find

Find locates files with ACLs when the -acl flag is specified.

ls

By default ls does not display ACL information. When the -v option is specified, a file's ACL is displayed.

mv

When a file is moved, all attributes are carried along with the renamed file. When a file is moved across a file system boundary, the ACLs are replicated. If the ACL information cannot be replicated, the move fails and the source file is not removed.

pack

When a file is packed, any ACL associated with the original file is preserved with the packed file.

rcp

rcp has been enhanced to support copying. A file's ACL is only preserved when the remote host supports ACLs.

tar

ACLs are preserved when the -p option is specified.

unpack

When a file with an ACL is unpacked, the unpacked file retains the ACL information.

The primary interfaces required to access file system ACLs at the programmatic level are the acl_get() and acl_set() functions. These functions support both POSIX-draft ACLs and NFSv4 ACLs. 


int acl_get(const char *path, int flag, acl_t **aclp);
int facl_get(int fd, int flag, acl_t **aclp);

The acl_get(3SEC) and facl_get(3SEC) functions retrieve an ACL on a file whose name is given by path or referenced by the open file descriptor fd. The flag argument specifies whether a trivial ACL should be retrieved. When the flag argument equals ACL_NO_TRIVIAL only ACLs that are not trivial are retrieved. The ACL is returned in the aclp argument.


void acl_free(acl_t *aclp);

The acl_free() function frees up memory allocated for the argument aclp.


int acl_set(const char *path, acl_t *aclp);
int facl_set(int fd, acl_t *aclp);

The acl_set(3SEC) and facl_get(3SEC) functions are used for setting an ACL on a file whose name is given by path or referenced by the open file descriptor fd. The aclp argument specifies the ACL to set. The acl_set(3SEC) function translates a POSIX-draft ACL into a NFSv4 ACL when the target file system supports NFSv4 ACLs. No translation is performed when trying to set an NFSv4 ACL on a POSIX-draft ACL supported file system.


int acl_trivial(const char *path);

The acl_trivial() function is used to determine whether a file has a trivial ACL.


int acl_strip(const char *path, uid_t uid, gid_t gid, mode_t mode);

The acl_strip() function removes all ACLs from a file and replaces them with a trivial ACL based off of the passed in argument mode. After replacing the ACL the owner and group of the file are set to the values specified in the uid and gid parameters.


int acl_fromtext(const char *path, acl_t **aclp);
char *acl_totext(acl_t *aclp, int flags);

The acl_totext() function converts an internal ACL representation pointed to by aclp into an external representation. See DESCRIPTION for details about external representation.

The acl_fromtext() function converts an external representation into an internal representation. See DESCRIPTION for details about external representation.

The following examples demonstrate how the API can be used to perform basic operations on ACLs.

Example 1 Retrieving and Setting an ACL

Use the following to retrieve an ACL and set it on another file:


error = acl_get("file", ACL_NO_TRIVIAL, &aclp);
if (error == 0 && aclp != NULL) {



error = acl_set("file2", aclp);
acl_free(aclp);



}
...

Example 2 Retrieving and Setting Any ACLs

Use the following to retrieve any ACL, including trivial ACLs, and set it on another file:


error = acl_get("file3", 0, &aclp);
if (error == 0) {



error = acl_set("file4", aclp);
acl_free(aclp);



}
...

Example 3 Determining if a File has a Trivial ACL

Use the following to determine if a file has a trivial ACL:


char *file = "file5";
istrivial = acl_trivial(file);
if (istrivial == 0)



printf("file %s has a trivial ACL\n", file);



else



printf("file %s has a NON-trivial ACL\n", file);



...

Example 4 Removing all ACLs from a File

Use the following to remove all ACLs from a file, and set a new mode, owner, and group:


error = acl_strip("file", 10, 100, 0644);
...

chgrp(1), chmod(1), chown(1), cp(1), cpio(1), find(1), ls(1), mv(1), tar(1), setfacl(1), chmod(2), acl(2), stat(2), acl_get(3SEC), aclsort(3SEC), acl_fromtext(3SEC), acl_free(3SEC), acl_strip(3SEC), acl_trivial(3SEC)
February 8, 2020