In certain situations, the access permissions may be too restrictive. Therefore, Linux has additional settings that enable the temporary change of the current user and group identity for a specific action. For example, the passwd program normally requires root permissions to access /etc/passwd. This file contains some important information, like the home directories of users and user and group IDs. Thus, a normal user would not be able to change passwd, because it would be too dangerous to grant all users direct access to this file. A possible solution to this problem is the setuid mechanism. setuid (set user ID) is a special file attribute that instructs the system to execute programs marked accordingly under a specific user ID. Consider the passwd command:

-rwsr-xr-x  1 root shadow 80036 2004-10-02 11:08 /usr/bin/passwd

You can see the s that denotes that the setuid bit is set for the user permission. By means of the setuid bit, all users starting the passwd command execute it as root.

The setuid bit applies to users. However, there is also an equivalent property for groups: the setgid bit. A program for which this bit was set runs under the group ID under which it was saved, no matter which user starts it. Therefore, in a directory with the setgid bit, all newly created files and subdirectories are assigned to the group to which the directory belongs. Consider the following example directory:

drwxrws--- 2 tux archive 48 Nov 19 17:12  backup

You can see the s that denotes that the setgid bit is set for the group permission. The owner of the directory and members of the group archive may access this directory. Users that are not members of this group are “mapped” to the respective group. The effective group ID of all written files will be archive. For example, a backup program that runs with the group ID archive is able to access this directory even without root privileges.

There is also the sticky bit. It makes a difference whether it belongs to an executable program or a directory. If it belongs to a program, a file marked in this way is loaded to RAM to avoid needing to get it from the hard disk each time it is used. This attribute is used rarely, because modern hard disks are fast enough. If this bit is assigned to a directory, it prevents users from deleting each other's files. Typical examples include the /tmp and /var/tmp directories:

drwxrwxrwt 2 root root 1160 2002-11-19 17:15 /tmp

Figure 10.1, “Minimum ACL: ACL Entries Compared to Permission Bits” and Figure 10.2, “Extended ACL: ACL Entries Compared to Permission Bits” illustrate the two cases of a minimum ACL and an extended ACL. The figures are structured in three blocks—the left block shows the type specifications of the ACL entries, the center block displays an example ACL, and the right block shows the respective permission bits according to the conventional permission concept (for example, as displayed by ls -l). In both cases, the owner class permissions are mapped to the ACL entry owner. Other class permissions are mapped to the respective ACL entry. However, the mapping of the group class permissions is different in the two cases.

Figure 10.1. Minimum ACL: ACL Entries Compared to Permission Bits

In the case of a minimum ACL—without mask—the group class permissions are mapped to the ACL entry owning group. This is shown in Figure 10.1, “Minimum ACL: ACL Entries Compared to Permission Bits”. In the case of an extended ACL—with mask—the group class permissions are mapped to the mask entry. This is shown in Figure 10.2, “Extended ACL: ACL Entries Compared to Permission Bits”.

Figure 10.2. Extended ACL: ACL Entries Compared to Permission Bits

This mapping approach ensures the smooth interaction of applications, regardless of whether they have ACL support. The access permissions that were assigned by means of the permission bits represent the upper limit for all other “fine adjustments” made with an ACL. Changes made to the permission bits are reflected by the ACL and vice versa.

With getfacl and setfacl on the command line, you can access ACLs. The usage of these commands is demonstrated in the following example.

Before creating the directory, use the umask command to define which access permissions should be masked each time a file object is created. The command umask 027 sets the default permissions by giving the owner the full range of permissions (0), denying the group write access (2), and giving other users no permissions at all (7). umask actually masks the corresponding permission bits or turns them off. For details, consult the umask man page.

mkdir mydir creates the mydir directory with the default permissions as set by umask. Use ls -dl mydir to check whether all permissions were assigned correctly. The output for this example is:

drwxr-x--- ... tux project3 ... mydir

With getfacl mydir, check the initial state of the ACL. This gives information like:

# file: mydir 
# owner: tux 
# group: project3 
user::rwx 
group::r-x 
other::---

The first three output lines display the name, owner, and owning group of the directory. The next three lines contain the three ACL entries owner, owning group, and other. In fact, in the case of this minimum ACL, the getfacl command does not produce any information you could not have obtained with ls.

Modify the ACL to assign read, write, and execute permissions to an additional user geeko and an additional group mascots with:

setfacl -m user:geeko:rwx,group:mascots:rwx mydir

The option -m prompts setfacl to modify the existing ACL. The following argument indicates the ACL entries to modify (multiple entries are separated by commas). The final part specifies the name of the directory to which these modifications should be applied. Use the getfacl command to take a look at the resulting ACL.

# file: mydir 
# owner: tux 
# group: project3 
user::rwx 
user:geeko:rwx 
group::r-x
group:mascots:rwx 
mask::rwx 
other::---

In addition to the entries initiated for the user geeko and the group mascots, a mask entry has been generated. This mask entry is set automatically so that all permissions are effective. setfacl automatically adapts existing mask entries to the settings modified, unless you deactivate this feature with -n. mask defines the maximum effective access permissions for all entries in the group class. This includes named user, named group, and owning group. The group class permission bits displayed by ls -dl mydir now correspond to the mask entry.

drwxrwx---+ ... tux project3 ... mydir

The first column of the output contains an additional + to indicate that there is an extended ACL for this item.

According to the output of the ls command, the permissions for the mask entry include write access. Traditionally, such permission bits would mean that the owning group (here project3) also has write access to the directory mydir. However, the effective access permissions for the owning group correspond to the overlapping portion of the permissions defined for the owning group and for the mask—which is r-x in our example (see Table 10.2, “Masking Access Permissions”). As far as the effective permissions of the owning group in this example are concerned, nothing has changed even after the addition of the ACL entries.

Edit the mask entry with setfacl or chmod. For example, use chmod g-w mydir. ls -dl mydir then shows:

drwxr-x---+ ... tux project3 ... mydir

getfacl mydir provides the following output:

# file: mydir 
# owner: tux 
# group: project3 
user::rwx 
user:geeko:rwx          # effective: r-x
group::r-x 
group:mascots:rwx       # effective: r-x 
mask::r-x 
other::---

After executing the chmod command to remove the write permission from the group class bits, the output of the ls command is sufficient to see that the mask bits must have changed accordingly: write permission is again limited to the owner of mydir. The output of the getfacl confirms this. This output includes a comment for all those entries in which the effective permission bits do not correspond to the original permissions, because they are filtered according to the mask entry. The original permissions can be restored at any time with chmod g+w mydir.

Directories can have a default ACL, which is a special kind of ACL defining the access permissions that objects in the directory inherit when they are created. A default ACL affects both subdirectories and files.

A check algorithm is applied before any process or application is granted access to an ACL-protected file system object. As a basic rule, the ACL entries are examined in the following sequence: owner, named user, owning group or named group, and other. The access is handled in accordance with the entry that best suits the process. Permissions do not accumulate.

Things are more complicated if a process belongs to more than one group and would potentially suit several group entries. An entry is randomly selected from the suitable entries with the required permissions. It is irrelevant which of the entries triggers the final result “access granted”. Likewise, if none of the suitable group entries contain the required permissions, a randomly selected entry triggers the final result “access denied”.