File system permissions

From Wikipedia, the free encyclopedia

Most modern file systems have methods of administering permissions or access rights to specific users and groups of users. These systems control the ability of the users affected to view or make changes to the contents of the file system.

1 Differences between operating systems
2 Traditional Unix permissions
3 Permission notation
4 See also
5 External links

[edit] Differences between operating systems

Unix-like and otherwise POSIX-compliant systems, including Linux-based systems and Mac OS X (through version 10.3), have a simple system for managing individual file permissions. POSIX also specifies a system of access control lists, but it is only implemented by certain file systems and operating systems.

DOS variants (including the Microsoft products MS-DOS, Windows 95, Windows 98, and Windows Me) do not have permissions. There is a "read-only" attribute that can be set or unset on a file by any user or program.

Microsoft Windows NT and its derivatives (including Windows 2000 and Windows XP), as well as VMS and OpenVMS use access control lists (ACLs) to administer a more complex and varied set of permissions.

Mac OS X, beginning with version 10.4 ("Tiger"), supports both the use of complex access control lists (ACLs) and the traditional POSIX-compliant Unix-like simple system for managing individual file permissions.

[edit] Traditional Unix permissions

Permissions on Unix-like systems are managed in three distinct classes. These classes are known as user, group, and others. In effect, Unix permissions are a simplified form of access control lists (ACLs).

[edit] Classes

On Unix file systems, every file and directory is owned by a specific user. The owner of an object comprises its user class. Permissions assigned to the user class only apply to that specific user.

A file or directory is also assigned a group, which comprises its group class. Permissions assigned to the group class only apply to members of that group (other than the owner).

Users who are not otherwise represented by the other two classes comprise a file's others class.

The effective permissions that have applied to a specific user in relation to a file are determined in logical precedence. For example, the user who owns the file will have the effective permissions given to the user class regardless of those assigned to the group or others class.

[edit] Basic Permissions

There are three specific permissions on Unix-like systems that apply to every class:

The read permission, which grants the ability to read a file. When set for a directory, this permission grants the ability to read the names of files in the directory (but not to find out any further information about them, including file type, size, ownership, permissions, etc.)
The write permission, which grants the ability to modify a file. When set for a directory, this permission grants the ability to modify entries in the directory. This includes creating files, deleting files, changing file permissions, and renaming files.
The execute permission, which grants the ability to execute a file. This permission must be set for executable binaries in order to allow the operating system to run them. When set for a directory, this permission grants the ability to traverse its tree in order to access files or subdirectories, but not see files inside the directory (unless read is set).

When a permission is not set, the rights it would grant are denied. Unlike ACL-based systems, permissions on a Unix-like system are not inherited. Files created within a directory will not necessarily have the same permissions as that directory. The permissions to be assigned are determined using umasks.

[edit] Additional Permissions

Unix-like systems typically employ three additional permissions or modes. These special permissions are set for a file or directory overall, not by a class.

The set user ID, setuid, or SUID permission. When a file for which this permission has been set is executed, the resulting process will assume the effective user ID given to the user class.
The set group ID, setgid, or SGID permission. When a file for which this permission has been set is executed, the resulting process will assume the group ID given to the group class. When setgid is applied to a directory, new files and directories created under that directory will inherit the group from that directory. (Default behaviour is to use the primary group of the effective user when setting the group of new files and directories.)
The sticky permission. The typical behaviour of the sticky bit on executable files encourages the kernel to retain the resulting process image beyond termination. On a directory, the sticky permission prevents users from renaming, moving or deleting contained files owned by users other than themselves, even if they have write permission to the directory. Only the directory owner and superuser are exempt from this.

These additional permissions are also referred to as setuid bit, setgid bit, and sticky bit respectively, due to the fact that they each occupy only one bit.

[edit] Permission notation

[edit] Symbolic notation

There are many ways by which Unix permission schemes are represented. The most common form is symbolic notation. This scheme represents permissions as a series of 10 characters.

First Character
`-`	a regular file
`d`	a directory
`l`	a symbolic link
Three groups of three
first	what the owner can do
second	what the group members can do
third	what other users can do
The triplet
first	`r`: readable.
second	`w`: writable.
third	`x`: executable.
third	`s`: setuid/setgid.

The first character indicates the file type:

- denotes a regular file
d denotes a directory
b denotes a block special file
c denotes a character special file
l denotes a symbolic link
p denotes a named pipe
s denotes a domain socket

Each class of permissions is represented by three characters. The first set of characters represents the user class. The second set represents the group class. The third and final set of three characters represents the others class.

Each of the three characters represent the read, write, and execute permissions respectively:

r if the read bit is set, - if it is not.
w if the write bit is set, - if it is not.
x if the execute bit is set, - if it is not.
The x will be an s if the setuid or setgid bit is also set, and in the third, it will be a t if the sticky bit is set. If these are set but the execute bit is not, the letter will be in uppercase.

The following are some examples of symbolic notation:

"-rwxr-xr-x" for a regular file whose user class has full permissions and whose group and others classes have only the read and execute permissions.
"crw-rw-r--" for a character special file whose user and group classes have the read and write permissions and whose others class has only the read permission.
"dr-x------" for a directory whose user class has read and execute permissions and whose group and others classes have no permissions.

[edit] Symbolic notation and additional permission

The three additional permissions are indicated by changing one of the three "execute" characters as shown in the following table:

Permission	Class	Executable¹	Non-executable²
Set User ID (`setuid`)	User	`s`	`S`
Set Group ID (`setgid`)	Group	`s`	`S`
Sticky	Others	`t`	`T`

The character that will be used to indicate that the execute bit is also set.
The character that will be used when the execute bit is not set.

Here is an example:

"-rwsr-Sr-x" for a file whose user class has read, write and execute permissions; whose group class has read permission; whose others class has read and execute permissions; and which has setuid and setgid permissions set.

[edit] Octal notation

Another common method for representing Unix permissions is octal notation. Octal notation consists of a three- or four-digit base-8 value.

With three-digit octal notation, each numeral represents a different component of the permission set: user class, group class, and "others" class respectively.

Each of these digits is the sum of its component bits (see also Binary numeral system). As a result, specific bits add to the sum as it is represented by a numeral:

The read bit adds 4 to its total,
The write bit adds 2 to its total, and
The execute bit adds 1 to its total.

These values never produce ambiguous combinations; each sum represents a specific set of permissions.

These are the examples from the Symbolic notation section given in octal notation:

"-rwxr-xr-x" would be represented as 755 in three-digit octal.
"-rw-rw-r--" would be represented as 664 in three-digit octal.
"-r-x------" would be represented as 500 in three-digit octal.

[edit] Octal notation and additional permissions

There is also a four-digit form of octal notation. In this scheme, the standard three digits described above become the last three digits. The first digit represents the additional permissions. On some systems, this first digit cannot be omitted; it is therefore common to use all four digits (where the first digit is zero).

This first digit is also the sum of component bits:

The setuid bit adds 4 to the total,
The setgid bit adds 2 to the total, and
The sticky bit adds 1 to the total.

The example from the Symbolic notation and additional permissions section, "-rwsr-Sr-x" would be represented as 6745 in four-digit octal. In addition, the examples in the previous section would be represented as 0755, 0664, and 0500 respectively in four-digit octal notation.