User privilege

In computing, privilege is defined as the delegation of authority over a computer system. A privilege allows a user to perform an action. Examples of various privileges include the ability to create a file in a directory, or to read or delete a file, access a device, or have read or write permission to a socket for communicating over the Internet.

Users who have been delegated extra levels of control are called privileged. Users who lack most privileges are defined as unprivileged, regular, or normal users.

Privileges can either be automatic, granted, or applied for.

An automatic privilege exists when there is no requirement to have permission to perform an action. For example, on systems where people are required to log into a system to use it, logging out will not require a privilege. Systems that do not implement file protection - such as MS-DOS - essentially give unlimited privilege to perform any action on a file.

A granted privilege exists as a result of presenting some credential to the privilege granting authority. This is usually accomplished by logging on to a system with a username and password, and if the username and password supplied are correct, the user is granted additional privileges.

A privilege is applied for by either an executed program issuing a request for advanced privileges, or by running some program to apply for the additional privileges. An example of a user applying for additional privileges is provided by the sudo command to run a command as the root user, or by the authentication system.

Modern processor architectures have multiple CPU modes that allows the OS to run at different privilege levels. Some processors have two levels (such as user and supervisor); i386+ processors have four levels (#0 with the most, #3 with the least privileges). Tasks are tagged with a privilege level. Resources (segments, pages, ports, etc.) and the privileged instructions are tagged with a demanded privilege level. When a task tries to use a resource, or execute a privileged instruction, the processor determines whether it has the permission (if not, a "protection fault" interrupt is generated). This prevents user tasks from damaging the OS or each other.

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