29.3 Why Change the Persona of a Process?

The most obvious situation where it is necessary for a process to change its user and/or group IDs is the login program. When login starts running, its user ID is root. Its job is to start a shell whose user and group IDs are those of the user who is logging in. (To accomplish this fully, login must set the real user and group IDs as well as its persona. But this is a special case.)

The more common case of changing persona is when an ordinary user program needs access to a resource that wouldn't ordinarily be accessible to the user actually running it.

For example, you may have a file that is controlled by your program but that shouldn't be read or modified directly by other users, either because it implements some kind of locking protocol, or because you want to preserve the integrity or privacy of the information it contains. This kind of restricted access can be implemented by having the program change its effective user or group ID to match that of the resource.

Thus, imagine a game program that saves scores in a file. The game program itself needs to be able to update this file no matter who is running it, but if users can write the file without going through the game, they can give themselves any scores they like. Some people consider this undesirable, or even reprehensible. It can be prevented by creating a new user ID and login name (say, games) to own the scores file, and make the file writable only by this user. Then, when the game program wants to update this file, it can change its effective user ID to be that for games. In effect, the program must adopt the persona of games so it can write the scores file.