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| <h3 class="section">29.3 Why Change the Persona of a Process?</h3> |
| |
| <p>The most obvious situation where it is necessary for a process to change |
| its user and/or group IDs is the <code>login</code> program. When |
| <code>login</code> starts running, its user ID is <code>root</code>. 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, <code>login</code> must set the real |
| user and group IDs as well as its persona. But this is a special case.) |
| |
| <p>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. |
| |
| <p>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. |
| |
| <p>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, <code>games</code>) 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 <code>games</code>. In effect, the program must |
| adopt the persona of <code>games</code> so it can write the scores file. |
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