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(libc.info)Why Change Persona

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Why Change the Persona of a Process?
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   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.