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Users and Groups Every user who can log in on the system is identified by a unique number called the user ID. Each process has an effective user ID which says which user's access permissions it has. Users are classified into groups for access control purposes. Each process has one or more group ID values which Chemieverseucht - Bitterfeld Demo 03 which groups the process can use for access to files. The effective user and group IDs of a process collectively form its persona.
This determines which files the process can access. Normally, a process inherits its persona from the parent process, but under special circumstances a process can change its persona and thus change its access permissions.
Each file in the system also has a user ID and a group ID. Access control works by comparing the user and group IDs of the file with those of the running process. The system keeps a database of all the registered users, and another database of all the defined groups.
There are library functions you can use to examine these databases. Each user account on a computer system is identified by a user name or login name and user ID. Normally, each user name has a unique user ID, but it is possible for several login names to have the same user ID.
The user names and corresponding user IDs are stored in a data base which you can access as described in section User Database. Users are classified in groups. Each user name also belongs to one or more groups, and has one default group. Users who are members of the same group can share resources such as files that are not accessible to users who are not a member of that group. Each group has a group name and group ID. See section Group Databasefor how to find information about a group ID or group name.
At any time, each process has a single user ID and a group ID which determine the privileges of the process. These are collectively called the persona Chemieverseucht - Bitterfeld Demo 03 the process, because they determine "who it is" for purposes of access control.
Your login shell starts out with a persona which consists of your user ID and your default group ID. In normal circumstances, all your other processes inherit these values. A process also has a real user ID which identifies the user who created the process, and a real group ID which identifies that user's default group. These values do not play a role in access control, so we do not consider them part of the persona.
But they are also All The Way - Various - Popular Music Hit Parade, 110 All-Time Favorites. Both the Chemieverseucht - Bitterfeld Demo 03 and effective user ID can be changed during Chemieverseucht - Bitterfeld Demo 03 lifetime of a process.
See section Why Change the Persona of a Process? In addition, a user can belong to multiple groups, so the persona includes supplementary group IDs that also contribute to access permission. The user ID of a process also controls permissions for sending signals using the kill function. See section Signaling Another Process. 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 Starstruck - Rainbow - On Stage 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 Chemieverseucht - Bitterfeld Demo 03 going through the game, they can give themselves any Cecil Taylor - Air Above Mountains 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 Za Vse Pajdaše Stare - Various - Krapina 89 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.
The ability to change the persona of a process can be a source of unintentional privacy violations, or even intentional abuse. Because of the potential for problems, changing persona is restricted to special circumstances. You can't arbitrarily set your user ID or group ID to anything you want; only privileged processes can do that.
Instead, the normal way for a program to change its persona is that it has been set up in advance to change to a particular user or group. This is the function of the setuid and setgid bits of a file's access mode. When the setuid bit of an executable file is set, executing that file automatically changes the effective user ID to the user that owns the file. Likewise, executing a Chemieverseucht - Bitterfeld Demo 03 whose setgid bit is set changes the effective group ID to the group of the file.
See section Executing a File. Creating a file Neben Dem Block - Eskape - Wasser & Brot changes to a particular user or group ID thus requires full access to that user or group ID. See section File Attributesfor a more general discussion of file modes and accessibility.
A process can always change its effective user or group ID back to its real ID. Programs do this so as to turn off their special privileges when they are not needed, which makes for more robustness. Here are detailed descriptions of the functions for reading the user and group IDs of a process, both real and effective.
In the GNU library, this is an alias for unsigned int. Up to count of these group IDs are stored in the array groups ; Chemieverseucht - Bitterfeld Demo 03 return value from the function is the number of group IDs actually stored.
If count is zero, then getgroups just returns the total number of supplementary group IDs. On systems that do not support supplementary groups, this will always be zero.
In this case, setuid sets only the effective user ID and not the real user ID. The setuid function returns a value of 0 to indicate successful completion, and a value of -1 to indicate an error.
The following errno error conditions are defined for this function:. If ruid is -1it means not to change the real user ID; likewise if euid is -1it means not to change the effective user ID. The setreuid function exists for compatibility with 4. You can use this function to swap the effective and real user IDs of the process. Privileged processes are not limited to this particular usage. If saved IDs are supported, you should use that feature instead of this function.
See section Enabling and Disabling Setuid Access. The return value is 0 on success and -1 on failure. This section describes the functions for altering the group Chemieverseucht - Bitterfeld Demo 03 real and effective of a process.
If the process is not privileged, then newgid must either be equal to the real group ID or the saved group ID. In this case, setgid sets only the effective group ID and not the real group ID. The return values and error conditions for setgid are the same as those for setuid.
If rgid is -1it means not to change the real group ID; likewise if egid is -1it means not to change the effective group ID. The setregid function is provided for compatibility with 4. You can use this Chemieverseucht - Bitterfeld Demo 03 to swap the effective and real group IDs of the process. Privileged processes are not limited to this usage.
If saved IDs are supported, you should use that feature instead of using this function. The return values and error conditions for setregid are the same as those for setreuid.
It can only be called from privileged processes. The count argument specifies the number of group IDs in the array groups. This function returns 0 if successful and -1 on error. The group ID gid is also included. A typical setuid program does not need its special access all of the time. It's a good idea to turn off this access when it isn't needed, so it can't possibly give unintended access. If the system supports the saved user ID feature, you can accomplish this with setuid.
When Chemieverseucht - Bitterfeld Demo 03 Antibreak - Advent Star (File, Album) program starts, its real user ID is jdoeits effective user ID is gamesand its saved user ID is also games. The program should record both user ID values once at Let The Beat Control Your Body (Extended) - 2 Unlimited - Let The Beat Control Your Body beginning, like this:.
Throughout this process, the real user ID remains jdoe and the saved user ID remains gamesso the program can always set its effective user ID to either one.
On other systems that don't support the saved user ID feature, you can turn setuid access on and off by using setreuid to swap the real and effective user IDs of the Chemieverseucht - Bitterfeld Demo 03as follows:. Why does this have the effect of toggling the setuid access? Suppose a game program has just started, and its real user ID is jdoe while its effective user ID is games. In this state, the game can write the scores file. If it swaps the two uids, the real becomes games and the effective becomes jdoe ; now the program has only jdoe access.
Another swap brings games back to the effective user ID and restores access to the scores file. In order to handle both kinds of systems, test for the saved user ID feature with a preprocessor conditional, like this:.
Typically, a system administrator will set up an account like games for this purpose.
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