(libc.info.gz) Enable/Disable Setuid

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(libc.info.gz) Setuid Program Example

 
 30.8 Enabling and Disabling Setuid Access
 =========================================
 
 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 '_POSIX_SAVED_IDS' feature, you can
 accomplish this with 'seteuid'.  When the game program starts, its real
 user ID is 'jdoe', its effective user ID is 'games', and its saved user
 ID is also 'games'.  The program should record both user ID values once
 at the beginning, like this:
 
      user_user_id = getuid ();
      game_user_id = geteuid ();
 
    Then it can turn off game file access with
 
      seteuid (user_user_id);
 
 and turn it on with
 
      seteuid (game_user_id);
 
 Throughout this process, the real user ID remains 'jdoe' and the file
 user ID remains 'games', so the program can always set its effective
 user ID to either one.
 
    On other systems that don't support file user IDs, you can turn
 setuid access on and off by using 'setreuid' to swap the real and
 effective user IDs of the process, as follows:
 
      setreuid (geteuid (), getuid ());
 
 This special case is always allowed--it cannot fail.
 
    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:
 
      #ifdef _POSIX_SAVED_IDS
        seteuid (user_user_id);
      #else
        setreuid (geteuid (), getuid ());
      #endif
 

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(libc.info.gz) Setting Groups

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(libc.info.gz) Setuid Program Example