Toggling between Identities

A process can change its current USERCODE to either of those stored in the REALUSERCODE or SAVEDUSERCODE attribute, without having to specify a password. However, the process cannot do this toggling with ordinary task attribute assignments. Instead, the process must use certain specialized functions that are available in three forms:

  • As ALGOL procedures in the file SYMBOL/POSIX/ALGOL/PROPERTIES. These procedures are documented in the ALGOL and MCP Interfaces to POSIX Features Programming Reference Manual.

  • As C language functions accessible through a header file. These functions are documented in the C Programming Reference Manual, Volume 2: Headers and Functions.

  • As library procedures exported by the MCPSUPPORT library. These procedures are documented in the ALGOL and MCP Interfaces to POSIX Features Programming Reference Manual.

These functions and procedures refer to user identities by numeric values that correspond to the UID usercode attribute. Therefore, changing to the REALUSERCODE or SAVEDUSERCODE is a two-step process:

  1. Retrieve the user ID associated with the REALUSERCODE or SAVEDUSERCODE.

  2. Change the process's current user ID to the user ID that was retrieved.

Similarly, a process can change its current GROUPCODE to either of those stored in the REALGROUPCODE or SAVEDGROUPCODE attribute. The process uses procedures or functions that are available from the SYMBOL/POSIX/ALGOL/PROPERTIES file, the C header file, or the MCPSUPPORT library.

The following table lists the procedures and functions used to toggle between the real and saved usercode or group code:

Action to Perform

ALGOL Procedure

C Function

MCPSUPPORT Procedure

Retrieve user ID of REALUSERCODE

GETUID

getuid()

POSIX_INTEGERIDS

Retrieve user ID of USERCODE

GETEUID

geteuid()

POSIX_INTEGERIDS

Change current user ID

SETUID

setuid()

POSIX_SETIDS

Retrieve group ID of REALGROUPCODE

GETGID

getgid()

POSIX_INTEGERIDS

Retrieve group ID of GROUPCODE

GETEGID

getegid()

POSIX_INTEGERIDS

Change current group ID

SETGID

setgid()

POSIX_SETIDS

The following ALGOL program uses procedures from SYMBOL/POSIX/ALGOL/ PROPERTIES to change its usercode and group code between the “real” and “saved” values: 

BEGIN                                                           
$INCLUDE “SYMBOL/POSIX/ALGOL/PROPERTIES.”                       
INTEGER ERRNO,                                                  
        REAL_GID,                                               
        REAL_UID,                                               
        RSLT,                                                   
        SAVED_GID,                                              
        SAVED_UID;                                              
                                                                
% RECORD SAVED USERCODE AND GROUPCODE                           
% Because process has not yet changed its identity, these values
% are the same as the effective usercode and group code.        
SAVED_UID := GETEUID(ERRNO);                                    
SAVED_GID := GETEGID(ERRNO);                                    
                                                                
% RECORD REAL USERCODE AND GROUPCODE                            
REAL_UID := GETUID(ERRNO);                                      
REAL_GID := GETGID(ERRNO);                                      
                                                                
% CHANGE USERCODE TO REALUSERCODE                               
RSLT := SETUID(REAL_UID, ERRNO);                                
                                                                
% CHANGE GROUPCODE TO REALGROUPCODE                             
RSLT := SETGID(REAL_GID, ERRNO);        
                                        
% CHANGE USERCODE TO SAVEDUSERCODE      
RSLT := SETUID(SAVED_UID, ERRNO);       
                                        
% CHANGE GROUPCODE TO SAVEDGROUPCODE    
RSLT := SETGID(SAVED_GID, ERRNO);       
                                        
END.

Note that the system performs security checking on the SETUID function. The system permits the SETUID to succeed only if the requested user ID corresponds to the USERCODE, REALUSERCODE, or SAVEDUSERCODE of the process.

Similarly, the system performs checking on the SETGID function. The system permits the SETGID to succeed only if the requested group ID corresponds to the value of the GROUPCODE, REALGROUPCODE, or SAVEDGROUPCODE of the process.

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