UNIX for Advanced & Expert Users

$ groups kirk spock
  kirk :  human starfleet
  spock : vulcan starfleet
  
$ ls -lR /tmp/.z:
  total 16
  -rw-rw-r--   1 kirk    human             0 Mar 17 22:37 aa
  drwxrwxr-x   2 sarek   vulcan          117 Mar 17 22:38 adir
  
  /tmp/.z/adir:
  total 0

$ /bin/id
  uid=5418(kirk) gid=29(human)

$ runas spock /bin/id
  uid=7643(spock) gid=35(vulcan)

$ runas spock /bin/touch /tmp/.z/adir/zz
  touch: /tmp/.z/adir/zz cannot create

$ cat runas.c

     #include <stdio.h>
     #include <sys/types.h>
     #include <unistd.h>
     #include <pwd.h>
     #include <grp.h>
     
     main(int argc, char *argv[])
     {
         struct passwd *pw;
         struct group *grp;
         char *args[argc];
         char command[60];
         int i;
         char user[20];
         
         if (argc >= 3) {
             strcpy(user, argv[1]);
             strcpy(command, argv[2]);
         } else {
             exit(1);
         }
         
         if (setuid(0) != 0) {
             printf("Cannot proceed...This program doesn't have the proper permissions!\n"); 
             exit(2);
         }
         
         /* additional security checks go here... */
         
         pw = getpwnam(user);          /* get password record for specified user. */
         if (pw == NULL)
            {
             printf("%s is an invalid username.\n", user );
             exit(3);
            }
         
         i = setreuid(pw->pw_uid, pw->pw_uid);  /* become the specified user! */
         if (i != 0) 
            {
             printf("setreuid() failed...\n");
             exit(5);
            }
         
         for (i = 2; i < argc; i++)  {    
              args[i - 2] = (char *)malloc(strlen (argv [i]) * sizeof(char) + 1); 
              strcpy(args[i - 2], argv[i]);     
         }
         args[argc - 2] = NULL;
          
         execv(command, args); 
         printf("%s: execv() failed!!!\n\n", argv[0]);
     
     } /* main() */