#include <iostream>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <pwd.h>
#include <sys/wait.h>

using namespace std;

void ReportUID(string which, uid_t value) {
    struct passwd *pwent;
    if(nullptr == (pwent = getpwuid(value))) {
       perror("getpwuid");
       return;
    }
    cout << "\t"<<which <<": " << pwent->pw_name << endl;

    return;
}

void ReportUIDS(void) {
    uid_t real, effective, saved;

    if( -1 == getresuid(&real, &effective, &saved)){
       perror("getresuid");
       exit(1);
    }
    ReportUID("Real", real);
    ReportUID("Effective", effective);
    ReportUID("Saved", saved);
    cout << endl;

    return;
}

void TrialAccess() {
    int fd;

    cout << "\tAttempting to open restrictedFile " << endl;
    fd = open("restrictedFile",O_RDONLY);
    if (fd == -1) {
       perror("\tOPEN of restrictedFile FAILED");
    } else {
       cout << "\trestrictedFile opened" << endl;
    }
    cout << endl;
    close(fd);
    return;
}

void TrialRun() {
   int stat=0;

   cout << "\tRunning the id system call" << endl;

   switch(fork()) {
      case 0:
         execl("myid","myid",nullptr);
	 perror("\tExec");
	 exit(1);
      case -1:
         perror("\tFork");
	 break;
      default:
         wait(&stat);
   }
   cout << endl;
   return;
}


int main() {

    uid_t real, effective;
    real = getuid();
    effective = geteuid();

    

    cout << "At program start " << endl;
    ReportUIDS();
    TrialAccess();
    TrialRun();

    cout << "Calling setuid(" << getuid() << ")" << endl;
    setuid(real);
    cout <<"\t Things should go wrong now ..... " << endl;
    ReportUIDS();
    TrialAccess();
    TrialRun();

    cout << "Reverting to " << effective << endl;
    setuid (effective);
    ReportUIDS();
    TrialAccess();
    TrialRun();

    cout << "Changing all three" << endl;
    setresuid(real, real, real);
    ReportUIDS();
    cout <<"\t Things should go wrong now ..... " << endl;
    TrialAccess();
    TrialRun();

    cout << "Trying to revert back to effective" << endl;
    if (setuid(effective) == -1) {
        perror("Setuid");
    }

    return 0;

}