How to bypass the check of setuid - c

Question

I am now taking a security class. And the professor asked us to exploit a program to gain higher privilege.
This is the code
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#define BUFSIZE 512
int main(int argc, char *argv[]) {
struct stat buf;
char cmd[BUFSIZE];
FILE *f = NULL;
if (argv[1] == NULL) {
fprintf(stderr, "Please provide an argument\n");
exit(1);
}
if (stat(argv[1], &buf)) {
fprintf(stderr, "Can't stat the file\n");
exit(1);
}
if (buf.st_gid != getegid()) {
fprintf(stderr, "The file must be owned by group %d\n", getegid());
exit(1);
}
sleep(1);
if ((f = fopen(argv[1], "r")) == NULL) {
fprintf(stderr, "Cannot open command file\n");
return 1;
}
while (fgets(cmd, BUFSIZE, f)) {
if ((cmd[0] == '\n') || (cmd[0] == 0x7f)) {
fprintf(stderr, "Found empty line, quitting!\n");
return 2;
}
system(cmd);
}
printf("Done!\n");
return 0;
}
The particular situation is:
There is a binary code for this program for us to run. And the SUID of the binary code is set to higher level(here I assume level2). However, buf.st_gid != getegid() will always check if the file we specified is owned by root. Most important thing is, I only have level1 privilege. There is only one file owned by level2 I can use which is provided by our professor. And I certainly tried to pass in that filename. But nothing happens. I don't know if there is anyway I can exploit this program.(To execute some commands during runtime using the privilege of level2)