Binary Exploitation

Question

I am trying to learn binary exploitation. I can modify the binary behaviour with gdb, but I have not understood how to exploit the binary in C.

I've found some references on "Hacking - the art of exploitation", but I cannot completely figure out how to exploit the binary. Could someone show me some examples or give me a guideline?

Below is the simple code I am trying to exploit (force the app to run execl):

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

int main(int argc, char *argv[]) 
{
  int shell= 0;


  if (shell==0) {
  printf("Bye\n");
  return 1;
  
  }

  execl("/bin/sh", "/bin/sh", "-p", (void *) NULL);
  perror("exec");
  return 1;
}
  

with objdump:

08048486 <main>:
 8048486:       55                      push   %ebp
 8048487:       89 e5                   mov    %esp,%ebp
 8048489:       83 ec 04                sub    $0x4,%esp
...
 80484b4:       68 68 85 04 08          push   $0x8048568
 80484b9:       68 68 85 04 08          push   $0x8048568
 80484be:       e8 8d fe ff ff          call   8048350 <execl@plt>
 80484c3:       83 c4 10                add    $0x10,%esp
...
 80484d8:       c9                      leave  
 80484d9:       c3                      ret    
 80484da:       66 90                   xchg   %ax,%ax
 80484dc:       66 90                   xchg   %ax,%ax
 80484de:       66 90                   xchg   %ax,%ax

Thank you