Frame pointer overwriting and FreeBSD

[<chris () cmc optus net au>]

Hi,

Been trying some frame pointer overwriting techniques
and it doesn't seem to be working like i'd expect on a
FreeBSD (x86) system, wondering if the more familiar
could clarify?

I'm basically doing a frame pointer overwrite by 1 byte
exploit.

Stepping through the code in GDB i can make eip run
into my NOPs, and then my shellcode, however, using
(multiple) standard execve /bin/sh shellcodes written
for FreeBSD, causes the vulnerable program to die with
numerous errors like SIGFPE, SIGILL, etc (strangely
enough, looking at what eip was in the core dump, it
was usually in the string "/bin/sh").

Is it correct that since i've altered ebp's value,  and
as BSD passes it's arguments on the stack, i wont be
able to get it working?

I'm taking a guess that it works on Linux because it's
syscall arguments get passed via registers?




Cheers,

Chris.