2010-9-16 ELF rewrite function

Main idea was to replace compiled in function with some other code and
run it. In default it is not possible. If you try to write some bytes
withmemcpy() in function location then segfault happens. Why? Programm
has different segments and they used for different program purpose.Our
code belongs to readonly-executable segment. And '.text' section.
We can se it with readelf -S main -l in previos post
there was program that can be used to make segment writable.After running
./textwriteble main

now segment with '.text' section becomes writable. When we try
use memcpy() there is no segfault now.
Second thing is how to make our function that will
replace compiled in functionposition independent for some data inside
function? First of all we should know our current position.It is in
eip register. push eip? mov eax, eip? it doesnt work. When we use
call in stack is saved return address. Now with this small functionit
can be saved in some location

get_ip:
    mov ecx, [esp]
    ret

At this moment we have converted segment to writable.Have writen
position detection function. If there would be data that will used
in replaced function than need detectposition of that data. For
example we will use

mov eax, sys_call ;we will use SYS_WRITE = 5
mov ebx, output_id ; output on terminal is STDOUT 1
mov ecx, pointer_to_msg
mov edx, size_of_msg
int 80h

if this was ordinary situation then define:

msg db "Hello",10
msg_size = $-msg

and our code becomes

mov eax, SYS_WRITE
mov ebx, STDOUT
mov ecx, msg
mov edx, msg_size
int 80h

but how to know position of msg if you dont know position where
function will placed?Use function get_it and you will know current
instruction position. And it will next instructionafter

call get_ip

Our code becomes

call get_ip    ;calling and detecting eip
saved_ip:      ;position that will be saved
jmp get_ip_end ;jump over function
get_ip:
    mov ecx, [esp] ;save return eip
    ret
get_ip_end:
mov eax, SYS_WRITE   
mov ebx, STDOUT
add ecx, msg-saved_ip  ;offset of msg
mov edx, msg_size
int 80h

ECX has position independent pointer to our text.For testing purposes
function fun() is filled with

asm(".byte 0x90, ... ,0x90");

hex 0x90 translates in nop instruction.

nop is No OPeration instruction.
And function does nothing.Function fun()  contains

push ebp
mov ebp, esp
start_overwrite_here:
nop
...
...
...
nop
pop ebp
ret

Nop instructions can be replaced with any binary code. There should
be enought nop instructions for our binary code. There is no check
on function size that way when overwriting can be problemsif binary
code size is larger then function size.Start function overwriting at
position (&fun+3) witn memcpy()

push ebp
mov ebp, esp
start_overwrite_here:
nop
...
...
...
nop
pop ebp
ret

Wuala function after enabling segment can be overwriten. Here is
used previous expirienceand we have mega trick with function replacment.
Compile:
make