Double Looping

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Wonfis

Joined: 23 Jul 2010
Posts: 1

Wonfis 23 Jul 2010, 11:30

I've just started getting to grips with assembly code in general and have been playing around with some commands.

I've got this loop that's supposed to simply count up to 5 from 0 while requiring a keyboard return between each number. It checks the return and if it sees "q" it exits. This works fine.

The problem is: every time I hit enter it performs two iterations of the loop. It did this with some earlier code I was playing around with too, which just used jumps instead of the loop command. The current stuff is below:

Code:

include 'win32ax.inc'

.data

inchar     DB   ?
numread    DD   ?
inhandle   DD   ?
outhandle  DD   ?
numwrit    DD   ?
numb       DB   "0"
quitmess   DB   "Enter 'q' to quit.", 10

.code

push    eax
push    ebx
push    ecx
push    edx

  start:

        invoke  AllocConsole
        invoke  GetStdHandle,STD_INPUT_HANDLE
        mov     [inhandle],eax
        invoke  GetStdHandle,STD_OUTPUT_HANDLE
        mov     [outhandle],eax
        xor     eax,eax
        mov     ecx,5

  lab:
        push    ecx
        invoke  WriteConsole,[outhandle],numb,1,numwrit,0
        invoke  ReadConsole,[inhandle],inchar,1,numread,0
        pop     ecx

        mov     bl,[inchar]
        cmp     bl,"q"
        jz      exitl
        inc     byte    [numb]

        loop    lab



  exitl:

  pop   edx
  pop   ecx
  pop   ebx
  pop   eax

        invoke ExitProcess,0

.end start

So why is it doing this? I'm running on XP Pro SP3.

23 Jul 2010, 11:30

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20524
Location: In your JS exploiting you and your system

revolution 23 Jul 2010, 11:54

ReadConsole returns more than one keystroke for some keys. Try displaying what ReadConsole returns to you and you will see. Alternatively you can try http://ollydbg.de to watch what happens.

23 Jul 2010, 11:54

bitshifter

Joined: 04 Dec 2007
Posts: 796
Location: Massachusetts, USA

bitshifter 23 Jul 2010, 23:22

You can trick it by reading 2 bytes from the stream.
Its because ReadConsole blocks (w/echo) until VK_RETURN is found.

Code:

format PE GUI 4.0
entry start

include 'win32a.inc'

section '.code' code readable executable

  start:
        invoke  AllocConsole
        invoke  GetStdHandle,STD_INPUT_HANDLE
        mov     [g_stdin],eax
        invoke  GetStdHandle,STD_OUTPUT_HANDLE
        mov     [g_stdout],eax
    .mloop:
        invoke  WriteConsole,[g_stdout],g_number,1,g_iobytes,0
        invoke  ReadConsole,[g_stdin],g_inchar,2,g_iobytes,0

        cmp     byte[g_inchar],'q'
        je      .quit
        inc     [g_number]
        cmp     [g_number],'5'
        jl      .mloop

    .quit:
        invoke ExitProcess,0

section '.data' data readable writeable

  g_stdin   dd ?
  g_stdout  dd ?
  g_iobytes dd ?
  g_inchar  dw ?
  g_number  db '0'

section '.idata' import data readable

  library kernel32,'kernel32.dll',\
          user32,'user32.dll'

  include 'api\kernel32.inc'
  include 'api\user32.inc'

23 Jul 2010, 23:22

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