lodsw Error Control

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Torrey

Joined: 12 Oct 2003
Posts: 78

Torrey 04 Mar 2004, 09:28

Is there any error control that is possible when using lodsw? Or any of the lodsx instructions?

Code:

        mov esi,0x00400000
Another:
        mov edx,esi
        lodsw
        cmp eax,'hi'
        je Continue
        jmp Another

That's a really crappy example there, but eventually that will reach a part of memory that will not contain anything, what I'd like it to do is use some form of error control if any to skip over that part of memory or jump to an error handler within my program. If you wonder why I had the "mov edx,esi" in there it's to watch where esi's current location is as I watch in the debugger. Any ideas?

Thanks for the help
~Torrey

04 Mar 2004, 09:28

roticv

Joined: 19 Jun 2003
Posts: 374
Location: Singapore

roticv 04 Mar 2004, 12:43

Try repnz(or repz) scasw depending on your needs.

04 Mar 2004, 12:43

pelaillo
Missing in inaction

Joined: 19 Jun 2003
Posts: 878
Location: Colombia

pelaillo 04 Mar 2004, 19:34

If you are looking for a word and you know it will be aligned, you can scan two by two bytes, but if not, you better go byte by byte or scan possibilities of having second byte or first byte. i.e

Code:

         ; 1 2 3 4 5 6 7 8 9 0 1 2 3 4
         ; ..  ..  ..  ..  ..  ..  ..
stuff1 db 'this is some stuff, ok?',0,0
stuff2 db 'this is new stuff, ok?',0,0

Start:
  mov   esi,stuff1
Another1:
  lodsw
  cmp   ax,'ok'
  jne   Another1 ; you will find it
ok_found:
  mov   esi,stuff2
Another2:
  lodsw
  cmp   ax,'ok'
  jne   Another2 ; you won't find it.
          ; It may run forever, depending on memory
          ; after stuff2
Another3:
  lodsw
  or    ax,ax
  je    not_found
  cmp   ax,'ok'
  jne   Another2 ; here it stops in a zero word

04 Mar 2004, 19:34

Torrey

Joined: 12 Oct 2003
Posts: 78

Torrey 05 Mar 2004, 00:05

@pelaillo & roticv

Originally the idea was to scan through a process' memory space for a specific byte pattern, using error control if possible to skip over parts of memory space that esi=??? (nothing) and just do a cmp with esi's current address so it stops at a certain point. As it seems there might not be error control for lodsx. Some colleagues told me to check out SEH for what I wanted. SEH will most likely be the last thing I test out. The first will be checking more into roticv's idea of using repnz or repz scasw. I'm expecting an access violation bomb out, but really hoping for the best.

05 Mar 2004, 00:05

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 05 Mar 2004, 01:45

Torrey wrote:

I'm expecting an access violation bomb out, but really hoping for the best.

Don't. You probably will get access violation. You need SEH. It is not exactly assembly related stuff, but Windows related. You have to read some tutorials about SEH - in generally it is not so complex.

Regards.

05 Mar 2004, 01:45

Torrey

Joined: 12 Oct 2003
Posts: 78

Torrey 05 Mar 2004, 02:26

Honestly I skipped over the scasw stuff earlier and went straight for SEH. Worked perfectly! The hardest part about it all as most of you know...the debugger can't use the new error handler when one is set.

05 Mar 2004, 02:26

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