flat assembler - Bug in DLL example

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ejamesr

Joined: 04 Feb 2011
Posts: 52
Location: Provo, Utah, USA

ejamesr 15 Feb 2011, 07:28

I copied the 'display' example from the documentation (section 2.2.5) into the ERRORMSG.ASM example file that comes with version 1.69.31. FASM displays the message "Error: invalid expression" at line 19 for the command:

d = '0' + $ shr (bits-%*4) and 0Fh

If I comment our the rest of the line after after "d = '0'" then it compiles - but of course, then it doesn't produce the desired answer. In a new blank .asm file, the entire display example works fine. Here's the example code, pulled directly from the documentation:

Code:

    bits = 16
    display 'Current offset is 0x'
    repeat bits/4
        d = '0' + $ shr (bits-%*4) and 0Fh
        if d > '9'
            d = d + 'A'-'9'-1
        end if
        display d
    end repeat
    display 13,10

Can anyone else confirm this problem? Is this a known bug? Or did I do something wrong? I've attached the asm file.

Thank you,

Eric

Description:	This is the Errormsg.asm example file (from \FASM\Examples\DLL), with display example inserted	Download
Filename:	ERRORMSG.ASM
Filesize:	1.37 KB
Downloaded:	132 Time(s)

15 Feb 2011, 07:28

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20461
Location: In your JS exploiting you and your system

revolution 15 Feb 2011, 08:41

You can't use the $ symbol like that in relocatable code. Since DLLs are relocatable, that means that you can't do assembly time program counter ($) arithmetic because the program counter is unknown at assembly time.

You can use RVA $ instead of just $ to show the relative offset of the program counter at assembly time.

15 Feb 2011, 08:41

SFeLi

Joined: 03 Nov 2004
Posts: 138

SFeLi 15 Feb 2011, 15:23

Code:

virtual
  dd $
  load x dword from $-4
end virtual

; then you can display x, but read revolution's post first.

15 Feb 2011, 15:23

ejamesr

Joined: 04 Feb 2011
Posts: 52
Location: Provo, Utah, USA

ejamesr 15 Feb 2011, 15:37

Revolution and SFeLi,

Thank you for your responses. I'll work on this some more, taking into account your information and suggestions.

Eric

15 Feb 2011, 15:37

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20461
Location: In your JS exploiting you and your system

revolution 15 Feb 2011, 15:42

SFeLi wrote:

Code:

virtual
  dd $
  load x dword from $-4
end virtual

This will give you the value of base_address + RVA $, but it won't give you the value of the PC at runtine. The OS can relocate the DLL.

15 Feb 2011, 15:42

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