[solved] strange problem with EDX register in Win32 Api

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Picnic

Joined: 05 May 2007
Posts: 1393
Location: Piraeus, Greece

Picnic 11 Apr 2021, 10:42

Hey everyone,

This is a piece of code from a procedure. The code works fine but if i replace eax with edx (and edx only) the program crashes. No other changes are made to the procedure.
Any idea why is this happening?

this works

Code:


    ; ... 

    mov ecx, dword [dwBufferSize]
    mov eax, [buffer]
    cmp word [eax], 0
    je .continue

    invoke WriteConsoleOutput, [hOut], eax, ecx, 0, addr lpWriteRegion

    ; ...

this fails

Code:


    ; ...

    mov ecx, dword [dwBufferSize]
    mov edx, [buffer]
    cmp word [edx], 0
    je .continue

    invoke WriteConsoleOutput, [hOut], edx, ecx, 0, addr lpWriteRegion

    ; ...

11 Apr 2021, 10:42

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20404
Location: In your JS exploiting you and your system

revolution 11 Apr 2021, 11:04

The macro for addr uses edx as a temporary register.

Code:

lea edx, [lpWriteRegion]
push edx

11 Apr 2021, 11:04

Picnic

Joined: 05 May 2007
Posts: 1393
Location: Piraeus, Greece

Picnic 11 Apr 2021, 11:10

oh, i see. Thanks revolution.

11 Apr 2021, 11:10

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20404
Location: In your JS exploiting you and your system

revolution 11 Apr 2021, 15:43

You could change the macro to do this:

Code:

push edx
lea edx, [lpWriteRegion]
xchg edx, [esp]

11 Apr 2021, 15:43

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