[ASM][c++] problem with asm address

forsatus · 12/30/2016, 00:17

Hi, i got a problem with the variable DWORD when i use asm

Code:

DWORD dwSendFunction = PathernSendFunc();
	DWORD tmp = PathernNetwork();

	__asm
	{
		MOV EAX, DWORD PTR DS : [tmp]
		MOV EAX, DWORD PTR DS : [EAX]
		MOV EDX, szPacket
		CALL dwSendFunction
	}

The pathern return the good value, but nostale crash.
I also trie to put value like this :

Code:

DWORD dwSendFunction = 0x00518AC4;
	DWORD tmp = 0x0069630C;

	__asm
	{
		MOV EAX, DWORD PTR DS : [tmp]
		MOV EAX, DWORD PTR DS : [EAX]
		MOV EDX, szPacket
		CALL dwSendFunction
	}

And same, nostale crash it work only when i use it

Code:

#define tmp 0x0069630C
#define dwSendFunction 0x00518AC4

Thanks in advance

xopy · 12/30/2016, 00:34

Quote:

Originally Posted by forsatus

Hi, i got a problem with the variable DWORD when i use asm

DWORD dwSendFunction = PathernSendFunc();
DWORD tmp = PathernNetwork();

__asm
{
MOV EAX, DWORD PTR DS : [tmp]
MOV EAX, DWORD PTR DS : [EAX]
MOV EDX, szPacket
CALL dwSendFunction
}

...

Thanks in advance

it's logical because you are referencing here not of the values themselves.

forsatus · 12/30/2016, 02:14

Quote:

Originally Posted by xopy

it's logical because you are referencing here not of the values themselves.

i don't understand what you mean

Jeoni · 12/30/2016, 02:29

With

Code:

DWORD tmp = PathernNetwork();

there is a part of memory somewhere that holds 4 byte (some address).
If you now execute

Code:

MOV EAX, DWORD PTR DS : [tmp]

you access that part of the memory and read it. So the content (value) of tmp is put into eax. With

Code:

MOV EAX, DWORD PTR DS : [EAX]

you're dereferencing and read the content located at the address specified by eax (which is the value of tmp). So on this part what you now did translated to C pseudocode is

Code:

eax = *(DWORD*)tmp

or

Code:

eax = *(DWORD*)PathernNetwork()

.

Now let's take a look why it works with defines. Defines does not translate to variables, so are not actually located somewhere in the memory. Instead it's like a synonym for the thing defined (addresses in this case). So with

Code:

MOV EAX, DWORD PTR DS : [tmp]

you're not accessing a variable first but directly dereferencing the "target" address. With the second dereferencation with

Code:

MOV EAX, DWORD PTR DS : [EAX]

you'd get something like this in pseudocode:

Code:

eax = **(DWORD**)tmp

(note that tmp is the define now not a variable) or

Code:

eax = **(DWORD**)0x0069630C

.

I guess you see the difference? One dereferencation. That's easy to fix, the solution is trivial and I hope you don't need spoonfeeding after this detailed explanation?

With best regards
Jeoni

forsatus · 12/30/2016, 03:21

Ok, it's a stupid error xD
Thanks to help me to see the error, know i understand how to work the DR

#closerequest