Set offset of members in struct?

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MUFOS

Joined: 17 Apr 2016
Posts: 47

MUFOS 13 May 2016, 14:28

Hello there,

I was wondering if it's possible to set the offsets of the members of a struct.

This could be useful when I have an array inside the struct.

For instance:

Code:

struct SomeStruct
SomeArray dd ? (on runtime, it will be allocated 10*4 (10 dwords))
SomeOtherValue dd 0
ends
 ; When accessing the value of SomeOtherValue when it's in a register, I would do something like:

mov edx, [eax + SomeStruct.SomeOtherValue] ;However, it will not work since it is an array. The structure has been created by allocating memory (40 + 4 bytes).

;Would it be possible to do something like:
struct SomeStruct
SomeArray dd ?
offset 40 SomeOtherValue dd 0 ;Set the offset of the value.
ends

13 May 2016, 14:28

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20299
Location: In your JS exploiting you and your system

revolution 13 May 2016, 14:30

Just use RB:

Code:

struct SomeStruct
SomeArray dd ?
rb 40
SomeOtherValue dd 0
ends

Or:

Code:

struct SomeStruct
SomeArray rd 11
SomeOtherValue dd 0
ends

13 May 2016, 14:30

MUFOS

Joined: 17 Apr 2016
Posts: 47

MUFOS 13 May 2016, 14:50

I may be picky, but doesn't this mean the program size would be +40 bytes?

You see I never assign struct on compiletime, so is there any way to do it without adding the extra bytes?

13 May 2016, 14:50

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20299
Location: In your JS exploiting you and your system

revolution 13 May 2016, 15:19

It depends upon how you allocate the structure. If you statically add it within your code section, or within the initialised data then it will take up space in the exe file. But you can put it at the end of the data into the uninitialised portion and it won't take up any space in the exe file.

13 May 2016, 15:19

MUFOS

Joined: 17 Apr 2016
Posts: 47

MUFOS 13 May 2016, 15:29

revolution wrote:

It depends upon how you allocate the structure. If you statically add it within your code section, or within the initialised data then it will take up space in the exe file. But you can put it at the end of the data into the uninitialised portion and it won't take up any space in the exe file.

I would like to allocate it on runtime using the heap.
Can you show me an example of uninitialized data?

13 May 2016, 15:29

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20299
Location: In your JS exploiting you and your system

revolution 13 May 2016, 23:35

What OS are you using? What file format?

13 May 2016, 23:35

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