flat assembler - PE Format

Index > Windows > PE Format - SizeOfImage

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wyvern

Joined: 08 Dec 2011
Posts: 27

wyvern 15 Dec 2011, 02:07

Hi, im trying to understand how to calculae the SizeOfImage value. From what i have read in different places:

First i have: "SizeOfImage = SectionAlignment * NumberOfSections" (assuming the sections are equal size). But in a PE from scratch in this forum i saw this:

Code:

;SA = SectionAlignment = 0x1000
;NA = NumberOfSections = 3
.SizeOfImage            dd SA*(NS+1)

The result here is: 16384 bytes
But why the "+1" ?? Is adding another section or this is for the round up?

Second i have: "SizeOfImage = VirtualAddress * VirtualSize" (values for the last section). But this doesnt return the same result than the first... so i dont know what to believe. However, In the PE Tools app i noted a "?" button next to the "SizeOfImage" data field, when i click it the value decreases and match this formula.

Can somebody explain me which is the correct method, or what i am doing wrong?. Thanks[/quote]

_________________
Thanks

15 Dec 2011, 02:07

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20413
Location: In your JS exploiting you and your system

revolution 15 Dec 2011, 02:15

I presume NS == NA? Assuming that is so, then you add one (+1) because the initial section (the first section, the header) is not included in the NA (NS) value.

15 Dec 2011, 02:15

DOS386

Joined: 08 Dec 2006
Posts: 1903

DOS386 15 Dec 2011, 05:48

> I presume NS == NA?

SS === SA ???

> Hi, im trying to understand how to calculae the SizeOfImage value.

Sum of all section sizes (every section padded up to multiple of 4 KiB) + 4 KiB.

> "SizeOfImage = SectionAlignment * NumberOfSections"

Consider a section > 4 KiB Shocked

Note: section alignment is always 4 KiB in EXE and DLL (Ring0 SYS at your own risk).

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

15 Dec 2011, 05:48

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20413
Location: In your JS exploiting you and your system

revolution 15 Dec 2011, 05:57

DOS386 wrote:

SS === SA ???

I don't see SS in the post. To what are you referring?

15 Dec 2011, 05:57

wyvern

Joined: 08 Dec 2011
Posts: 27

wyvern 15 Dec 2011, 11:48

DOS386 wrote:

Sum of all section sizes (every section padded up to multiple of 4 KiB) + 4 KiB.

Please, can you show me an example of that?

15 Dec 2011, 11:48

DOS386

Joined: 08 Dec 2006
Posts: 1903

DOS386 17 Dec 2011, 01:52

why don't you check my examples?

Code:

org $0040'0000 ; RVA=0 | AVA=$0040'0000
; bullshit (up to 4 KiB, some versions of Windaube do
; mmap the MZ stub and PE headers too for no valid reason)
org $0040'1000 ; RVA=$1000 | AVA=$0040'1000 
; a section with $1001 useful Byte's
  ...
  call dword [GetVirusEx]
  int1
  ...
; $0FFF Byte's wasted
org $0040'3000 ; RVA=$3000 | AVA=$0040'3000
; a section with $0800 useful Byte's
  ...
  . db "NTOSKRNL.EXE",0
  . db "LSASS.EXE",0
  ...
; $0800 Byte's wasted
org $0040'4000 ; RVA=$4000 | AVA=$0040'4000
; end of program
; we got 2 sections and SizeOfImage $4000 = 16 KiB

17 Dec 2011, 01:52

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