Author Topic: Disassembling and then reassembling a windows exe file with nasm?  (Read 19673 times)

Offline soren

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is this possible? i was reading through the docs and couldnt find a particular way to do this. im able to disassemble a exe but the output seems very different to what nasm looks like. the output was really huge even for a hello world c program i compiled myself.

Offline Frank Kotler

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Re: Disassembling and then reassembling a windows exe file with nasm?
« Reply #1 on: August 19, 2011, 08:09:55 AM »
Well... the output of Ndisasm is intended more for "examination" than "reassembly". It is possible, with some minor tweaking (delete the first 20 bytes of each line... and other unneeded stuff... and add some needed stuff), to get it to reassemble. But any attempt to modify it would most likely fail. With a whole lot more work, it is possible to get something that can be modified, reassembled, and still work. (reassembling the unmodified disassembly is pointless - you've got the .exe!) With a very small executable, this might be practical, but as you observe, even a simple executable bloats up fast. For practical purposes, "no, you can't do that". Easier to write the code that does what the program needs to do from scratch (consulting the disassembly if you have to).

Ndisasm doesn't attempt to be "clever" - doesn't know about "executable formats"... headers and all. Just attempts to disassemble everything as if it were code. Agner Fog's "objconv" knows "disassembly" as one of its object formats, and knows executable header formats! You might find it interesting to disassemble your simple C hello world with it. Objconv knows what all the parts are, and it will help explain why it's so big!

http://www.agner.org

Edit: more complete URL:

http://www.agner.org/optimize/#objconv

Best,
Frank

« Last Edit: August 19, 2011, 08:26:24 AM by Frank Kotler »

Offline soren

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Re: Disassembling and then reassembling a windows exe file with nasm?
« Reply #2 on: August 19, 2011, 09:29:05 AM »
Thanks for the thorough answer. Yeah, I'm aware of the "pointlessness" of disassembling a file and assembling it again. It was more of a curious question than anything else. I'm interested in how translators work and was wondering how much asm source could be recovered.

Offline Bryant Keller

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Re: Disassembling and then reassembling a windows exe file with nasm?
« Reply #3 on: August 19, 2011, 05:36:49 PM »
Some years ago I played around with a disassembler called "Borg", or something similar to that. Probably the most interesting part of it was that it generated assembler source which was almost completely compatible with MASM. The only thing you really had to work to fix in the disassembled code was the data types (the disassembler would make any section not the code section into a massive collection of DB statements which, for readability purposes had to be hand fixed.)

About Bryant Keller
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Offline soren

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Re: Disassembling and then reassembling a windows exe file with nasm?
« Reply #4 on: August 22, 2011, 01:46:24 PM »
good fun.

i emaciated the hello world program down to the smallest thing that would assemble and got it to this:


Code: [Select]
    global      _main
   
    section .text
_main:

then i assembled it and played around with disassembling it.

i had no idea how much "stuff" the linker puts into the exe file. for example, this is the output of from objconv of the .obj file for above code:


Quote
; Disassembly of file: nasm_bones.obj
; Mon Aug 22 23:28:05 2011
; Mode: 32 bits
; Syntax: YASM/NASM
; Instruction set: 80386


global _main

.absolut equ 00000000H                                  ; 0
@feat.00 equ 00000001H                                  ; 1


SECTION .text   align=16 execute                        ; section number 1, code

this is the output of it after being put through the linker:

Code: [Select]
; Disassembly of file: nasm_bones.exe
; Mon Aug 22 23:39:46 2011
; Mode: 32 bits
; Syntax: YASM/NASM
; Instruction set: SSE2


global Entry_point: function

extern GetCommandLineA                                  ; near; KERNEL32.dll
extern HeapSetInformation                               ; near; KERNEL32.dll
extern SetUnhandledExceptionFilter                      ; near; KERNEL32.dll
extern GetProcAddress                                   ; near; KERNEL32.dll
extern GetModuleHandleW                                 ; near; KERNEL32.dll
extern ExitProcess                                      ; near; KERNEL32.dll
extern DecodePointer                                    ; near; KERNEL32.dll
extern WriteFile                                        ; near; KERNEL32.dll
extern GetStdHandle                                     ; near; KERNEL32.dll
extern GetModuleFileNameW                               ; near; KERNEL32.dll
extern GetModuleFileNameA                               ; near; KERNEL32.dll
extern FreeEnvironmentStringsW                          ; near; KERNEL32.dll
extern WideCharToMultiByte                              ; near; KERNEL32.dll
extern GetEnvironmentStringsW                           ; near; KERNEL32.dll
extern SetHandleCount                                   ; near; KERNEL32.dll
extern InitializeCriticalSectionAndSpinCount            ; near; KERNEL32.dll
extern GetFileType                                      ; near; KERNEL32.dll
extern GetStartupInfoW                                  ; near; KERNEL32.dll
extern DeleteCriticalSection                            ; near; KERNEL32.dll
extern EncodePointer                                    ; near; KERNEL32.dll
extern TlsAlloc                                         ; near; KERNEL32.dll
extern TlsGetValue                                      ; near; KERNEL32.dll
extern TlsSetValue                                      ; near; KERNEL32.dll
extern TlsFree                                          ; near; KERNEL32.dll
extern InterlockedIncrement                             ; near; KERNEL32.dll
extern SetLastError                                     ; near; KERNEL32.dll
extern GetCurrentThreadId                               ; near; KERNEL32.dll
extern GetLastError                                     ; near; KERNEL32.dll
extern InterlockedDecrement                             ; near; KERNEL32.dll
extern HeapCreate                                       ; near; KERNEL32.dll
extern QueryPerformanceCounter                          ; near; KERNEL32.dll
extern GetTickCount                                     ; near; KERNEL32.dll
extern GetCurrentProcessId                              ; near; KERNEL32.dll
extern GetSystemTimeAsFileTime                          ; near; KERNEL32.dll
extern LeaveCriticalSection                             ; near; KERNEL32.dll
extern EnterCriticalSection                             ; near; KERNEL32.dll
extern LoadLibraryW                                     ; near; KERNEL32.dll
extern UnhandledExceptionFilter                         ; near; KERNEL32.dll
extern IsDebuggerPresent                                ; near; KERNEL32.dll
extern TerminateProcess                                 ; near; KERNEL32.dll
extern GetCurrentProcess                                ; near; KERNEL32.dll
extern HeapFree                                         ; near; KERNEL32.dll
extern Sleep                                            ; near; KERNEL32.dll
extern GetCPInfo                                        ; near; KERNEL32.dll
extern GetACP                                           ; near; KERNEL32.dll
extern GetOEMCP                                         ; near; KERNEL32.dll
extern IsValidCodePage                                  ; near; KERNEL32.dll
extern RtlUnwind                                        ; near; KERNEL32.dll
extern HeapSize                                         ; near; KERNEL32.dll
extern HeapAlloc                                        ; near; KERNEL32.dll
extern HeapReAlloc                                      ; near; KERNEL32.dll
extern LCMapStringW                                     ; near; KERNEL32.dll
extern MultiByteToWideChar                              ; near; KERNEL32.dll
extern GetStringTypeW                                   ; near; KERNEL32.dll
extern IsProcessorFeaturePresent                        ; near; KERNEL32.dll


SECTION .text   align=1 execute                         ; section number 1, code

?_0001: ; Local function
; Filling space: 2H
; Filler type: mov with same source and destination
;       db 8BH, 0FFH

ALIGN   2
        push    ebp                                     ; 00401002 _ 55
        mov     ebp, esp                                ; 00401003 _ 8B. EC
        cmp     dword [?_0982], 2                       ; 00401005 _ 83. 3D, 00408B28(d), 02
        jz      ?_0002                                  ; 0040100C _ 74, 05
        call    ?_0074                                  ; 0040100E _ E8, 00000691
?_0002: push    dword [ebp+8H]                          ; 00401013 _ FF. 75, 08
        call    ?_0063                                  ; 00401016 _ E8, 000004DA
        push    255                                     ; 0040101B _ 68, 000000FF
        call    ?_0025                                  ; 00401020 _ E8, 000001EA
        pop     ecx                                     ; 00401025 _ 59
        pop     ecx                                     ; 00401026 _ 59
        pop     ebp                                     ; 00401027 _ 5D
        ret                                             ; 00401028 _ C3

?_0003: ; Local function
        push    20                                      ; 00401029 _ 6A, 14
        push    ?_0922                                  ; 0040102B _ 68, 00407860(d)
        call    ?_0235                                  ; 00401030 _ E8, 000012EB
        xor     esi, esi                                ; 00401035 _ 33. F6
        cmp     dword [?_1059], esi                     ; 00401037 _ 39. 35, 004098BC(d)
        jnz     ?_0004                                  ; 0040103D _ 75, 0B
        push    esi                                     ; 0040103F _ 56
        push    esi                                     ; 00401040 _ 56
        push    1                                       ; 00401041 _ 6A, 01
        push    esi                                     ; 00401043 _ 56
        call    near [imp_HeapSetInformation]           ; 00401044 _ FF. 15, 00406004(d)
?_0004: mov     eax, 23117                              ; 0040104A _ B8, 00005A4D
        cmp     word [Unnamed_80000000_0], ax           ; 0040104F _ 66: 39. 05, 00400000(d)
        jz      ?_0006                                  ; 00401056 _ 74, 05
?_0005: mov     dword [ebp-1CH], esi                    ; 00401058 _ 89. 75, E4
        jmp     ?_0007                                  ; 0040105B _ EB, 36

?_0006: mov     eax, dword [Unnamed_80000000_0]         ; 0040105D _ A1, 0040003C(d)
        cmp     dword [Unnamed_80000000_0+eax], 17744   ; 00401062 _ 81. B8, 00400000(d), 00004550
        jnz     ?_0005                                  ; 0040106C _ 75, EA
        mov     ecx, 267                                ; 0040106E _ B9, 0000010B
        cmp     word [Unnamed_80000000_0+eax], cx       ; 00401073 _ 66: 39. 88, 00400018(d)
        jnz     ?_0005                                  ; 0040107A _ 75, DC
        cmp     dword [Unnamed_80000000_0+eax], 14      ; 0040107C _ 83. B8, 00400074(d), 0E
        jbe     ?_0005                                  ; 00401083 _ 76, D3
        xor     ecx,D, FC
        pop     edi                                     ; 00401698 _ 5F
        pop     esi                                     ; 00401699 _ 5E
        xor     ecx, ebp                                ; 0040169A _ 33. CD
        pop     ebx                                     ; 0040169C _ 5B
        call    ?_0416                                  ; 0040169D _ E8, 00001BF7
 

that is just a little bit of it! in all it goes 9000 lines!  :o




« Last Edit: August 22, 2011, 01:48:15 PM by soren »

Offline soren

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Re: Disassembling and then reassembling a windows exe file with nasm?
« Reply #5 on: August 22, 2011, 01:57:18 PM »
part of output from borg:

Code: [Select]
;
;             Created by Borg Disassembler
;                   written by Cronos

1000:00401000 ;-----------------------------------------------------------------------
1000:00401000 ;Segment : 1000h     Offset : 401000h     Size : 4400h
1000:00401000 ;32-bit Code
1000:00401000 ;-----------------------------------------------------------------------
1000:00401000 ;                                   XREFS First: 1000:0040109e Number : 3
1000:00401000 loc_00401000:
1000:00401000 8bff                 mov    edi, edi
1000:00401002 55                   push   ebp
1000:00401003 8bec                 mov    ebp, esp
1000:00401005 833d288b400002       cmp    dword ptr [loc_00408b28], 02h
1000:0040100c 7405                 jz     loc_00401013
1000:0040100e e891060000           call   loc_004016a4
1000:00401013 ;                                   XREFS First: 1000:0040100c Number : 1
1000:00401013 loc_00401013:
1000:00401013 ff7508               push   dword ptr [ebp+08h]
1000:00401016 e8da040000           call   loc_004014f5
1000:0040101b 68ff000000           push   0ffh
1000:00401020 e8ea010000           call   loc_0040120f
1000:00401025 59                   pop    ecx
1000:00401026 59                   pop    ecx
1000:00401027 5d                   pop    ebp
1000:00401028 c3                   ret   
1000:00401029 ;                                   XREFS First: 1000:0040118f Number : 1
1000:00401029 loc_00401029:
1000:00401029 6a14                 push   14h
1000:0040102b 6860784000           push   offset loc_00407860
1000:00401030 e8eb120000           call   loc_00402320
1000:00401035 33f6                 xor    esi, esi
1000:00401037 3935bc984000         cmp    dword ptr [loc_004098bc], esi
1000:0040103d 750b                 jnz    loc_0040104a
1000:0040103f 56                   push   esi
1000:00401040 56                   push   esi
1000:00401041 6a01                 push   01h
1000:00401043 56                   push   esi
1000:00401044 ff1504604000         call   dword ptr [HeapSetInformation]
1000:0040104a ;                                   XREFS First: 1000:0040103d Number : 1
1000:0040104a loc_0040104a:
1000:0040104a b84d5a0000           mov    eax, 5a4dh
1000:0040104f 66390500004000       cmp    word ptr [400000h], ax
1000:00401056 7405                 jz     loc_0040105d
1000:00401058 ;                                   XREFS First: 1000:0040106c Number : 3
1000:00401058 loc_00401058:
1000:00401058 8975e4               mov    [ebp-1ch], esi
1000:0040105b eb36                 jmp    loc_00401093
1000:0040105d ;                                   XREFS First: 1000:00401056 Number : 1
1000:0040105d loc_0040105d:
1000:0040105d a13c004000           mov    eax, dword ptr [40003ch]
1000:00401062 81b80000400050450000 cmp    dword ptr [eax+400000h], 4550h
1000:0040106c 75ea                 jnz    loc_00401058
1000:0040106e b90b010000           mov    ecx, 10bh
1000:00401073 66398818004000       cmp    [eax+400018h], cx
1000:0040107a 75dc                 jnz    loc_00401058
1000:0040107c 83b8740040000e       cmp    dword ptr [eax+400074h], 0eh
1000:00401083 76d3                 jbe    loc_00401058
1000:00401085 33c9                 xor    ecx, ecx
1000:00401087 39b0e8004000         cmp    [eax+4000e8h], esi
1000:0040108d 0f95c1               setnz  cl
1000:00401090 894de4               mov    [ebp-1ch], ecx
1000:00401093 ;                                   XREFS First: 1000:0040105b Number : 1
1000:00401093 loc_00401093:
1000:00401093 e85c120000           call   loc_004022f4
1000:00401098 85c0                 test   eax, eax
1000:0040109a 7508                 jnz    loc_004010a4
1000:0040109c 6a1c                 push   1ch
1000:0040109e e85dffffff           call   loc_00401000
1000:004010a3 59                   pop    ecx
1000:004010a4 ;                                   XREFS First: 1000:0040109a Number : 1
1000:004010a4 loc_004010a4:
1000:004010a4 e8d0100000           call   loc_00402179
1000:004010a9 85c0                 test   eax, eax
1000:004010ab 7508                 jnz    loc_004010b5
1000:004010ad 6a10                 push   10h
1000:004010af e84cffffff           call   loc_00401000
1000:004010b4 59                   pop    ecx
1000:004010b5 ;                                   XREFS First: 1000:004010ab Number : 1
1000:004010b5 loc_004010b5:
1000:004010b5 e87a0d0000           call   loc_00401e34
1000:004010ba 8975fc               mov    [ebp-04h], esi
1000:004010bd e82d0b0000           call   loc_00401bef
1000:004010c2 85c0                 test   eax, eax
1000:004010c4 7908                 jns    loc_004010ce

Offline Bryant Keller

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  • Posts: 360
  • Country: us
    • About Bryant Keller
Re: Disassembling and then reassembling a windows exe file with nasm?
« Reply #6 on: August 24, 2011, 03:35:19 AM »
Yep, you'll find that when dealing with C, there is a lot of behind the scenes stuff that needs to go on to get things setup the way the language expects it to be. These "stubs" are one of the main reasons I like writing in assembly rather than C. Your program doesn't actually start at _main, that is a function called by libc's startup files once they have configured the C environment. The true entry point is at _start and you can actually create your own to reduce code size. Check this out on a GNU/Linux system with NASM and C.

First, create the following two files (hello.c and loader.asm):

Code: (hello.c) [Select]
#include <stdio.h>

int main(int argc, char *argv[], char *envp[])
{
printf( "Hello, World!\n" );
return ( 0 );
}

Code: (loader.asm) [Select]
BITS 32
SECTION .text
EXTERN main
GLOBAL _start

_start:
pop ecx
mov esi, esp
push ecx
lea eax, [(esi+4)+(ecx*4)]
push eax
push esi
push ecx
call main
add esp, 12
xor ebx, ebx
xor eax, ebx
xor ebx, eax
xor eax, ebx
inc eax
int 0x80

Now compile and run the C version without the loader:

Code: [Select]
bash-4.1$ gcc -o hello1 hello.c
bash-4.1$ ./hello1
Hello, World!
bash-4.1$

Now assemble the loader and compile it against the C version (and remove the startup code from GCC):
Code: [Select]
bash-4.1$ nasm -f elf -o loader.o loader.asm
bash-4.1$ gcc -nostartfiles -o hello2 hello.c loader.o
bash-4.1$ ./hello2
Hello, World!
bash-4.1$

Great! So both versions seem to work the same. Lets take a look at the listing.
Code: [Select]
bash-4.1$ ls -o1
total 24
-rw-r--r-- 1 bkeller  116 2011-08-24 03:08 hello.c
-rwxr-xr-x 1 bkeller 5525 2011-08-24 03:15 hello1
-rwxr-xr-x 1 bkeller 2019 2011-08-24 03:19 hello2
-rw-r--r-- 1 bkeller  248 2011-08-24 03:12 loader.asm
-rw-r--r-- 1 bkeller  512 2011-08-24 03:19 loader.o
bash-4.1$

Wow! look at that, we actually shaved off 3506 bytes from the C source by just using a custom startup routine. ;) Being as you're on windows, the code will be a bit different. But the idea is the same. Basically libc needs to grab argc, argv, and sometimes envp (not standard) and invoke the main() C function, afterwards it needs to invoke the system shutdown routine. One thing you'll notice, win32's command line argument function (GetCommandLine) returns the command line as a single string, so your programs have to include a routine to parse that command line into an array of character pointers for main before it can start. This adds even more overhead to the final exectuable (in our Linux environment, the argc,argv,envp are available directly on the stack and only need to be located).

About Bryant Keller
bkeller@about.me

Offline NeonFlash

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Re: Disassembling and then reassembling a windows exe file with nasm?
« Reply #7 on: September 03, 2011, 05:01:06 PM »
Hi,

I have a question which fits well to this thread.

I am following the instructions provided in this online resource here:

http://www.phreedom.org/solar/code/tinype/

It talks about minimizing the PE file size by omitting unused header fields from MZ and PE Header.

However, I am stuck at this section: "Switching to assembly and removing the DOS stub".

How have they disassembled the executable C program into an assembly source mentioned there (tiny.asm)?

I have tried using:

ndisasm -b 32 tiny.exe, but it gives a different format.

Also, I used the tiny.asm file provided there and saved it.

Ran the command:

nasm -f bin -o tiny.exe tiny.asm

It throws the following error messages:

Code: [Select]
tiny.asm:71: warning: macro `sectalign' exists, but not taking 0 parameters
tiny.asm:69: error: symbol `sectalign' undefined
tiny.asm:71: error: symbol `sectalign' undefined
tiny.asm:80: error: symbol `sectalign' undefined
tiny.asm:106: error: symbol `sectalign' undefined

It would be great if you could get me past this step and help me in understanding the concept of disassembling better.

Regards,
NeonFlash

Offline Frank Kotler

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Re: Disassembling and then reassembling a windows exe file with nasm?
« Reply #8 on: September 03, 2011, 06:09:06 PM »
We don't do malware here, and the site you link to veers off in that direction. Your question does not involve the malware part, so you're okay. Do not discuss malware here - the moderation team is not in a tolerant mood!

The disassembly of the executable from "tiny.c" is most easily done using Agner Fog's "objconv". To do it in ndisasm:

Code: [Select]
ndisasm -b32 -e1D0h tiny.exe

"tiny.asm" was apparently written before "sectalign" was added to Nasm as a built-in macro.

http://www.nasm.us/xdoc/2.09.10/html/nasmdoc4.html#section-4.11.13

 It uses "sectalign" as:

Code: [Select]
sectalign equ 4

This apparently confuses Nasm. Call it something else (I used "salign"), and I think you'll find that it assembles without complaint. To disassemble this with ndisasm (no point to it):

Code: [Select]
ndisasm -b32 -e0Ch tiny.exe

Further options to ndisasm would prevent it from disassembling the cruft after the instructions. Because of the non-standard header (I assume), "objconv" segfaults(!) on this puppy.

Best,
Frank


Offline NeonFlash

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Re: Disassembling and then reassembling a windows exe file with nasm?
« Reply #9 on: September 03, 2011, 07:05:44 PM »
Thanks Frank for your help.

I apologize for that but my intention was not to post something related to malware here. I found that exercise interesting and it helped me learn more about the PE file format and what are the different header fields involved in it.

I was able to assemble the file, tiny.asm using NASM by replacing sectalign with salign. Thanks :)

I am still confused about the assembly source mentioned there.

When I disassemble the C program executable using ndisasm, it gives the output as shown below:

Code: [Select]
00000000  0000              add [eax],al
00000002  0000              add [eax],al
00000004  0000              add [eax],al
00000006  0000              add [eax],al
00000008  0000              add [eax],al
0000000A  0000              add [eax],al
0000000C  0000              add [eax],al
0000000E  0000              add [eax],al
00000010  0000              add [eax],al
00000012  0000              add [eax],al
00000014  0000              add [eax],al
00000016  0000              add [eax],al
00000018  0000              add [eax],al
0000001A  0000              add [eax],al
0000001C  0000              add [eax],al
0000001E  0000              add [eax],al
00000020  0000              add [eax],al
00000022  0000              add [eax],al

It's giving me the Memory Address Offset / Opcodes / Assembly Language Instruction format.

However, I am looking for a way to convert it to the following format:

Code: [Select]
; tiny.asm

BITS 32

;
; MZ header
;
; The only two fields that matter are e_magic and e_lfanew

mzhdr:
    dw "MZ"                       ; e_magic
    dw 0                          ; e_cblp UNUSED
    dw 0                          ; e_cp UNUSED
    dw 0                          ; e_crlc UNUSED
    dw 0                          ; e_cparhdr UNUSED
    dw 0                          ; e_minalloc UNUSED
    dw 0                          ; e_maxalloc UNUSED
    dw 0                          ; e_ss UNUSED
    dw 0                          ; e_sp UNUSED
    dw 0                          ; e_csum UNUSED
    dw 0                          ; e_ip UNUSED
    dw 0                          ; e_cs UNUSED
    dw 0                          ; e_lsarlc UNUSED
    dw 0                          ; e_ovno UNUSED
    times 4 dw 0                  ; e_res UNUSED
    dw 0                          ; e_oemid UNUSED
    dw 0                          ; e_oeminfo UNUSED
    times 10 dw 0                 ; e_res2 UNUSED
    dd pesig                      ; e_lfanew

;
; PE signature
;

pesig:
    dd "PE"

;
; PE header
;

pehdr:
    dw 0x014C                     ; Machine (Intel 386)
    dw 1                          ; NumberOfSections
    dd 0x4545BE5D                 ; TimeDateStamp UNUSED
    dd 0                          ; PointerToSymbolTable UNUSED
    dd 0                          ; NumberOfSymbols UNUSED
    dw opthdrsize                 ; SizeOfOptionalHeader
    dw 0x103                      ; Characteristics (no relocations, executable, 32 bit)

;
; PE optional header
;

filealign equ 1
sectalign equ 1

%define round(n, r) (((n+(r-1))/r)*r)

opthdr:
    dw 0x10B                      ; Magic (PE32)
    db 8                          ; MajorLinkerVersion UNUSED
    db 0                          ; MinorLinkerVersion UNUSED
    dd round(codesize, filealign) ; SizeOfCode UNUSED
    dd 0                          ; SizeOfInitializedData UNUSED
    dd 0                          ; SizeOfUninitializedData UNUSED
    dd start                      ; AddressOfEntryPoint
    dd code                       ; BaseOfCode UNUSED
    dd round(filesize, sectalign) ; BaseOfData UNUSED
    dd 0x400000                   ; ImageBase
    dd sectalign                  ; SectionAlignment
    dd filealign                  ; FileAlignment
    dw 4                          ; MajorOperatingSystemVersion UNUSED
    dw 0                          ; MinorOperatingSystemVersion UNUSED
    dw 0                          ; MajorImageVersion UNUSED
    dw 0                          ; MinorImageVersion UNUSED
    dw 4                          ; MajorSubsystemVersion
    dw 0                          ; MinorSubsystemVersion UNUSED
    dd 0                          ; Win32VersionValue UNUSED
    dd round(filesize, sectalign) ; SizeOfImage
    dd round(hdrsize, filealign)  ; SizeOfHeaders
    dd 0                          ; CheckSum UNUSED
    dw 2                          ; Subsystem (Win32 GUI)
    dw 0x400                      ; DllCharacteristics UNUSED
    dd 0x100000                   ; SizeOfStackReserve UNUSED
    dd 0x1000                     ; SizeOfStackCommit
    dd 0x100000                   ; SizeOfHeapReserve
    dd 0x1000                     ; SizeOfHeapCommit UNUSED
    dd 0                          ; LoaderFlags UNUSED
    dd 16                         ; NumberOfRvaAndSizes UNUSED

I am not sure what this format is called specifically. I assume this is the assembly source. Is this written manually or is there a way to get this type of format by disassembling?

Thanks for your patience in taking the time to help me out :)

Regards,
NeonFlash

Offline Frank Kotler

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Re: Disassembling and then reassembling a windows exe file with nasm?
« Reply #10 on: September 03, 2011, 09:06:52 PM »
No apology necessary. There's no actual malicious code there. Unfortunately(?) they chose "download and execute" as the "challenge"... and they use a "trick" to do it, so there's really no "code" involved...

What you show above is plain assembly source, and it's "hand-written" - there's no way a disassembler could know that some fields are supposed to be "dw" and others "dd"... let alone "rounded"... In the executable, it's just a big clump of bytes. A lot of information is lost in going from source (any language) to an executable. Agner Fog's "objconv" knows about ELF and PE headers 'cause Agner told it - the information is not actually in the executable. Also, "objconv" puts the address and opcode bytes on the right, as a comment, and the disassembled instructions on the left - so the thing can actually be assembled. Neat tool!

That "Tiny PE" page says:

Quote
We'll disassemble our 468 byte C program and convert it to assembly source that can be assembled with NASM.

They don't mention that "convert" involves rewriting it "by hand". For practical purposes, the answer remains: "no, you can't do that".

Best,
Frank