Display the result of a registry addition

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Mino

Joined: 14 Jan 2018
Posts: 163

Mino 24 Jan 2018, 16:00

Hello

I have one more question if you do not mind:

How to display a value or a string without using printf or a C function, without anything in fact?

I know that there is the "int" statement to use system interrupts. I try this from another website (using x86 intel asm):

Code:

...
push msg
int 21h
...

But it does not work :'(

What would you advise me?

24 Jan 2018, 16:00

Furs

Joined: 04 Mar 2016
Posts: 2493

Furs 24 Jan 2018, 16:02

int instruction is for DOS or 32-bit Linux. It's a system call. You don't need a system call to print on Windows.

You can use WriteFile (Windows API, kernel32.dll) to write to the console directly, without printf. But it's more complicated. (you also have to obtain the stdout handle). That's the "lowest level" you can get, probably. Since Windows is based on user-mode APIs (libraries, DLLs) and all system calls are hidden (and they change amongst Windows versions! but APIs don't).

printf probably ends up calling WriteFile anyway.

BTW, "call" is also an instruction, just like "int". The difference is that the latter switches to kernel mode (and thus calls a system function).

24 Jan 2018, 16:02

Mino

Joined: 14 Jan 2018
Posts: 163

Mino 24 Jan 2018, 16:21

OK thanks a lot Smile

And when we use "int" then?

24 Jan 2018, 16:21

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20300
Location: In your JS exploiting you and your system

revolution 24 Jan 2018, 16:57

Mino wrote:

OK thanks a lot
And when we use "int" then?

That depends upon your OS. In Windows you wouldn't ever need to use it (in theory at aleast), in Linux you can use either syscall or int 0x80. If you are writing some BIOS interface code then you use int for everything. If you are programming bare-metal then you don't use it at all unless for your functions.

24 Jan 2018, 16:57

yeohhs

Joined: 19 Jan 2004
Posts: 195
Location: N 5.43564° E 100.3091°

yeohhs 24 Jan 2018, 22:26

Mino wrote:

How to display a value or a string without using printf or a C function, <snipped>?
What would you advise me?

There is FASMLIB where many useful functions (including I/O) are implemented in asm. FASMLIB comes with source.
You can download it from here. http://fasmlib.x86asm.net/

There is also BASELIB from fasmnewbie.
https://sourceforge.net/projects/baselibs/files/
or
https://plus.google.com/108856344528527240894

24 Jan 2018, 22:26

Mino

Joined: 14 Jan 2018
Posts: 163

Mino 25 Jan 2018, 17:25

Hello again!
I would like to know why, after compiling my file, it appears as such:

The executable works, but it's a little annoying to have to be in Admin to run it.

Do you have any idea Smile

?

PS: Thank you for the answer, I hadn't seen it Embarassed

25 Jan 2018, 17:25

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20300
Location: In your JS exploiting you and your system

revolution 25 Jan 2018, 17:50

What code are you running? What OS are you using? Does your code try to write to any protected directories? Are you trying to access low level functions?

25 Jan 2018, 17:50

Mino

Joined: 14 Jan 2018
Posts: 163

Mino 26 Jan 2018, 07:22

Quote:

What code are you running?

I tried with the sent program, during the 1st post

Quote:

What OS are you using?

Windows 10

Quote:

Does your code try to write to any protected directories? Are you trying to access low level functions?

It's possible, but frankly, I don't know.

26 Jan 2018, 07:22

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20300
Location: In your JS exploiting you and your system

revolution 26 Jan 2018, 07:31

The first post has this code:

Code:

mov eax, 7
add eax, 3
; Print what's in eax

But did you wrap it in a DOS format or a Windows (GUI or console) format? Or some other format?

Normally programs won't need to be admin unless you are accessing protected resources. If you post the code you assembled we can probably help you.

26 Jan 2018, 07:31

Mino

Joined: 14 Jan 2018
Posts: 163

Mino 26 Jan 2018, 11:36

The code is exactly the same as the file sent, but here it is anyway:

Code:

;----------------------------------------------------------
; Program Name: integeradd
; Purpose     : integer addition
; Date        : August 2008
; Author      : Yeoh HS
; FASM        : Built using Flat Assembler version 1.67.27
;               edited and compiled with FASM's IDE.
;----------------------------------------------------------
format PE CONSOLE 4.0
entry start

include 'win32a.inc'

section '.data' data readable writeable
    val1      dd  7
    val2      dd  3

section '.code' code readable executable
start:
    ;mov eax, [val1]
    mov eax, [val2]
    add [val1], eax

    stdcall show_me, dfmt,intval,[val1]

    stdcall waitforchar
    invoke  ExitProcess,0

;---------------------------------------------------------
;   Proc to show values of data variables.
;---------------------------------------------------------
proc show_me, fmt, prompt, val
    cinvoke printf, [fmt], [prompt], [val]
    cinvoke printf, strfmt, CRLF
    ret
endp

;----------------------------------------------------------
;  Proc to prevent console window from closing immediately
;----------------------------------------------------------
proc waitforchar
   cinvoke printf, strfmt, CRLF
   cinvoke printf, strfmt, msg
   cinvoke getchar
   ret
endp

section '.data' data readable writeable

    CRLF      db '',13,10,0  ; carriage return and linefeed

    dfmt      db '%s = %d',0
    intval    db 'Integer value',0

    strfmt    db  '%s',0
    msg       db  'Press the Enter key...',0


section '.idata' import data readable writeable

library kernel32,'kernel32.dll',\
        user32,  'user32.dll',\
        msvcrt,  'msvcrt.dll'

include 'api\kernel32.inc'
include 'api\user32.inc'

import msvcrt,\
       printf, 'printf',\
       getchar,'getchar'

; end of file =================================================================

26 Jan 2018, 11:36

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20300
Location: In your JS exploiting you and your system

revolution 26 Jan 2018, 18:58

AFAICT there is nothing in that code that will need admin privilege.

Can you run the example "hello world!" code that comes with fasm?

26 Jan 2018, 18:58

Mino

Joined: 14 Jan 2018
Posts: 163

Mino 26 Jan 2018, 20:51

An "Hello, world!" GUI program, from the FASM examples:

Code:

include 'win32ax.inc'

.code

  start:
        invoke  MessageBox,HWND_DESKTOP,"Hello, world!",invoke GetCommandLine,MB_OK
        invoke  ExitProcess,0

.end start

What's strange is that if I change (drag and drop) the generated application, ADMIN mode is no longer required.

26 Jan 2018, 20:51

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