Simple Runtime Framework by Example
Written by Alexey Lyashko   
Wednesday, 06 June 2012
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Simple Runtime Framework by Example
Example software framework

 

In the case of .NET framework (at least as far as I know), the loader identifies a file as a .NET executable, reads in the meta header, and initializes the mscoree.dll appropriately. We will not go through all those complications and will use a regular PE file:

 

frame2

Figure 2

Customized PE file

  • PE Header - regular PE Header, no modification needed;
  • Code Section- simply invokes the core function of the framework:push pseudo_code_base_address
    call [core]
  • Import Section - regular import section that only imports one function from the framework.dll - framework.core(unsigned int);
  • Data Section - this section contains the actual compiled pseudo assembly code and whatever headers you may come up with, that may instruct the core() function to correctly initialize the application.

Example Executable Source Code

The following is the source code of the example executable. It may be compiled with FASM (Flat Assembler).

include 'win32a.asm'
 ;we need the 'import' macro

include
'asm.asm'    
 ;pseudo assembly commands and constants

format PE console
entry start
section '.text' readable executable
start:
  push _base  
  call [core_func]
 

section
'.idata' data import writeable

library
  framework, 'framework.dll'

import framework,\  
     core_func, 'Core'

section '.data' readable writeable
_base
:
    loadi A, _base
   loadi B, 0x31
   _add A, B
   loadr B, A
   loadi A, _data.string
   loadi C, _data.string_len
   _call _func
   loadi A, 1
   loadi B, _data.string
   loadi C, _data.str_len
   _int sys_write
   loadi A, 1
   loadi B, _data.msg
   loadi C, _data.msg_len
   _int sys_write
   _int sys_exit


_func:
   ; A = string address
   ; B = key
   ; C = counter
.decode:
   loadr D, A
   xorr D, B
   storr A, D
   loadi D, 4
   _add A, D
   _loop .decode
   _ret



_data:
  .string db 'Hello, developer!', 10, 13
  .str_len = $-.string
  db 0
  .string_len = ($-.string)/4
  .msg db 'The program will now exit.',
                                  10, 13
  .msg_len = $-.msg

;Encrypt one string
load k dword from _base + 0x31
repeat 5
load a dword from _data.string+(% - 1)*4
a = a xor k
store dword a at _data.string + (% - 1)*4
end repeat

The code above produces a tiny executable which invokes framework's core() function. Pseudo assembly code simply prints two messages (the first one is decoded prior to being printed). Full source code is available, see link at the end of the article. 

The good thing is that you do not have to start the interpreter and load this executable (or specify it as a command line parameter) - you may simply run this executable, Windows loader will bind it with the framework.dll automatically. 

The bad thing is that you would, most probably, have to write your own compiler, because writing assembly is fun, dealing with pseudo assembly is fun as well, BUT, only when done for fun. It is not as pleasant when dealing with production code.

Possible uses

Unless you are trying to create a framework that would improve on existing software frameworks, you could use such approach to increase the protection of your applications by, for example, virtualizing cryptography algorithms or any other part of your program which is not crucial in terms of execution speed, but represents a sensitive intellectual property.

 

Source code for this article may be found here.

 

This article originally appeared on Alexey Lyashko's blog System Programming and is reproduced here with kind permission of the author.

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Last Updated ( Wednesday, 06 June 2012 )
 
 

   
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