How to get Type / LengthOf / SizeOf info for Proc Arguments

[tarka]

The code below shows what I am trying to achieve. This code works (single stepping in OllyDbg) but only because the arguments used for the Type and SizeOf statements are those of the actual variables. For a generalised solution I need to obtain these values dynamically for the real variables via the DWORD pointers passed by the INVOKE statement. I have searched extensively and tried all the variations I can generate without success. I assume that the solution needs to be something like "mov  ah,Type [esi]", whilst this statement compiles OK it returns a value of 0 rather than 1 at runtime. All the other variations that I have tried return the error "A2166 Structure Field expected"

It would be appreciated if somebody could help me with this and provide a sample of the instructions that I need to use.

CODE
~~~~
.386
.model flat, stdcall
option casemap:none
include \masm32\include\windows.inc
include \masm32\include\kernel32.inc
includelib \masm32\lib\kernel32.lib
include \masm32\include\user32.inc
includelib \masm32\lib\user32.lib

testpro proto :DWORD,:DWORD

.data
Msg1        db "This is my first message string",0
Msg2        db "                               ",0
Wrd1        dw 25,26
DWd1        dd 100,200
DQw1        dq 1000,2000
DTw1        dt 12345,67890

.code
start:
    invoke  testpro, addr Msg1, addr Msg1           
    invoke ExitProcess,NULL

testpro proc Sstr:DWORD, Dstr:DWORD
    mov     esi,Sstr
    mov     edi,Dstr
    ;Check both parameters are of type(byte)
    mov     ah,Type Msg1
    mov     al,Type Msg2
    cmp     ah,al
    jz      testpro1
    mov     eax,1
    jmp     testpro5
testpro1:
    ;Check Sizes are equal, if unequal use shortest for Count
    mov     ah,SizeOf Msg1
    mov     al,SizeOf Msg2
    cmp     al,ah
    jg      testpro2
    jmp     testpro3
testpro2:
    xchg    al,ah
testpro3:   
    and     eax,000000FFh
    dec     eax
    mov     ecx,eax
testpro4:   
    lodsb
    stosb
    loop    testpro4   
testpro5:   
    ret
testpro endp

end start

Cheers and Thanks

Tarka
 

[Mirno]

Unfortunately this isn't possible using native types. I assume you're familiar with languages like Java that have type information as an intrinsic part of the classes, and this is where you're getting the idea from...

As I said, this is not the case - languages like java have a whole lot of hidden code and information in order to do this.
Programming is like working under a contract - you cannot expect a given result unless you abide by the rules of the contract. For example, you can't use the strlen function with a number, and expect a reasonable result (the decimal length of the number?) - the contract states you pass in a pointer to a string, if you pass a pointer to anything else the contract is void.

Mirno

[tarka]

Thanks Mirno

If type checking as I was trying to implement it is not available, how do you prevent someone passing the wrong arguments and causing a mal function. i.e. a buffer overrun. I had considered calculating the string length by scanning for the "0" terminator. However if the routine were passed two quadbytes instead of strings, or even a byte string without a terminator, there would not be any terminators and the first zero that would be found would be a random event and belong to something else. In my case that would cause me to incorrectly overwrite an area of memory with a potentially nasty result.

How are arguments validated in say a .dll. I assume there must be a way otherwise it would be a simple to crash any program . Or do we assume that all users of shared code will only use it in the correct manner?

Comments Please

Cheers & Thanks

Tarka   

[jj2007]

You might use a macro and check the opattr directive. This allows to call the proc based on the info available at assembly time. See here for an example.

[Vortex]

tarka,

Here is where you can find information about the OPATTR expression :

http://webster.cs.ucr.edu/Page_TechDocs/MASMDoc/ProgrammersGuide/Chap_09.htm

[tarka]

Thanks jj2007 & Vortex

I have read the info about OPATTR and found it helpful. Unfortunately, it does not provide exactly what I want but it has given me some more ideas. I now think its a case of back to the drawing board and design it again!

Thanks for your help.

Cheers & Thanks

Tarka