Inline Asm w VB6


Author: Dave
Date: 02.04.10 - 6:07am



Just a quick post for documentation sake of using inline asm with VB6 (or at least as close as we can get to it without an external c dll.

In terms of development of the asm to put in. You can use your C Compiler to generate it for you..here are the tips.

  • CallwindowsProc has 5 arguments max and can return a long The first argument is already used so your args start at [EBP+0x0C] this means use a dummy int arg first in your prototype to line up args

  • do not use any sub functions from your code do things in blocks in you have to.

  • once you generate your code, you can extract the opcodes from VC in debug mode viewing mixed mode disasm (develop as an exe usually although you may have to as a dll to use with vb as standard call dll)

  • you need to strip all the function prolog and epilog asm from the compiler generated block (or use naked declspec) your ret should be RETN 10h

  • keep a couple nops (&H90) in place at start in case you need room to add a breakpoint (&hCC) manually to stop on your asm in a debugger to debug it. yes you will have to use ollydbg to debug it in asm most likley.

  • you can twiddle with the CallWindowProc prototypes more based on what you are using it for..see last example.



Full example here:
http://sandsprite.com/CodeStuff/vb_cdecl.zip

Note: All single quotes for comments are stripped by my blog script
same as default CallWindowProc except
param 1 is now "ByRef lpBytes As Any"
or you can use the default like this: CallWindowProc( Varptr( asmBytes(0) ) ,
Private Declare Function CallAsm Lib "user32" 
    Alias "CallWindowProcA" _
    (ByRef lpBytes As Any, 
    ByVal hWnd As Long, 
    ByVal Msg As Long, 
    ByVal wParam As Long, 
    ByVal lParam As Long) As Long

Function Shl(x As Long) As Long
    8B45 0C        MOV EAX,DWORD PTR SS:[EBP+12]
    D1E0           SHL EAX,1
    C2 10 00       RETN 10h
    Dim o() As Byte
    Const sl As String = "8B 45 0C D1 E0 C2 10 00"
    o() = toBytes(sl)
    Shl = CallAsm(o(0), x, 0, 0, 0)
End Function

Function Shr(x As Long) As Long
    8B45 0C        MOV EAX,DWORD PTR SS:[EBP+12]
    D1E8           SHR EAX,1
    C2 10 00       RETN 10h
    Dim o() As Byte
    Const sr As String = "8B 45 0C D1 E8 C2 10 00"
    o() = toBytes(sr)
    Shr = CallAsm(o(0), x, 0, 0, 0)
End Function

Private Function ShlX(x As Long, shift As Byte) As Long
    8B45 0C        MOV EAX,DWORD PTR SS:[EBP+12]
    C1E0 12        SHL EAX,12
    C2 10 00       RETN 10h
    Dim o() As Byte
    Const sl As String = "8B 45 0C C1 E0 __ C2 10 00"
    o() = toBytes(Replace(sl, "__", Hex(shift)))
    Shl = CallAsm(o(0), x, 0, 0, 0)
End Function

Private Function ShrX(x As Long, shift As Byte) As Long
    8B45 0C        MOV EAX,DWORD PTR SS:[EBP+12]
    C1E8 12        SHR EAX,12
    C2 10 00       RETN 10h
    Dim o() As Byte
    Const sr As String = "8B 45 0C C1 E8 __ C2 10 00"
    o() = toBytes(Replace(sr, "__", Hex(shift)))
    ShrX = CallAsm(o(0), x, 0, 0, 0)
End Function

Function getESP() As Long

    Dim C As Currency 8 bytes, initially all 0s
    
    	his isnt a naked function, so the esp is actually local to this
    function frame, but since we only use it for relative size differences
    etween before and after and there are no stack mods in vb after prolog
    
ormally, this is fine as a standalone function
    
    8BC4           MOV EAX,ESP
    C2 10 00       retn 10h
    
    CopyMemory C, &H10C2C48B, 4
    getESP = CallAsm(C, 0, 0, 0, 0)

End Function

Function toBytes(x As String) As Byte()
    Dim tmp() As String
    Dim fx() As Byte
    Dim i As Long
    
    tmp = Split(x, " ")
    ReDim fx(UBound(tmp))
    
    For i = 0 To UBound(tmp)
        fx(i) = CInt("&h" & tmp(i))
    Next
    
    toBytes = fx()

End Function




Another example of working on a byte buffer in your asm Private Declare Function CallAsm2 Lib "user32" Alias "CallWindowProcA" _ (ByRef lpBytes As Any, ByRef chararray As Any, ByVal length As Long, ByVal unused1 As Long, ByVal unused2 As Long) As Long

Const opcodes As String = "909090C745F800000000EB098B4DF88" & _ "3C101894DF88B55F83B55107D258B45" & _ "0C0345F88A08884DFC8B45F833D28A5" & _ "5FC2AD08855FC8B550C0355F88A45FC" & _ "8802EBCA9090C21000" fx() = toBytes2(opcodes) CallAsm2 fx(0), _ byteBufferToWorkOn(0), _ UBound(byteBufferToWorkOn), _ 0, 0 opcodes are for the following C with the prolog and epilog stripped out.. void __stdcall fnDecode(int dummy, char* b, int len) { char x; for(int i=0; i len; i++){ x = b[i]; _asm{ //do stuff to x here } b[i] = x; //update vb byte buffer } } Function toBytes2(x As String, Optional debugit As Boolean = False) As Byte() Dim tmp() As String Dim fx() As Byte Dim i As Long Dim y ReDim fx(Len(x) / 2) For i = 1 To Len(x) Step 2 fx(y) = CByte(CLng("&h" & Mid(x, i, 2))) y = y + 1 Next If debugit Then fx(0) = &HCC toBytes2 = fx() End Function
- more C Source - Const asm_rol32 = "x55x8BxECx56x8Bx4Dx10x83xE1x1Fx8Bx45x0C" & _ "xD3xE0x8Bx4Dx10x83xE1x1FxBAx20x00x00x00" & _ "x2BxD1x83xE2x1Fx8Bx75x0Cx8BxCAxD3xEEx0B" & _ "xC6x5Ex5DxC2x10x00" Const asm_add32 = "x55x8BxECx8Bx45x0Cx03x45x10x5dxC2x10x00" Function rol32(base As Long, bits As Long) rol32 = CallWindowProc(VarPtr(asm(1)), 0, base, bits, 0) End Function Function add32(v1 As Long, v2 As Long) 'no overflow, allows wrap add32 = CallWindowProc(VarPtr(asm2(1)), 0, v1, v2, 0) End Function






Comments: (3)

On 09.04.11 - 4:17pm gheo wrote:
Golden code mate ! Thanks for this knowledge.

On 01.31.20 - 5:12pm Dave wrote:
another example of writing/debugging your asm in C thats compatible with the CallWindowProc prototype: http://sandsprite.com/blogs/index.php?uid=7&pid=144

On 02.05.20 - 7:45am Dave wrote:
Dep safe of the above:
https://github.com/dzzie/libs/blob/master/pe_lib3/modAsm.bas

 
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