ExtTextOut
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Thread: ExtTextOut

  1. #1
    Join Date
    Jan 2011
    Posts
    4

    ExtTextOut

    Hi all. I am using the MS Detours to intercept 4 API functions, DrawText, DrawTextEx, TextOut and ExtTextOut. This is all occurring in a small Unmanaged C++ Dll that I put together (after much Google time, C++ not my forte). From the intercepted functions I call a function that puts together a small COPYDATASTRUCT and ships it off the my main, originating VB.Net app. In the VB.Net app I am retrieving the struct, interpreting and obtaining all text that passes through the target app.

    Everything seems to work like a charm for the two DrawText APIs but not for the two TextOut APIs. First off, whenever I close the VB.Net app (WinForm exe) the target app (ANY app, I have tested on countless programs) crashes whenever I hook the TextOut functions. Secondly, all text retrieved from these functions is completely jumbled. Text is perfect from the DrawText functions, and they do not cause errors or crashes (with the TextOut hooks commented out.

    Can anybody please direct me to what I may be doing wrong? Any help/advice is greatly appreciated as I have spent weeks scouring Google and the web for an answer.

    Code snippets below. Please ask and I will happily provide more info/code.

    TIA

    C++ code:
    Code:
    enum Win32API
    {
        API_TextOut = 1,
        API_ExtTextOut = 2,
        API_DrawText = 3,
        API_DrawTextEx = 4
    };
    
    void TransmitToGUI(int Len, LPCWSTR szText, Win32API sourceAPI)
    {
    	HWND hWnd = ::FindWindow(NULL, L"AutoERP::");
      if (!hWnd)
    	{
        return;
    	}
    	else
    	{
    		if(!szText)
    			return;
    
    		wchar_t Text[255];
    		wcscpy_s(Text, szText);
    
    		COPYDATASTRUCT cds;
    		::ZeroMemory(&cds, sizeof(COPYDATASTRUCT));
    		cds.dwData = sourceAPI;
    		cds.cbData = (Len+1)*2;
    		cds.lpData = &Text;
    		
    		::SendMessage(hWnd, WM_COPYDATA, NULL, (LPARAM)&cds);
    
    	}
    }
    
    BOOL (WINAPI * Real_TextOut)(HDC a0, int a1, int a2, LPCWSTR a3, int a4) = TextOutW;
    BOOL (WINAPI * Real_ExtTextOut)(HDC a0, int a1, int a2, UINT a3, CONST RECT* a4, LPCWSTR a5, UINT a6, CONST INT* a7) = ExtTextOutW;
    int (WINAPI * Real_DrawText)(HDC a0, LPCWSTR a1, int a2, LPRECT a3, UINT a4) = DrawTextW;
    int (WINAPI * Real_DrawTextEx)(HDC a0, LPWSTR a1, int a2, LPRECT a3, UINT a4, LPDRAWTEXTPARAMS a5) = DrawTextExW;
    
    BOOL WINAPI Mine_TextOut(HDC hdc,int X,int Y,LPCWSTR text,int textLen)
    {
      BOOL rv = Real_TextOut(hdc, X, Y, text, textLen);
    	if (text)
    	{
    		TransmitToGUI(textLen, text, API_TextOut);
    	}
    	return rv;
    }
    BOOL WINAPI Mine_ExtTextOut(HDC hdc, int X, int Y, UINT options, RECT* lprc, LPCWSTR text, UINT cbCount, INT* lpSpacingValues)
    {
      BOOL rv = Real_ExtTextOut(hdc, X, Y, options, lprc, text, cbCount, lpSpacingValues);
    	if (text)
    	{
    		TransmitToGUI(cbCount, text, API_ExtTextOut);
    	}
    	return rv;
    }
    int WINAPI Mine_DrawText(HDC hdc, LPCWSTR text,  int nCount, LPRECT lpRect, UINT uOptions)
    {
      int rv = Real_DrawText(hdc, text, nCount, lpRect, uOptions);
    	if (text)
    	{
    		TransmitToGUI(nCount, text, API_DrawText);
    	}
    	return rv;
    }
    int WINAPI Mine_DrawTextEx(HDC hdc, LPWSTR text, int textLength, LPRECT lpRect, UINT uFormat, LPDRAWTEXTPARAMS lpParams)
    {
      int rv = Real_DrawTextEx(hdc, text, textLength, lpRect, uFormat, lpParams);
    	if (text)
    	{
    		TransmitToGUI(textLength, text, API_DrawTextEx);
    	}
    	return rv;
    }
    VB.Net code:
    Code:
    	Public Structure COPYDATASTRUCT
    		Public dwData As Integer
    		Public cbData As Integer
    		Public lpData As IntPtr
    	End Structure
    
    	Friend Enum Win32API
    		API_TextOut = 1
    		API_ExtTextOut = 2
    		API_DrawText = 3
    		API_DrawTextEx = 4
    	End Enum
    
    	<System.Security.Permissions.PermissionSetAttribute(System.Security.Permissions.SecurityAction.Demand, Name:="FullTrust")> _
    	 Protected Overrides Sub WndProc(ByRef m As Message)
    
    		Select Case (m.Msg)
    			Case WM_HOTKEY '// hotkey detected, install hook
    				SetCBTHook()
    
    			Case WM_COPYDATA '// data coming from [API] hook(s)
    				DataReceived(m)
    
    			Case Else
    				MyBase.WndProc(m)
    		End Select
    
    	End Sub
    
    	Private Sub DataReceived(ByRef m As Message)
    		Try
    
    			'// marshal and extract COPYDATASTRUCT from pointer
    			Dim cds As New COPYDATASTRUCT()
    			cds = DirectCast(Marshal.PtrToStructure(m.LParam, GetType(COPYDATASTRUCT)), COPYDATASTRUCT)
    
    			'// if our call, process data
    			If ((cds.dwData >= 1) AndAlso (cds.dwData <= 4)) Then
    				'// marshal and extract string from pointer
    				Dim tms As String = Marshal.PtrToStringAuto(cds.lpData, cds.cbData)
    				'// display text (if present)
    				If (tms.Replace(" ", String.Empty).Length > 0) Then
    					If ((cds.dwData = 1) OrElse (cds.dwData = 2)) Then
    						ListBox1.Items.Add([Enum].GetName(GetType(Win32API), cds.dwData) & ": " & tms)
    						ListBox1.SelectedIndex = ListBox1.Items.Count - 1
    					Else
    						ListBox2.Items.Add([Enum].GetName(GetType(Win32API), cds.dwData) & ": " & tms)
    						ListBox2.SelectedIndex = ListBox2.Items.Count - 1
    					End If
    				End If
    				Else
    					MyBase.WndProc(m)
    				End If
    
    		Catch ex As Exception
    			'MsgBox(ex.Message)
    		End Try
    	End Sub
    Last edited by ovidiucucu; January 14th, 2011 at 06:49 AM. Reason: Corrected [CODE] tags

  2. #2
    Join Date
    Apr 2000
    Location
    Belgium (Europe)
    Posts
    4,100

    Re: ExtTextOut

    Length in Drawtext can be -1 indicating it's a zero terminated string.
    Your transmit code seems to assume it'll always be properly filled in.

    In the case of textout, the length is always provided, but you cannot assume the string will be terminated, thus simply doing length+1 is incorrect.

  3. #3
    Join Date
    Jan 2011
    Posts
    4

    Re: ExtTextOut

    Thanks for reply.

    Forgive my less than ideal knowledge of C++, but what is the best way to handle these various situations?

    If I test for -1 in the drawtext methods then I know if it is zero terminated, but then how to I get the length in the VB side to pull string from ptr?

    As far as the textout, I can understand how this can cause errors, but will this have something to do with the garbled text as well? I was reading about unicode and glyph tables, but I am getting symbols and characters on every textout and exttextout call from every target app I test. Seems unlikely that the entire system is using glyphs? Or is that likely?

    Thanks again.

  4. #4
    Join Date
    Jan 2011
    Posts
    4

    Re: ExtTextOut

    Any further help is greatly appreciated here please.

    If I test for a zero terminated string then I would know whether to pass ((len+1)*2) or (len*2). Is this assumption correct?

    If so, and given that the LPCWSTR is a pointer, how exactly can I test if zero terminated??

    Again, many TIA!

  5. #5
    Join Date
    Jan 2011
    Posts
    4

    Re: ExtTextOut

    Small *bump* for any further assistance from anyone plz?

    TIA

  6. #6
    Join Date
    Jan 2011
    Posts
    4

    Re: ExtTextOut

    Nice sharing.................

  7. #7
    Join Date
    Jun 2011
    Posts
    2

    Re: ExtTextOut

    Hi,

    I'm also having the problem that the hooked ExtTextOutW function produces garbled text. I'm on Windows 7 with Microsoft Detours 2.1 and Visual Studio C++ 2010 Express.

    Did you manage to solve the problem, msfiend? The only observation I was able to make is that there seems to be a mapping from the garbled characters to the real ones. In my case for example, what should be a lower case "i" is always displayed as "L" and "e" is displayed as "H". I tried using WideCharToMultiByte but without any luck.

    I'll buy the person who helps us a beer!

    Cheers,
    Michael

  8. #8
    VictorN's Avatar
    VictorN is offline Super Moderator Power Poster
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    Location
    Wallisellen (ZH), Switzerland
    Posts
    17,605

    Re: ExtTextOut

    Quote Originally Posted by mherrmann View Post
    I'm also having the problem that the hooked ExtTextOutW function produces garbled text.
    ,,,
    I tried using WideCharToMultiByte but without any luck.
    Perhaps you should use MultiByteToWideChar to obtain a wchat_t* string to be passed in ExtTextOutW function?
    Victor Nijegorodov

  9. #9
    Join Date
    Jun 2011
    Posts
    2

    Re: ExtTextOut

    Thanks for your reply VictorN,

    I think the "problem" is that the string passed to ExtTextOut is not a string of characters but a string of glyphs, as per the ETO_GLYPH_INDEX parameter to ExtTextOut.

    Does anybody have an idea where the glyphs that are passed to ExtTextOut are computed? The documentation for ExtTextOut mentions GetCharacterPlacement but in my case that doesn't seem to be it (I hooked GetCharacterPlacement and it's never called).

    TIA!

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