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The bug in MFC tool tip control

, 13 Sep 2010 CPOL
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The bug in MFC tool tip control

Yesterday, I encountered the bug in MFC’s tool tip control implementation.
The bug dozes in the following code:

LRESULT CToolTipCtrl::OnAddTool(WPARAM wParam, LPARAM lParam)
    TOOLINFO ti = *(LPTOOLINFO)lParam;       <---------------------------- HERE
    if ((ti.hinst == NULL) && (ti.lpszText != LPSTR_TEXTCALLBACK)
        && (ti.lpszText != NULL))
        void* pv;
        if (!m_mapString.Lookup(ti.lpszText, pv))
            m_mapString.SetAt(ti.lpszText, NULL);
        // set lpszText to point to the permanent memory associated
        // with the CString
        VERIFY(m_mapString.LookupKey(ti.lpszText, ti.lpszText));
    return DefWindowProc(TTM_ADDTOOL, wParam, (LPARAM)&ti);

If you look at the definition of TOOLINFO structure closely, you’ll find that this structure has a variable size which depends on the compilation macros defined:

typedef struct {
  UINT      cbSize;
  UINT      uFlags;
  HWND      hwnd;
  UINT_PTR  uId;
  RECT      rect;
  HINSTANCE hinst;
  LPTSTR    lpszText;
#if (_WIN32_IE >= 0x0300)
  LPARAM    lParam;
#if (_WIN32_WINNT >= Ox0501)
  void      *lpReserved;

That is, a structure pointed to by lParam can have a size from 40 to 48 bytes long depending on the compilation macros defined and can’t be just copied to structure created inside the MFC module. Dealing with variable-size structures, you can copy only a common part of a structure or analyze its version first (that is the cbSize field was made for!).

We can easily figure out what TOOLINFO’s size can be expected by MFC:

7c1a21e5 55              push    ebp
7c1a21e6 8bec            mov     ebp,esp
7c1a21e8 83ec30          sub     esp,30h
7c1a21eb 53              push    ebx
7c1a21ec 56              push    esi
7c1a21ed 8b750c          mov     esi,dword ptr [ebp+0Ch]
7c1a21f0 57              push    edi
7c1a21f1 8bd9            mov     ebx,ecx
7c1a21f3 6a0c            push    0Ch
7c1a21f5 59              pop     ecx
7c1a21f6 8d7dd0          lea     edi,[ebp-30h]
7c1a21f9 f3a5            rep movs dword ptr es:[edi],dword ptr [esi]  

The size of TOOLINFO inside the MFC module is equal to 0xc*4 = 0x30 = 48 bytes. And the size of TOOLINFO structure inside our product’s module was equal to 44 bytes!

Of course, in most cases nothing criminal will happen. CToolTipCtrl::OnAddTool just copies 4 extra bytes – it's eventually not even used. The only case when the problem may arise – if those 4 extra bytes don’t have read access. Though this situation is hardly possible if your TOOLINFO structure resides on the stack, what about heap memory? With probability far from zero, your structure can occupy last free bytes in a memory page and next page can be not committed yet. This is exactly the case which I encountered. As a result – access violation.

Note that the problem can appear only if you allocate memory for TOOLINFO structure inside your own module, that is, use SendMessage approach.

I found the bug in MFC 7.1 library, but it’s not fixed even in MFC 10.0.

The Ways to Workaround

There are several obvious ways to workaround the problem:

  1. Do not use SendMessage approach to register a tool with a tool tip control. Use CToolTipCtrl::AddTool method instead.

    In case you cannot avoid SendMessage approach (for static controls, for example), you can do the following:

  2. Compile your module which uses tool tip control with all macros available (_WIN32_IE and _WIN32_WINNT).
  3. Reserve extra bytes (4 or 8, depending on your options) which can be copied without impact.

    For example like this:

    // Extend TOOLINFO on 8 bytes
    struct TOOLINFO_EX : public TOOLINFO
      DWORD m_dwReserve1;
      DWORD m_dwReserve2;
    // Allocate memory from heap (remember, it’s just example :))
    TOOLINFO* pti = new TOOLINFO_EX;
    // Initialization
    // Register a tool
    toolTipCtrl.SendMessage(TTM_ADDTOOL, 0, (LPARAM) pti); 


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Andrey Permamedov
Software Developer (Senior) Transas Technologies
Russian Federation Russian Federation
No Biography provided

Comments and Discussions

GeneralHarmless IMHO PinmemberMartin Richter [MVP C++]23-Sep-10 20:43 
GeneralRe: Harmless IMHO PinmemberAndrey Permamedov23-Sep-10 22:00 
GeneralRe: Harmless IMHO PinmemberMartin Richter [MVP C++]23-Sep-10 22:51 
IE? You are joking? Big Grin | :-D
AFAIK all version since W2K (even dead Win98) implies IE>Version 3.0.
So again: Why should you care? The version of the OS might be a problem.
I can say nothing about Pats comment, but if we look into the requirements of VS-2010 I see no need to change anything... Wink | ;) We will see what will happen with this code.
Again: You only have a problem in developing for pre-XP!
But everything before XP is dead and no longer supported. Keep this in mind!
For MFC 7.1 it is a problem when developing for pre XP, but also only when the block is alligned on a page boundary. When the block is located on the stack (as usual) you don't have a problem.
IN all cases cbSize isn't change and transfered to the window class as it is. Just a few bytes to much are copied...
Martin Richter (MVP for C++) WWJD
"A well-written program is its own heaven; a poorly written program is its own hell!" The Tao of Programming

GeneralRe: Harmless IMHO PinmemberAndrey Permamedov23-Sep-10 23:24 
GeneralRe: Harmless IMHO [modified] PinmemberMartin Richter [MVP C++]23-Sep-10 23:41 
GeneralRe: Harmless IMHO PinmemberAndrey Permamedov24-Sep-10 0:48 
GeneralMy vote of 5 Pinmemberflyingxu21-Sep-10 19:23 
GeneralRe: My vote of 5 PinmemberAndrey Permamedov21-Sep-10 22:55 
General[My vote of 2] There is many ways to solve this circs PinmemberCarlos_never17-Sep-10 19:39 
GeneralRe: [My vote of 2] There is many ways to solve this circs PinmemberAndrey Permamedov18-Sep-10 1:57 
GeneralRe: [My vote of 2] There is many ways to solve this circs PinmemberCarlos_never21-Sep-10 17:42 
GeneralRe: [My vote of 2] There is many ways to solve this circs PinmemberAndrey Permamedov21-Sep-10 19:12 
GeneralMy vote of 5 PinmemberBlake Miller13-Sep-10 9:19 
GeneralRe: My vote of 5 PinmemberGrump24-Jan-11 22:43 

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