求助：boundschecker报告这个函数有GDI泄露，为什么？

void CSizingControlBar::OnNcPaint()
{
    // get window DC that is clipped to the non-client area
    CWindowDC dc(this);    CRect rcClient, rcBar;
    GetClientRect(rcClient);
    ClientToScreen(rcClient);
    GetWindowRect(rcBar);
    rcClient.OffsetRect(-rcBar.TopLeft());
    rcBar.OffsetRect(-rcBar.TopLeft());    CDC mdc;
    mdc.CreateCompatibleDC(&dc);
    
    CBitmap bm;
    bm.CreateCompatibleBitmap(&dc, rcBar.Width(), rcBar.Height());
    CBitmap* pOldBm = mdc.SelectObject(&bm);    // draw borders in non-client area
    CRect rcDraw = rcBar;
    DrawBorders(&mdc, rcDraw);    // erase the NC background
    mdc.FillRect(rcDraw, CBrush::FromHandle(
        (HBRUSH) GetClassLong(m_hWnd, GCL_HBRBACKGROUND)));    if (m_dwSCBStyle & SCBS_SHOWEDGES)
    {
        CRect rcEdge; // paint the sizing edges
        for (int i = 0; i < 4; i++)
            if (GetEdgeRect(rcBar, GetEdgeHTCode(i), rcEdge))
                mdc.Draw3dRect(rcEdge, ::GetSysColor(COLOR_BTNHIGHLIGHT),
                    ::GetSysColor(COLOR_BTNSHADOW));
    }    NcPaintGripper(&mdc, rcClient);    // client area is not our bussiness :)
    dc.IntersectClipRect(rcBar);
    dc.ExcludeClipRect(rcClient);    dc.BitBlt(0, 0, rcBar.Width(), rcBar.Height(), &mdc, 0, 0, SRCCOPY);    ReleaseDC(&dc);    mdc.SelectObject(pOldBm);
    bm.DeleteObject();
    mdc.DeleteDC();
}