Add type cast definitions for CPDF_Stream.

core/src/fpdfapi/fpdf_render/fpdf_render_loadimage.cpp

Index: core/src/fpdfapi/fpdf_render/fpdf_render_loadimage.cpp
diff --git a/core/src/fpdfapi/fpdf_render/fpdf_render_loadimage.cpp b/core/src/fpdfapi/fpdf_render/fpdf_render_loadimage.cpp
index fecd6da9b24616629894e38bcd5f472a3a08e057..297fc07657715e735b2957754c38b3b3caa67663 100644
--- a/core/src/fpdfapi/fpdf_render/fpdf_render_loadimage.cpp
+++ b/core/src/fpdfapi/fpdf_render/fpdf_render_loadimage.cpp
@@ -765,7 +765,7 @@ CPDF_DIBSource* CPDF_DIBSource::LoadMask(FX_DWORD& MatteColor) {
   CPDF_Stream* pSoftMask = m_pDict->GetStream(FX_BSTRC("SMask"));
   if (pSoftMask) {
     CPDF_Array* pMatte = pSoftMask->GetDict()->GetArray(FX_BSTRC("Matte"));
-    if (pMatte != NULL && m_pColorSpace &&
+    if (pMatte && m_pColorSpace &&
         (FX_DWORD)m_pColorSpace->CountComponents() <= m_nComponents) {
       FX_FLOAT* pColor = FX_Alloc(FX_FLOAT, m_nComponents);
       for (FX_DWORD i = 0; i < m_nComponents; i++) {
@@ -779,14 +779,12 @@ CPDF_DIBSource* CPDF_DIBSource::LoadMask(FX_DWORD& MatteColor) {
     }
     return LoadMaskDIB(pSoftMask);
   }
-  CPDF_Object* pMask = m_pDict->GetElementValue(FX_BSTRC("Mask"));
-  if (pMask == NULL) {
-    return NULL;
-  }
-  if (pMask->GetType() == PDFOBJ_STREAM) {
-    return LoadMaskDIB((CPDF_Stream*)pMask);
-  }
-  return NULL;
+
+  if (CPDF_Stream* pStream =
+          ToStream(m_pDict->GetElementValue(FX_BSTRC("Mask"))))
+    return LoadMaskDIB(pStream);
+
+  return nullptr;
 }
 int CPDF_DIBSource::StratLoadMask() {
   m_MatteColor = 0XFFFFFFFF;
@@ -807,13 +805,12 @@ int CPDF_DIBSource::StratLoadMask() {
     }
     return StartLoadMaskDIB();
   }
+
   m_pMaskStream = m_pDict->GetElementValue(FX_BSTRC("Mask"));
-  if (m_pMaskStream == NULL) {
+  if (!m_pMaskStream)

[Tom Sepez, 2015/10/22 17:16:45]

nit: maybe 
  return ToStream(m_pMaskStream) ? StartLoadMaskDIB() : 1;

[dsinclair, 2015/10/22 17:42:56]

Done.

     return 1;
-  }
-  if (m_pMaskStream->GetType() == PDFOBJ_STREAM) {
+  if (m_pMaskStream->IsStream())
     return StartLoadMaskDIB();
-  }
   return 1;
 }
 int CPDF_DIBSource::ContinueLoadMaskDIB(IFX_Pause* pPause) {
@@ -849,17 +846,16 @@ CPDF_DIBSource* CPDF_DIBSource::LoadMaskDIB(CPDF_Stream* pMask) {
 }
 int CPDF_DIBSource::StartLoadMaskDIB() {
   m_pMask = new CPDF_DIBSource;
-  int ret = m_pMask->StartLoadDIBSource(
-      m_pDocument, (CPDF_Stream*)m_pMaskStream, FALSE, NULL, NULL, TRUE);
+  int ret = m_pMask->StartLoadDIBSource(m_pDocument, ToStream(m_pMaskStream),

[Tom Sepez, 2015/10/22 17:16:45]

I worry about this.  At line 809, we may have assigned a non-stream to
m_pMaskStream.  Now we will test type and pass NULL for that case.  Later on,
some clever code may check that the pointer is some other type, and sloppily
cast it to the type.  Please confirm.  Can we make the m_MaskStream be a stream,
not an object?  Can we force ToStream() on it at the time of assignment?

[dsinclair, 2015/10/22 17:42:56]

Done. There are 2 assignments and this one usage. The two assignments are 791
and 809 and 791 is GetStream

[Tom Sepez, 2015/10/22 19:11:55]

Ok.  Looks like were type safe.

+                                        FALSE, nullptr, nullptr, TRUE);
   if (ret == 2) {
-    if (m_Status == 0) {
+    if (m_Status == 0)
       m_Status = 2;
-    }
     return 2;
   }
   if (!ret) {
     delete m_pMask;
-    m_pMask = NULL;
+    m_pMask = nullptr;
     return 1;
   }
   return 1;