Reland of lax a couple checks to allow certain non-standard PDF files. (patchset #1 id:1 of https://codereview.chromium.org/1946693002/ )

Reason for revert:
The culprit was found and confirmed, not this one.

Original issue's description:
> Revert of Relax a couple checks to allow certain non-standard PDF files. (patchset #1 id:1 of https://codereview.chromium.org/1926823002/ )
>
> Reason for revert:
> Speculatively revert due to high volume of crashes on Chromium.
>
> Original issue's description:
> > Relax a couple checks to allow certain non-standard PDF files.
> >
> > Some non-standard PDF files misuse the size of cross reference table,
> > and reuse some object number which the old one is still in use. PDFium
> > can relax the reusing of xref objects only since it is not referred in
> > the pdf document. When the size of cross reference table is larger
> > than defined, PDFium will try to continue other than abort.
> >
> > BUG=chromium:596947
> >
> > Committed: https://pdfium.googlesource.com/pdfium/+/cd1e9ff4f432cbc29ed279e6891fb7ddc2ea3734
>
> TBR=thestig@chromium.org,dsinclair@chromium.org
> # Not skipping CQ checks because original CL landed more than 1 days ago.
> BUG=chromium:596947
>
> Committed: https://pdfium.googlesource.com/pdfium/+/5fc4f31285c3a88fc157fd2d9b9cf2eb5c7cabed

TBR=thestig@chromium.org,dsinclair@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:596947

Review-Url: https://codereview.chromium.org/1947983002

1 files changed, 16 insertions, 4 deletions

diff --git a/core/fpdfapi/fpdf_parser/cpdf_indirect_object_holder.cpp b/core/fpdfapi/fpdf_parser/cpdf_indirect_object_holder.cpp
index ef3395d3ae..4020b003bb 100644
--- a/core/fpdfapi/fpdf_parser/cpdf_indirect_object_holder.cpp
+++ b/core/fpdfapi/fpdf_parser/cpdf_indirect_object_holder.cpp
@@ -6,6 +6,7 @@
 
 #include "core/fpdfapi/fpdf_parser/include/cpdf_indirect_object_holder.h"
 
+#include "core/fpdfapi/fpdf_parser/include/cpdf_dictionary.h"
 #include "core/fpdfapi/fpdf_parser/include/cpdf_object.h"
 #include "core/fpdfapi/fpdf_parser/include/cpdf_parser.h"
 
@@ -24,17 +25,28 @@ CPDF_Object* CPDF_IndirectObjectHolder::GetIndirectObject(uint32_t objnum) {
   if (objnum == 0)
     return nullptr;
 
+  CPDF_Object* result_obj = nullptr;
   auto it = m_IndirectObjs.find(objnum);
-  if (it != m_IndirectObjs.end())
-    return it->second->GetObjNum() != CPDF_Object::kInvalidObjNum ? it->second
-                                                                  : nullptr;
+  if (it != m_IndirectObjs.end()) {
+    CPDF_Object* obj = it->second;
+    result_obj =
+        obj->GetObjNum() != CPDF_Object::kInvalidObjNum ? it->second : nullptr;
+    // Xref object is not used by the pdf document itself. Some software thus
+    // reuse an object number for xref object. So when we get an xref object,
+    // try again to see whether another object with the same number is defined.
+    // If so, use that object instead. See chromium:596947.
+    CPDF_Dictionary* dict =
+        obj->IsStream() ? obj->GetDict() : obj->AsDictionary();
+    if (!dict || dict->GetStringBy("Type") != "XRef")
+      return result_obj;
+  }
 
   if (!m_pParser)
     return nullptr;
 
   CPDF_Object* pObj = m_pParser->ParseIndirectObject(this, objnum);
   if (!pObj)
-    return nullptr;
+    return result_obj;
 
   pObj->m_ObjNum = objnum;
   m_LastObjNum = std::max(m_LastObjNum, objnum);