Add a simple check for V4 cross reference table

Some unknown software generates cross reference table with the
claimed object numbers are all off by one. Add a simple verification
function to detect this scenario, thus have a chance to rebuild
the correct cross reference table.

To avoid unnecessary checks and potiential performance hit, we only
check for the very first non free entry.

BUG=602650

Review URL: https://codereview.chromium.org/1910063004

3 files changed, 47 insertions, 0 deletions

diff --git a/core/fpdfapi/fpdf_parser/cpdf_parser.cpp b/core/fpdfapi/fpdf_parser/cpdf_parser.cpp
index 1ec912ebee..acf51de1ea 100644
--- a/core/fpdfapi/fpdf_parser/cpdf_parser.cpp
+++ b/core/fpdfapi/fpdf_parser/cpdf_parser.cpp
@@ -316,6 +316,32 @@ FX_FILESIZE CPDF_Parser::GetObjectOffset(uint32_t objnum) const {
   return 0;
 }
 
+// Ideally, all the cross reference entries should be verified.
+// In reality, we rarely see well-formed cross references don't match
+// with the objects. crbug/602650 showed a case where object numbers
+// in the cross reference table are all off by one.
+bool CPDF_Parser::VerifyCrossRefV4() {
+  for (const auto& it : m_ObjectInfo) {
+    if (it.second.pos == 0)
+      continue;
+    // Find the first non-zero position.
+    FX_FILESIZE SavedPos = m_pSyntax->SavePos();
+    m_pSyntax->RestorePos(it.second.pos);
+    bool is_num = false;
+    CFX_ByteString num_str = m_pSyntax->GetNextWord(&is_num);
+    m_pSyntax->RestorePos(SavedPos);
+    if (!is_num || num_str.IsEmpty() ||
+        FXSYS_atoui(num_str.c_str()) != it.first) {
+      // If the object number read doesn't match the one stored,
+      // something is wrong with the cross reference table.
+      return false;
+    } else {
+      return true;
+    }
+  }
+  return true;
+}
+
 FX_BOOL CPDF_Parser::LoadAllCrossRefV4(FX_FILESIZE xrefpos) {
   if (!LoadCrossRefV4(xrefpos, 0, TRUE))
     return FALSE;
@@ -366,6 +392,8 @@ FX_BOOL CPDF_Parser::LoadAllCrossRefV4(FX_FILESIZE xrefpos) {
   for (size_t i = 0; i < CrossRefList.size(); ++i) {
     if (!LoadCrossRefV4(CrossRefList[i], XRefStreamList[i], FALSE))
       return FALSE;
+    if (i == 0 && !VerifyCrossRefV4())
+      return FALSE;
   }
   return TRUE;
 }
diff --git a/core/fpdfapi/fpdf_parser/cpdf_parser_embeddertest.cpp b/core/fpdfapi/fpdf_parser/cpdf_parser_embeddertest.cpp
index f427ec5d81..042b221554 100644
--- a/core/fpdfapi/fpdf_parser/cpdf_parser_embeddertest.cpp
+++ b/core/fpdfapi/fpdf_parser/cpdf_parser_embeddertest.cpp
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "public/fpdf_text.h"
 #include "testing/embedder_test.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -38,3 +39,18 @@ TEST_F(CPDFParserEmbeddertest, Bug_325) {
   EXPECT_FALSE(OpenDocument("bug_325_a.pdf"));
   EXPECT_FALSE(OpenDocument("bug_325_b.pdf"));
 }
+
+TEST_F(CPDFParserEmbeddertest, Bug_602650) {
+  // Test the case that cross reference entries, which are well formed,
+  // but do not match with the objects.
+  EXPECT_TRUE(OpenDocument("bug_602650.pdf"));
+  FPDF_PAGE page = LoadPage(0);
+  EXPECT_NE(nullptr, page);
+  FPDF_TEXTPAGE text_page = FPDFText_LoadPage(page);
+  EXPECT_NE(nullptr, text_page);
+  // The page should not be blank.
+  EXPECT_LT(0, FPDFText_CountChars(text_page));
+
+  FPDFText_ClosePage(text_page);
+  UnloadPage(page);
+}
diff --git a/core/fpdfapi/fpdf_parser/include/cpdf_parser.h b/core/fpdfapi/fpdf_parser/include/cpdf_parser.h
index 5331ccafb1..48511e9263 100644
--- a/core/fpdfapi/fpdf_parser/include/cpdf_parser.h
+++ b/core/fpdfapi/fpdf_parser/include/cpdf_parser.h
@@ -114,6 +114,9 @@ class CPDF_Parser {
   FX_BOOL IsLinearizedFile(IFX_FileRead* pFileAccess, uint32_t offset);
   void SetEncryptDictionary(CPDF_Dictionary* pDict);
   void ShrinkObjectMap(uint32_t size);
+  // A simple check whether the cross reference table matches with
+  // the objects.
+  bool VerifyCrossRefV4();
 
   CPDF_Document* m_pDocument;
   std::unique_ptr<CPDF_SyntaxParser> m_pSyntax;