Add support for the VS2017 built binaries.

There's 2 things that had to be fixed:
- There's a new flag value for the 32 bit offset fixups, we used to always see a flag value of 0 but it's now set to 0x2000 in the VS2017 binaries.
- Add a workaround for what seems to be a toolchain bug in VS2017, the PDB contain some invalid symbols, see https://developercommunity.visualstudio.com/content/problem/47386/invalid-symbols-when-using-rtti.html

With these 2 changes we fully support the VS2017 built binaries.

BUG=chromium:683729

Review-Url: https://codereview.chromium.org/2856933003

syzygy/pdb/pdb_data.cc

@@ -30,8 +30,7 @@ bool PdbFixup::ValidHeader() const {
 
     case TYPE_OFFSET_32BIT:
     case TYPE_OFFSET_8BIT: {
-      // We've only ever seen offset fixups without flags.
-      return flags == 0;
+      return (flags & ~FLAG_OFFSET_32BIT_VS2017) == 0;
     }
 
     default: {

syzygy/pdb/pdb_data.h

@@ -289,6 +289,9 @@ struct PdbFixup {
   };
 
   enum Flags {
+    // Flag value that we observe for the TYPE_OFFSET_32BIT fixups produced by
+    // VS2017.
+    FLAG_OFFSET_32BIT_VS2017 = 0x2000,
     FLAG_IS_DATA = 0x4000,
     FLAG_REFERS_TO_CODE = 0x8000,
     FLAG_UNKNOWN = 0x3fff,

syzygy/pe/decomposer.cc

@@ -1792,6 +1792,7 @@ DiaBrowser::BrowserDirective Decomposer::OnDataSymbol(
 
   // Verify that the data symbol does not exceed the size of the block.
   if (addr + length > block_addr + block->size()) {
+    base::StringPiece spname(name);
     // The data symbol can exceed the size of the block in the case of data
     // imports. For some reason the toolchain emits a global data symbol with
     // type information equal to the type of the data *pointed* to by the import
@@ -1802,10 +1803,24 @@ DiaBrowser::BrowserDirective Decomposer::OnDataSymbol(
     // generated. This won't be part of the IAT, so we can't even filter based
     // on that. Instead, we simply ignore global data symbols that exceed the
     // block size.
-    base::StringPiece spname(name);
-    if (sym_tags.size() == 1 && spname.starts_with("_imp_")) {
+    bool is_imported_data_symbol = (sym_tags.size() == 1 &&
+                                    spname.starts_with("_imp_"));
+    // In VS2017 we've noticed that the size returned by IDiaSymbol::get_length
+    // function is invalid for the objects using RTTI in VS2017. This has been
+    // reported here:
+    // https://developercommunity.visualstudio.com/content/problem/47386/invalid-symbols-when-using-rtti.html
+    //
+    // In this situation the data symbol that we get always starts 4 bytes after
+    // the beginning of its parent block and has an identical size.
+    bool is_vtable_symbol = spname.ends_with("::`vftable'") &&
+                            (addr - block_addr == 4) &&
+                            length == block->size();
+    if (is_imported_data_symbol) {
       VLOG(1) << "Encountered an imported data symbol \"" << name << "\" that "
               << "extends past its parent block \"" << block->name() << "\".";
+    } else if (is_vtable_symbol) {
+      VLOG(1) << "Encountered a vtable data symbol \"" << name << "\" that "
+              << "extends past its parent block \"" << block->name() << "\".";
     } else {
       LOG(ERROR) << "Received data symbol \"" << name << "\" that extends past "
                  << "its parent block \"" << block->name() << "\".";