Tell MSan to ignore uninitialized padding when writing snapshots

src/snapshot/mksnapshot.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <errno.h>
 #include <signal.h>
 #include <stdio.h>
 
 #include "include/libplatform/libplatform.h"
 #include "src/assembler.h"
 #include "src/base/platform/platform.h"
 #include "src/flags.h"
 #include "src/list.h"
+#include "src/msan.h"
 #include "src/snapshot/natives.h"
 #include "src/snapshot/partial-serializer.h"
 #include "src/snapshot/startup-serializer.h"
 
 using namespace v8;
 
 class SnapshotWriter {
  public:
   SnapshotWriter() : snapshot_cpp_path_(NULL), snapshot_blob_path_(NULL) {}
 
   void SetSnapshotFile(const char* snapshot_cpp_file) {
     snapshot_cpp_path_ = snapshot_cpp_file;
   }
 
   void SetStartupBlobFile(const char* snapshot_blob_file) {
     snapshot_blob_path_ = snapshot_blob_file;
   }
 
   void WriteSnapshot(v8::StartupData blob) const {
     // TODO(crbug/633159): if we crash before the files have been fully created,
     // we end up with a corrupted snapshot file. The build step would succeed,
     // but the build target is unusable. Ideally we would write out temporary
     // files and only move them to the final destination as last step.
+
+    // Tell MSan to ignore uninitialized padding in the blob.
+    MSAN_MEMORY_IS_INITIALIZED(blob.data, blob.raw_size);
+
     i::Vector<const i::byte> blob_vector(
         reinterpret_cast<const i::byte*>(blob.data), blob.raw_size);
     MaybeWriteSnapshotFile(blob_vector);
     MaybeWriteStartupBlob(blob_vector);
   }
 
  private:
   void MaybeWriteStartupBlob(const i::Vector<const i::byte>& blob) const {
     if (!snapshot_blob_path_) return;
 
@@ skipping 134 matching lines @@
     CHECK(blob.data);
     writer.WriteSnapshot(blob);
     delete[] blob.data;
   }
 
   V8::Dispose();
   V8::ShutdownPlatform();
   delete platform;
   return 0;
 }