src: make sure that memcpy-ed structs in snapshot have no padding

To make the snapshots reproducible, this patch updates the size
of a few types and adds some static assertions to ensure that
there are no padding in the memcpy-ed structs.

PR-URL: #50983
Refs: nodejs/build#3043
Reviewed-By: Daniel Lemire <daniel@lemire.me>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/aliased_buffer-inl.h

@@ -8,7 +8,7 @@
 
 namespace node {
 
-typedef size_t AliasedBufferIndex;
+typedef uint64_t AliasedBufferIndex;
 
 template <typename NativeT, typename V8T>
 AliasedBufferBase<NativeT, V8T>::AliasedBufferBase(

src/aliased_buffer.h

@@ -9,7 +9,7 @@
 
 namespace node {
 
-typedef size_t AliasedBufferIndex;
+typedef uint64_t AliasedBufferIndex;
 
 /**
  * Do not use this class directly when creating instances of it - use the

src/base_object_types.h

@@ -42,10 +42,13 @@ namespace node {
   SERIALIZABLE_NON_BINDING_TYPES(V)
 
 #define V(TypeId, NativeType) k_##TypeId,
-enum class BindingDataType : uint8_t { BINDING_TYPES(V) kBindingDataTypeCount };
+// To avoid padding, the enums are uint64_t.
+enum class BindingDataType : uint64_t {
+  BINDING_TYPES(V) kBindingDataTypeCount
+};
 // Make sure that we put the bindings first so that we can also use the enums
 // for the bindings as index to the binding data store.
-enum class EmbedderObjectType : uint8_t {
+enum class EmbedderObjectType : uint64_t {
   BINDING_TYPES(V) SERIALIZABLE_NON_BINDING_TYPES(V)
   // We do not need to know about all the unserializable non-binding types for
   // now so we do not list them.

src/encoding_binding.h

@@ -18,6 +18,12 @@ class BindingData : public SnapshotableObject {
     AliasedBufferIndex encode_into_results_buffer;
   };
 
+  // Make sure that there's no padding in the struct since we will memcpy
+  // them into the snapshot blob and they need to be reproducible.
+  static_assert(sizeof(InternalFieldInfo) ==
+                    sizeof(InternalFieldInfoBase) + sizeof(AliasedBufferIndex),
+                "InternalFieldInfo should have no padding");
+
   BindingData(Realm* realm,
               v8::Local<v8::Object> obj,
               InternalFieldInfo* info = nullptr);

src/node_file.h

@@ -64,6 +64,12 @@ class BindingData : public SnapshotableObject {
     AliasedBufferIndex statfs_field_bigint_array;
   };
 
+  // Make sure that there's no padding in the struct since we will memcpy
+  // them into the snapshot blob and they need to be reproducible.
+  static_assert(sizeof(InternalFieldInfo) == sizeof(InternalFieldInfoBase) +
+                                                 sizeof(AliasedBufferIndex) * 4,
+                "InternalFieldInfo should have no padding");
+
   enum class FilePathIsFileReturnType {
     kIsFile = 0,
     kIsNotFile,

src/node_process.h

@@ -54,6 +54,12 @@ class BindingData : public SnapshotableObject {
     AliasedBufferIndex hrtime_buffer;
   };
 
+  // Make sure that there's no padding in the struct since we will memcpy
+  // them into the snapshot blob and they need to be reproducible.
+  static_assert(sizeof(InternalFieldInfo) ==
+                    sizeof(InternalFieldInfoBase) + sizeof(AliasedBufferIndex),
+                "InternalFieldInfo should have no padding");
+
   static void AddMethods(v8::Isolate* isolate,
                          v8::Local<v8::ObjectTemplate> target);
   static void RegisterExternalReferences(ExternalReferenceRegistry* registry);

src/node_snapshotable.cc

@@ -282,9 +282,9 @@ size_t SnapshotSerializer::Write(const PropInfo& data) {
 }
 
 // Layout of AsyncHooks::SerializeInfo
-// [ 4/8 bytes ]  snapshot index of async_ids_stack
-// [ 4/8 bytes ]  snapshot index of fields
-// [ 4/8 bytes ]  snapshot index of async_id_fields
+// [ 8 bytes ]  snapshot index of async_ids_stack
+// [ 8 bytes ]  snapshot index of fields
+// [ 8 bytes ]  snapshot index of async_id_fields
 // [ 4/8 bytes ]  snapshot index of js_execution_async_resources
 // [ 4/8 bytes ]  length of native_execution_async_resources
 // [   ...     ]  snapshot indices of each element in
@@ -387,9 +387,9 @@ size_t SnapshotSerializer::Write(const ImmediateInfo::SerializeInfo& data) {
 }
 
 // Layout of PerformanceState::SerializeInfo
-// [ 4/8 bytes ]  snapshot index of root
-// [ 4/8 bytes ]  snapshot index of milestones
-// [ 4/8 bytes ]  snapshot index of observers
+// [ 8 bytes ]  snapshot index of root
+// [ 8 bytes ]  snapshot index of milestones
+// [ 8 bytes ]  snapshot index of observers
 template <>
 performance::PerformanceState::SerializeInfo SnapshotDeserializer::Read() {
   Debug("Read<PerformanceState::SerializeInfo>()\n");

src/node_snapshotable.h

@@ -4,6 +4,8 @@
 
 #if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
 
+#include <cassert>  // For static_assert
+#include <cstddef>  // For offsetof
 #include "aliased_buffer.h"
 #include "base_object.h"
 #include "util.h"
@@ -33,13 +35,13 @@ bool WithoutCodeCache(const SnapshotConfig& config);
 // and pass it into the V8 callback as the payload of StartupData.
 // The memory chunk looks like this:
 //
-// [   type   ] - EmbedderObjectType (a uint8_t)
-// [  length  ] - a size_t
+// [   type   ] - EmbedderObjectType (a uint64_t)
+// [  length  ] - a uint64_t
 // [    ...   ] - custom bytes of size |length - header size|
 struct InternalFieldInfoBase {
  public:
   EmbedderObjectType type;
-  size_t length;
+  uint64_t length;
 
   template <typename T>
   static T* New(EmbedderObjectType type) {
@@ -71,14 +73,35 @@ struct InternalFieldInfoBase {
   InternalFieldInfoBase() = default;
 };
 
+// Make sure that there's no padding in the struct since we will memcpy
+// them into the snapshot blob and they need to be reproducible.
+static_assert(offsetof(InternalFieldInfoBase, type) == 0,
+              "InternalFieldInfoBase::type should start from offset 0");
+static_assert(offsetof(InternalFieldInfoBase, length) ==
+                  sizeof(EmbedderObjectType),
+              "InternalFieldInfoBase::type should have no padding");
+
 struct EmbedderTypeInfo {
-  enum class MemoryMode : uint8_t { kBaseObject, kCppGC };
+  // To avoid padding, the enum is uint64_t.
+  enum class MemoryMode : uint64_t { kBaseObject = 0, kCppGC };
   EmbedderTypeInfo(EmbedderObjectType t, MemoryMode m) : type(t), mode(m) {}
   EmbedderTypeInfo() = default;
+
   EmbedderObjectType type;
   MemoryMode mode;
 };
 
+// Make sure that there's no padding in the struct since we will memcpy
+// them into the snapshot blob and they need to be reproducible.
+static_assert(offsetof(EmbedderTypeInfo, type) == 0,
+              "EmbedderTypeInfo::type should start from offset 0");
+static_assert(offsetof(EmbedderTypeInfo, mode) == sizeof(EmbedderObjectType),
+              "EmbedderTypeInfo::type should have no padding");
+static_assert(sizeof(EmbedderTypeInfo) ==
+                  sizeof(EmbedderObjectType) +
+                      sizeof(EmbedderTypeInfo::MemoryMode),
+              "EmbedderTypeInfo::mode should have no padding");
+
 // An interface for snapshotable native objects to inherit from.
 // Use the SERIALIZABLE_OBJECT_METHODS() macro in the class to define
 // the following methods to implement:
@@ -150,6 +173,12 @@ class BindingData : public SnapshotableObject {
     AliasedBufferIndex is_building_snapshot_buffer;
   };
 
+  // Make sure that there's no padding in the struct since we will memcpy
+  // them into the snapshot blob and they need to be reproducible.
+  static_assert(sizeof(InternalFieldInfo) ==
+                    sizeof(InternalFieldInfoBase) + sizeof(AliasedBufferIndex),
+                "InternalFieldInfo should have no padding");
+
   BindingData(Realm* realm,
               v8::Local<v8::Object> obj,
               InternalFieldInfo* info = nullptr);

src/node_v8.h

@@ -23,6 +23,13 @@ class BindingData : public SnapshotableObject {
     AliasedBufferIndex heap_space_statistics_buffer;
     AliasedBufferIndex heap_code_statistics_buffer;
   };
+
+  // Make sure that there's no padding in the struct since we will memcpy
+  // them into the snapshot blob and they need to be reproducible.
+  static_assert(sizeof(InternalFieldInfo) == sizeof(InternalFieldInfoBase) +
+                                                 sizeof(AliasedBufferIndex) * 3,
+                "InternalFieldInfo should have no padding");
+
   BindingData(Realm* realm,
               v8::Local<v8::Object> obj,
               InternalFieldInfo* info = nullptr);