Is is possible to add a field to FunctionCallbackInfo ?

[L TY]

Hi everyone,

For some research reason, I need to log all property access to Blink in v8. Since all the property load information is passed to Blink through class `FunctionCallbackInfo`, I tried to add a pointer field in this class to hold the property name (represented as std::string) like follow:

      V8_INLINE FunctionCallbackInfo(internal::Address* implicit_args,

                                 internal::Address* values, int length);

  internal::Address* implicit_args_;

  internal::Address* values_;

  int length_;

  std::string* property_info_; // The field I add.

}

The `property_info_` field is initialized in the constructor function through c++ `new`.

I also added a const setter function for the `property_info_` field. The setter function like follow:

void FunctionCallbackInfo<T>::SetPropertyInfo(::std::string s) const {

  *property_info_ = s;

}

In Blink, I modified the template file to set this field every time an attribute in Blink is visited.

In code snips I write by myself,  it works well. However, when I try to run the code on some real-world websites, I found that the value of the field property_info_ is set to a strange value 0x38, so the pointer to the string is overlapped by some other value.

I tried to mark the property_info_ field as private and even moved it to class `ReturnValue<T>`, the overlapping case still happens:

template<typename T>

class ReturnValue {

 public:

  template <class S> V8_INLINE ReturnValue(const ReturnValue<S>& that)

      : value_(that.value_) {

    TYPE_CHECK(T, S);

  }

  // Local setters

  template <typename S>

  V8_INLINE void Set(const Global<S>& handle);

  template <typename S>

  V8_INLINE void Set(const TracedGlobal<S>& handle);

  template <typename S>

  V8_INLINE void Set(const Local<S> handle);

  // Fast primitive setters

  V8_INLINE void Set(bool value);

  V8_INLINE void Set(double i);

  V8_INLINE void Set(int32_t i);

  V8_INLINE void Set(uint32_t i);

  // Fast JS primitive setters

  V8_INLINE void SetNull();

  V8_INLINE void SetUndefined();

  V8_INLINE void SetEmptyString();

  // Convenience getter for Isolate

  V8_INLINE Isolate* GetIsolate() const;

  // Pointer setter: Uncompilable to prevent inadvertent misuse.

  template <typename S>

  V8_INLINE void Set(S* whatever);

  // Getter. Creates a new Local<> so it comes with a certain performance

  // hit. If the ReturnValue was not yet set, this will return the undefined

  // value.

  V8_INLINE Local<Value> Get() const;

  // use for taint map by n0b0dy

  V8_INLINE internal::Address GetValueAddr() const;

  V8_INLINE void SetProperty(std::string) const; // The setter function I add

 private:

  template<class F> friend class ReturnValue;

  template<class F> friend class FunctionCallbackInfo;

  template<class F> friend class PropertyCallbackInfo;

  template <class F, class G, class H>

  friend class PersistentValueMapBase;

  V8_INLINE void SetInternal(internal::Address value) { *value_ = value; }

  V8_INLINE internal::Address GetDefaultValue();

  V8_INLINE explicit ReturnValue(internal::Address* slot);

  internal::Address* value_;

  internal::Address* property_info_; // the field I add

};

I read the in https://source.chromium.org/chromium/chromium/src/+/master:v8/src/builtins/x64/builtins-x64.cc;drc=34ad93361f96ad875bf350ced27e15b7aa126113;bpv=1;bpt=1;l=3317 and found that v8 will arrange the stack frame with its own call convention, But I didn't find why the pointer value is still overlapped in class ReturnValue.

Does anyone have ideas about what should I do?

Thanks a lot!

[Jakob Kummerow]

This is expected. FunctionCallbackInfo instances are rarely constructed via C++ `new`; instead they are usually created manually on the stack. So adding a field to the class definition is going to be a lot of work, and that field's type can't be a std::string -- it would have to be something where you have precise control over the object layout.

I would suggest that you explore alternative approaches.

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[L TY]

Thank you very much for your advice, really appreciate it.

在 2020年3月31日星期二 UTC+8下午5:36:59，Jakob Kummerow写道：

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