对于从虚拟基类继承的虚函数，什么是“虚拟thunk”？

Posted 2021-04-12

当我尝试访问从虚拟基类继承的派生类对象的内存布局时，出了点问题。 编程环境：GNU / Linux 3.19.0-32-generic，x86_64 编译器：gcc 4.8.4

//virtual base class
class Base {
public :
    virtual void f() {
        cout << "Base::f()" << endl;
    }
private:
    long x;
};

//derived class
class Derived : public virtual Base {
public:
    virtual void f() {
        cout << "Derived::f()" << endl;
    }
private:
    long y;
};

int main() {
    typedef void (*FUNC)(void);
    Derived d;

    //In my machine, sizeof(long) == sizeof(pointers). My code below is neither portable nor concise. You can just read the annotation.

    //dereference the first element of the first virtual function table(equals to *(vptr1->slot[0]))
    cout << hex << *((long*)*((long*)(&d) + 0) + 0) << endl;
    ((FUNC)*((long*)*((long*)(&d) + 0) + 0))();//invoke Derived::f()

    //dereference the first element of the second virtual function table(equals to *(vptr2->slot[0]))
    cout << hex << *((long*)*((long*)(&d) + 2) + 0) << endl;
    ((FUNC)*((long*)*((long*)(&d) + 2) + 0))();//maybe Derived::f()?

    return 0;
}

当我运行代码时，我得到了“段错误”：

400c12
Derived::f()
400c3c
segment fault

所以我反汇编了可执行文件。 我在0x400c3c中找到了函数<_ZTv0_n24_N7Derived1fEv>：

0000000000400c3c <_ZTv0_n24_N7Derived1fEv>:
  400c3c:       4c 8b 17                mov    (%rdi),%r10
  400c3f:       49 03 7a e8             add    -0x18(%r10),%rdi
  400c43:       eb cd                   jmp    400c12 <_ZN7Derived1fEv>
  400c45:       90                      nop

在我的终端中解码符号：

> c++filt _ZTv0_n24_N7Derived1fEv
virtual thunk to Derived::f()

然后什么是Derived :: f（）的虚拟thunk？它为什么存在？

答案

某些函数的虚拟thunk是一个辅助函数，它在调用实际函数之前修复了this参数。看看这个例子：

Derived *d = new Derived();
// d now points to some address, e.g. 0x6eac40

d->f(); // This calls _ZN7Derived1fEv (Derived::f() directly)

Base *b = d;
// b now points to some other address (!), e.g. 0x6eac50

b->f(); // This calls _ZTv0_n24_N7Derived1fEv (the virtual thunk
        // of Derived::f()), which subtracts some amount from `this`
        // and then jumps to the _ZN7Derived1fEv (Derived::f())

内存中的Base对象看起来像这样：

      * Pointer to part of Base vtable with Base's virtual functions.
          This vtable contains Base::f()

      * Data of Base class (variable `x`)

内存中的Derived对象看起来像这样：

   |> * Pointer to part of Derived vtable with Derived's virtual functions.
   |>     This vtable contains the Derived::f()
   |>
|> |> * Pointer to part of Derived vtable with the same layout as Base vtable.
|> |>     This vtable contains the thunk of Derived::f()
|> |>
|> |> * Data of Base class (variable `x`)
|  |>
|  |> * Data of Derived class (variable `y`)
|  |
|   This is complete Derived object.
|    The `d` pointer points at the beginning of this.
|
 This is the part of Derived object that can act as a Base object.
  The `b` pointer points at beginning of this.

PS：现在也应该清楚为什么在_ZTv0_n24_N7Derived1fEv指针上调用d会崩溃。该函数仅在指定this对象内部的Derived指针时才有效 - 进入可以像Base对象一样使用的部分。

另一答案

这个问题有一个不明确的地方。在“虚拟thunk到Derived :: f（）”中，我认为“添加-0x18（％r10），％rdi”无法修复此指针，因为Derive对象的开头与其子对象（Base）之间的偏移量不是24（0x18）。