在C标准库中，仅使用std:：function和std:：shared_pointer进行双重释放

Question

最近，我在一个程序中捕获lambda中的shared_ptr时，遇到了一个奇怪的双重免费bug。我可以用以下最小的例子来减少它：

#include <memory>
#include <functional>

struct foo {
    std::function<void(void)> fun;
};

foo& get() {
    auto f = std::make_shared<foo>();
    // Create a circular reference by capturing the shared pointer by value
    f->fun = [f]() {};
    return *f;

}

int main(void) {
    get().fun = nullptr;
    return 0;
}

使用GCC 12.2.0和地址清理器编译并运行它，会在std::function中生成一个双空闲：

$ g++ -fsanitize=address -g -Wall -Wextra -o main main.cpp && ./main
=================================================================
==2401674==ERROR: AddressSanitizer: attempting double-free on 0x602000000010 in thread T0:
    #0 0x7f7064ac178a in operator delete(void*, unsigned long) /usr/src/debug/gcc/libsanitizer/asan/asan_new_delete.cpp:164
    #1 0x556a00865b9d in _M_destroy /usr/include/c++/12.2.0/bits/std_function.h:175
    #2 0x556a00865abe in _M_manager /usr/include/c++/12.2.0/bits/std_function.h:203
    #3 0x556a008658b9 in _M_manager /usr/include/c++/12.2.0/bits/std_function.h:282
    #4 0x556a00866623 in std::function<void ()>::operator=(decltype(nullptr)) /usr/include/c++/12.2.0/bits/std_function.h:505
    #5 0x556a008654b5 in main /tmp/cpp/main.cpp:16
    #6 0x7f706443c28f  (/usr/lib/libc.so.6+0x2328f)
    #7 0x7f706443c349 in __libc_start_main (/usr/lib/libc.so.6+0x23349)
    #8 0x556a008651b4 in _start ../sysdeps/x86_64/start.S:115

0x602000000010 is located 0 bytes inside of 16-byte region [0x602000000010,0x602000000020)
freed by thread T0 here:
    #0 0x7f7064ac178a in operator delete(void*, unsigned long) /usr/src/debug/gcc/libsanitizer/asan/asan_new_delete.cpp:164
    #1 0x556a00865b9d in _M_destroy /usr/include/c++/12.2.0/bits/std_function.h:175
    #2 0x556a00865abe in _M_manager /usr/include/c++/12.2.0/bits/std_function.h:203
    #3 0x556a008658b9 in _M_manager /usr/include/c++/12.2.0/bits/std_function.h:282
    #4 0x556a00866215 in std::_Function_base::~_Function_base() /usr/include/c++/12.2.0/bits/std_function.h:244
    #5 0x556a00866579 in std::function<void ()>::~function() /usr/include/c++/12.2.0/bits/std_function.h:334
    #6 0x556a00868337 in foo::~foo() /tmp/cpp/main.cpp:4
    #7 0x556a00868352 in void std::_Destroy<foo>(foo*) /usr/include/c++/12.2.0/bits/stl_construct.h:151
    #8 0x556a0086830d in void std::allocator_traits<std::allocator<void> >::destroy<foo>(std::allocator<void>&, foo*) /usr/include/c++/12.2.0/bits/alloc_traits.h:648
    #9 0x556a008680fa in std::_Sp_counted_ptr_inplace<foo, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>::_M_dispose() /usr/include/c++/12.2.0/bits/shared_ptr_base.h:613
    #10 0x556a00866005 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release() /usr/include/c++/12.2.0/bits/shared_ptr_base.h:346
    #11 0x556a008664c5 in std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count() /usr/include/c++/12.2.0/bits/shared_ptr_base.h:1071
    #12 0x556a00866235 in std::__shared_ptr<foo, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr() /usr/include/c++/12.2.0/bits/shared_ptr_base.h:1524
    #13 0x556a00866251 in std::shared_ptr<foo>::~shared_ptr() /usr/include/c++/12.2.0/bits/shared_ptr.h:175
    #14 0x556a008652ad in ~<lambda> /tmp/cpp/main.cpp:10
    #15 0x556a00865b90 in _M_destroy /usr/include/c++/12.2.0/bits/std_function.h:175
    #16 0x556a00865abe in _M_manager /usr/include/c++/12.2.0/bits/std_function.h:203
    #17 0x556a008658b9 in _M_manager /usr/include/c++/12.2.0/bits/std_function.h:282
    #18 0x556a00866623 in std::function<void ()>::operator=(decltype(nullptr)) /usr/include/c++/12.2.0/bits/std_function.h:505
    #19 0x556a008654b5 in main /tmp/cpp/main.cpp:16
    #20 0x7f706443c28f  (/usr/lib/libc.so.6+0x2328f)

previously allocated by thread T0 here:
    #0 0x7f7064ac0672 in operator new(unsigned long) /usr/src/debug/gcc/libsanitizer/asan/asan_new_delete.cpp:95
    #1 0x556a00865906 in _M_create<get()::<lambda()> > /usr/include/c++/12.2.0/bits/std_function.h:161
    #2 0x556a008657e3 in _M_init_functor<get()::<lambda()> > /usr/include/c++/12.2.0/bits/std_function.h:215
    #3 0x556a00865719 in function<get()::<lambda()> > /usr/include/c++/12.2.0/bits/std_function.h:449
    #4 0x556a00865578 in operator=<get()::<lambda()> > /usr/include/c++/12.2.0/bits/std_function.h:534
    #5 0x556a008653aa in get() /tmp/cpp/main.cpp:10
    #6 0x556a008654a8 in main /tmp/cpp/main.cpp:16
    #7 0x7f706443c28f  (/usr/lib/libc.so.6+0x2328f)

SUMMARY: AddressSanitizer: double-free /usr/src/debug/gcc/libsanitizer/asan/asan_new_delete.cpp:164 in operator delete(void*, unsigned long)
==2401674==ABORTING

一旦get函数返回，foo结构中的std::function拥有唯一拥有封闭的foo对象的shared_ptr。这意味着，将nullptr分配给它应该会破坏shared_ptr，这反过来会释放foo对象。

这里发生的似乎是std_function中的delete调用。h： 175首先运行lambda的析构函数，它在释放内存之前销毁shared_ptr、fooobject及其附带的 std:：function对象现在已经释放了该内存位置，从而导致双空闲。

我现在试图弄清楚这是标准库实现（libstdc）中的一个bug，还是程序在某处触发了未定义的行为。

这可能是< code>libstdc 错误的一个标志是，在< code>clang 和< code>libc 14.0.6中，没有双释放(或者至少没有检测到)，但是< code > clang with < code > libstdc 也有双释放问题。

这个程序是否违反了任何规则/根据任何C标准触发未定义的行为？

我在x86-64 linux机器上复制了所有这些。

Answer 1

我认为该标准的相关部分是[物体上的物体]，其中规定

如果访问了标准库类型的对象，并且对象生存期的开始在访问之前不会发生，或者访问未在对象的生存期结束之前发生，则除非另有指定，否则行为未定义。

在您的示例中，您可以通过分配给 std：： 函数来访问它。在此访问期间，将调用 std：：函数的析构函数，结束 std：：函数的生存期。但是访问尚未完成，因此访问不会在对象的生存期结束之前发生。

因此，代码具有未定义的行为。