c++ - 链接器选项 -Bsymbolic

Question

我在共享库中替换了 operator new 和 operator delete。我希望当我在这个共享库上执行 dlopen 替换共享库中的 operator new 和 operator delete 版本时(不是来自运行 dlopen() 的二进制文件)。为了实现这一点，我将 -Bsymbolic 选项传递给链接器，但它似乎不起作用。

这是 MWE:

主要.cpp

#include <dlfcn.h>

#include <iostream>

#include "library.h"

int main(int argc, const char* argv[])
{
    std::cout << "going call library function" << std::endl;

    using func_t = decltype(library_function);
    auto handle = dlopen("./libshared.so", RTLD_NOW);
    auto fnc = reinterpret_cast<func_t*>(dlsym(handle, "library_function"));
    fnc();
    dlclose(handle);
}

库.h

#ifndef LIBRARY_H
#define LIBRARY_H

extern "C" void library_function();

#endif // LIBRARY_H

库.cpp:

#include <iostream>
#include <string>

extern "C" void library_function()
{
    std::string str;

    str.resize(10);

    std::cout << "inside library function: " << str.size() << std::endl;
}

新.cpp

#include <iostream>
#include <new>

void* operator new(std::size_t size)
{
    std::cout << "operator new from shared library" << std::endl;

    void* ptr = malloc(size);

    if(!ptr) throw std::bad_alloc();

    return ptr;
}

void* operator new(std::size_t size, const std::nothrow_t&) noexcept
{
    std::cout << "operator new from shared library" << std::endl;

    return malloc(size);
}

void* operator new[](std::size_t size)
{
    return ::operator new(size);
}

void* operator new[](std::size_t size, const std::nothrow_t& nothrow) noexcept
{
    return ::operator new(size, nothrow);
}

void operator delete(void* ptr) noexcept
{
    std::cout << "operator delete from shared library" << std::endl;

    return free(ptr);
}

void operator delete(void* ptr, std::size_t size) noexcept
{
    ::operator delete(ptr);
}

void operator delete(void* ptr, const std::nothrow_t&) noexcept
{
    return ::operator delete(ptr);
}

void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept
{
    return ::operator delete(ptr);
}

void operator delete[](void* ptr) noexcept
{
    return ::operator delete(ptr);
}

void operator delete[](void* ptr, std::size_t size) noexcept
{
    return ::operator delete(ptr);
}

void operator delete[](void* ptr, const std::nothrow_t&) noexcept
{
    return ::operator delete(ptr);
}

void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept
{
    return ::operator delete(ptr);
}

生成文件:

all:
    c++ -std=c++14 -g2 -O0 -shared -fPIC -o libshared.so library.cpp new.cpp -Wl,-Bsymbolic

    c++ -std=c++14 -g2 -O0 main.cpp -o main -ldl

主输出:

$ ./main 
going call library function
inside library function: 10

预期输出:输出包含“operator new from shared library”、“operator delete from shared library”。

第二个问题 - 当我显式链接到这个库(-lshared 用于 main.cpp)时，如何实现相同的行为。

更新:

看起来实际上链接器进行了更改 w/或 w/o -Bsymbolic。考虑来自 readelf -r 的 diff 在带和不带 -Bsymbolic 的共享库上:

-Relocation section '.rela.plt' at offset 0xeb0 contains 20 entries:
+Relocation section '.rela.plt' at offset 0xeb0 contains 15 entries:
   Offset          Info           Type           Sym. Value    Sym. Name + Addend
-000000202018  002700000007 R_X86_64_JUMP_SLO 00000000000014b7 operator new(unsigned long, std::nothrow_t const&) + 0
-000000202020  000300000007 R_X86_64_JUMP_SLO 0000000000000000 _ZNSt7__cxx1112basic_s@GLIBCXX_3.4.21 + 0
-000000202028  003000000007 R_X86_64_JUMP_SLO 000000000000142c operator new(unsigned long) + 0
-000000202030  000600000007 R_X86_64_JUMP_SLO 0000000000000000 std::ios_base::Init::Init@GLIBCXX_3.4 + 0
-000000202038  000700000007 R_X86_64_JUMP_SLO 0000000000000000 malloc@GLIBC_2.2.5 + 0
-000000202040  003100000007 R_X86_64_JUMP_SLO 000000000000153f operator delete(void*) + 0
-000000202048  000800000007 R_X86_64_JUMP_SLO 0000000000000000 __cxa_atexit@GLIBC_2.2.5 + 0
-000000202050  000b00000007 R_X86_64_JUMP_SLO 0000000000000000 _ZStlsISt11char_traits@GLIBCXX_3.4 + 0
-000000202058  000c00000007 R_X86_64_JUMP_SLO 0000000000000000 _ZNSt7__cxx1112basic_s@GLIBCXX_3.4.21 + 0
-000000202060  000d00000007 R_X86_64_JUMP_SLO 0000000000000000 free@GLIBC_2.2.5 + 0
-000000202068  000f00000007 R_X86_64_JUMP_SLO 0000000000000000 _ZNSt7__cxx1112basic_s@GLIBCXX_3.4.21 + 0
-000000202070  002e00000007 R_X86_64_JUMP_SLO 00000000000016b8 std::exception::exception() + 0
-000000202078  001200000007 R_X86_64_JUMP_SLO 0000000000000000 std::basic_ostream<char, std::char_traits<char> >::operator<<(unsigned long)@GLIBCXX_3.4 + 0
-000000202080  002f00000007 R_X86_64_JUMP_SLO 00000000000016d6 std::bad_alloc::bad_alloc() + 0
-000000202088  001500000007 R_X86_64_JUMP_SLO 0000000000000000 __stack_chk_fail@GLIBC_2.4 + 0
-000000202090  001600000007 R_X86_64_JUMP_SLO 0000000000000000 __cxa_allocate_excepti@CXXABI_1.3 + 0
-000000202098  001700000007 R_X86_64_JUMP_SLO 0000000000000000 _ZNKSt7__cxx1112basic_@GLIBCXX_3.4.21 + 0
-0000002020a0  001800000007 R_X86_64_JUMP_SLO 0000000000000000 __cxa_throw@CXXABI_1.3 + 0
-0000002020a8  001900000007 R_X86_64_JUMP_SLO 0000000000000000 std::basic_ostream<char, std::char_traits<char> >::operator<<(std::basic_ostream<char, std::char_traits<char> >& (*)(std::basic_ostream<char, std::char_traits<char> >&))@GLIBCXX_3.4 + 0
-0000002020b0  001c00000007 R_X86_64_JUMP_SLO 0000000000000000 _Unwind_Resume@GCC_3.0 + 0
+000000202018  000300000007 R_X86_64_JUMP_SLO 0000000000000000 _ZNSt7__cxx1112basic_s@GLIBCXX_3.4.21 + 0
+000000202020  000600000007 R_X86_64_JUMP_SLO 0000000000000000 std::ios_base::Init::Init@GLIBCXX_3.4 + 0
+000000202028  000700000007 R_X86_64_JUMP_SLO 0000000000000000 malloc@GLIBC_2.2.5 + 0
+000000202030  000800000007 R_X86_64_JUMP_SLO 0000000000000000 __cxa_atexit@GLIBC_2.2.5 + 0
+000000202038  000b00000007 R_X86_64_JUMP_SLO 0000000000000000 _ZStlsISt11char_traits@GLIBCXX_3.4 + 0
+000000202040  000c00000007 R_X86_64_JUMP_SLO 0000000000000000 _ZNSt7__cxx1112basic_s@GLIBCXX_3.4.21 + 0
+000000202048  000d00000007 R_X86_64_JUMP_SLO 0000000000000000 free@GLIBC_2.2.5 + 0
+000000202050  000f00000007 R_X86_64_JUMP_SLO 0000000000000000 _ZNSt7__cxx1112basic_s@GLIBCXX_3.4.21 + 0
+000000202058  001200000007 R_X86_64_JUMP_SLO 0000000000000000 std::basic_ostream<char, std::char_traits<char> >::operator<<(unsigned long)@GLIBCXX_3.4 + 0
+000000202060  001500000007 R_X86_64_JUMP_SLO 0000000000000000 __stack_chk_fail@GLIBC_2.4 + 0
+000000202068  001600000007 R_X86_64_JUMP_SLO 0000000000000000 __cxa_allocate_excepti@CXXABI_1.3 + 0
+000000202070  001700000007 R_X86_64_JUMP_SLO 0000000000000000 _ZNKSt7__cxx1112basic_@GLIBCXX_3.4.21 + 0
+000000202078  001800000007 R_X86_64_JUMP_SLO 0000000000000000 __cxa_throw@CXXABI_1.3 + 0
+000000202080  001900000007 R_X86_64_JUMP_SLO 0000000000000000 std::basic_ostream<char, std::char_traits<char> >::operator<<(std::basic_ostream<char, std::char_traits<char> >& (*)(std::basic_ostream<char, std::char_traits<char> >&))@GLIBCXX_3.4 + 0
+000000202088  001c00000007 R_X86_64_JUMP_SLO 0000000000000000 _Unwind_Resume@GCC_3.0 + 0

在使用 -Bsymbolic 构建时，operator new/operator delete 似乎未标记为可重定位 - 因此必须使用内部版本？

更新:事实上，我看到了调用 operator new 的区别:

w/o -B 符号调用通过 PLT 和 GOT:

   0x000000000000109c <+28>:    callq  0xed0 <_Znam@plt>

w/-B 符号调用不通过 PLT 和 GOT:

   0x0000000000000f7c <+28>:    callq  0x110a <operator new[](unsigned long)>

更新:

实际上，似乎调用了 operator new/operator delete 的正确版本。 GDB 中的步骤说明显示了它。问题是 operator<< 从共享库中 cout stream 不工作。

更新:

问题似乎是由以下事实引起的:当我在 std::string 上调用 resize - 该符号是从二进制文件而不是从库中使用的。在二进制文件中的 std::string resize 函数中，对 operator new 的调用被路由到其本地定义——这就是为什么我没有看到替换的 operator new/operator delete 的调用。当我直接使用 operator new/operator delete from library 时它开始工作。

更新:

是的，当静态链接libstdc++(libc++) 时问题消失了。

Answer 1

提供的示例确实有效。问题是问题的验证场景(通过调用 std::string resize 方法分配内存)是不好的。因为对 resize 方法的调用是由 PLT/GOT 动态路由到二进制版本的。二进制文件中的 std::string resize 方法调用它自己的 operator new/operator delete(不是来自共享库的版本)。为了解决这个问题——我们可以使用 libstdc++(-static-libstdc++) 进行静态编译。