c++ - 如何通过 DbgHelp 获取局部变量的值

Question

如何通过 DbgHelp 获取局部变量的值？我尝试使用以下代码

#include <boost/scope_exit.hpp>

#include <Windows.h>
#include <Dbghelp.h>

#include <iostream>
#include <map>
#include <string>

#pragma comment(lib, "Dbghelp.lib")

BOOL CALLBACK enum_symbols_callback(PSYMBOL_INFO pSymInfo, ULONG SymbolSize, PVOID UserContext)
{
  if (SymbolSize == 0)
  {
    return TRUE;
  }

  auto* local_variables_info = reinterpret_cast<std::map<std::string, std::string>*>(UserContext);

  std::unique_ptr<unsigned char[]> bytes_read(new unsigned char[SymbolSize]);
  HANDLE cur_process_handle = GetCurrentProcess();
  SIZE_T number_of_bytes_actually_read;
  if (ReadProcessMemory(
    cur_process_handle
    , reinterpret_cast<void*>(pSymInfo->Address)
    , bytes_read.get()
    , SymbolSize
    , &number_of_bytes_actually_read) != 0)
  {
    unsigned char* bytes = bytes_read.get();
    local_variables_info->operator[](pSymInfo->Name) = std::to_string(pSymInfo->Value);
  }

  return TRUE; // Continue enumeration
}

std::map<std::string, std::string> get_local_variables_info()
{
  std::map<std::string, std::string> local_variables_info;

  HANDLE cur_process_handle = GetCurrentProcess();
  if (SymInitialize(cur_process_handle, NULL, TRUE) == FALSE)
  {
    return local_variables_info;
  }
  BOOST_SCOPE_EXIT_ALL(cur_process_handle)
  {
    SymCleanup(cur_process_handle);
  };

  const ULONG frames_to_skip = 0;
  const ULONG frames_to_capture = 1;
  void* stack[frames_to_capture];

  const USHORT frames = CaptureStackBackTrace(
    frames_to_skip
    , frames_to_capture
    , stack
    , NULL
  );
  if (frames != 1)
  {
    return local_variables_info;
  }

  IMAGEHLP_STACK_FRAME sf;
  sf.InstructionOffset = reinterpret_cast<DWORD_PTR>(stack[0]);
  if (SymSetContext(
    cur_process_handle,
    &sf,                  // The context 
    0                     // Not used 
  ) == FALSE)
  {
    return local_variables_info;
  }

  if (SymEnumSymbols(
    cur_process_handle,
    0,                      // 0 -> SymEnumSymbols will use the context set with SymSetContext
    0,                      // Mask must also be 0 to use the context
    enum_symbols_callback,
    &local_variables_info   // User-defined context
  ) == FALSE)
  {
    return local_variables_info;
  }

  return local_variables_info;
}

int main()
{
  int foo = 0;
  const auto& local_variables_info = get_local_variables_info();
  for (const std::pair<std::string, std::string> e : local_variables_info)
  {
    std::cout << e.first << ' ' << e.second << '\n';
  }
}

但是reinterpret_cast<void*>(pSymInfo->Address)总是返回类似 FFFFFFD4 的地址。

为什么？我究竟做错了什么？我该如何解决？

提前致谢。

Answer 1

您正在寻找的变量值的地址位于函数的开始地址(可从执行上下文的 EBP 寄存器访问)，从堆栈中函数的此变量位置的偏移量应用。在您的情况下(函数的局部变量)，pSymInfo->Address 成员存储变量相对于堆栈帧的偏移量。

(这里有一个解释:http://eli.thegreenplace.net/2011/02/04/where-the-top-of-the-stack-is-on-x86/)

所以你要找的地址在 (EBP + pSymInfo->Address)

要启动函数，您可以使用:

1. CONTEXT -> EBP 寄存器
2. StackWalk64() -> 启用遍历调用堆栈
3. GetThreadContext() -> 获取上下文