带有LD_PRELOAD和boost :: interprocess的mmap()不起作用

Question

我试图mmap()在预先标识的fd上替换原始系统调用LD_PRELOAD,以便调用它的进程可以读取由另一个进程先前创建的共享内存对象boost::interprocess.一切顺利,除非我最终尝试读取mmap的内存.在这种情况下,第一个进程以分段错误中止.原因是什么？我不需要对共享内存对象的写权限.

这是预加载库中的代码:

void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset) {
    static void* (*o_mmap) ( void *, size_t, int, int, int, off_t ) =
       o_mmap = (void*(*)( void *, size_t, int, int, int, off_t )) dlsym(RTLD_NEXT, "mmap");
    if (!o_mmap)
        std::cout << "mmap() preload failed\n";
    if (fd != my_fd)
        return (*o_mmap)( start, length, prot, flags, fd, offset );
    interprocess::shared_memory_object shm (interprocess::open_only, "obj", interprocess::read_only);
    interprocess::mapped_region region(shm, interprocess::read_only, 0, length, start);
    std::cout << "mmap() overridden. addr =" << region.get_address()  << " length: " << region.get_size() << " start: " << start << "\n";
    return region.get_address();
}

创建共享内存对象的程序代码是:

  //Create a shared memory object.
  shared_memory_object shm (create_only, "obj", read_write);

  //Set size
  shm.truncate(1000);

  //Map the whole shared memory in this process
  mapped_region region(shm, read_write);

  //Write all the memory to 1
  std::memset(region.get_address(), 1, region.get_size());

试图读取上面共享内存的程序代码(段错误)是:

  int fd = open(my_file, O_RDONLY);

  void* addr = mmap(0, 1000, PROT_READ, MAP_SHARED, fd, 0); // Okay

  //Check that memory was initialized to 1
  char *mem = static_cast<char*>(addr); 
  for(std::size_t i = 0; i < 1000; ++i)
     if(*mem++ != 1) // SEGFAULT!
        return 1;   //Error checking memory

Answer 1

您的问题是您有效地返回对本地的引用,但是以一种稍微混淆的方式.你的mmap()覆盖有一个interprocess::shared_memory_object和interprocess::mapped_region堆栈上的,当你返回到客户端时会被销毁.在销毁期间,boost包装器将取消映射内存区域,因此在客户端代码中访问它不再有效.作为一个简单的修复,将这些变量设置为静态可以防止seg错误,但是根据应用程序的结构,可能需要更复杂的解决方案.