Question

我的目的很简单：当我启动mongoose服务器时，服务器将初始化一个由我定义的变量。为此，我迷上了__libc_start_main。然后，当服务器收到请求时，它将打印出该初始化变量。为此，我迷上了recv。以下是我的代码。

#include <string>
#include <stdio.h>
#include <dlfcn.h>
#include <stdlib.h>
#include "common-structure.h"

#  define dprintf(fmt...)

data datainfo; //defined in common-structure.h

typedef int (*main_type)(int, char**, char**);

struct arg_type
{
  char **argv;
  int (*main_func) (int, char **, char **);
};

main_type saved_init_func = NULL;
void tern_init_func(int argc, char **argv, char **env){
  dprintf("%04d: __tern_init_func() called.\n", (int) pthread_self());
  if(saved_init_func)
    saved_init_func(argc, argv, env);
  datainfo.age = 10;
}

typedef void (*fini_type)(void*);
fini_type saved_fini_func = NULL;

extern "C" int my_main(int argc, char **pt, char **aa)
{
  int ret;
  arg_type *args = (arg_type*)pt;
  dprintf("%04d: __libc_start_main() called.\n", (int) pthread_self());
  ret = args->main_func(argc, args->argv, aa);
  return ret;
}

extern "C" int __libc_start_main(
  void *func_ptr,
  int argc,
  char* argv[],
  void (*init_func)(void),
  void (*fini_func)(void),
  void (*rtld_fini_func)(void),
  void *stack_end)
{
  typedef void (*fnptr_type)(void);
  typedef int (*orig_func_type)(void *, int, char *[], fnptr_type,
                                fnptr_type, fnptr_type, void*);
  orig_func_type orig_func;
  arg_type args;

  void * handle;
  int ret;

  // Get lib path.
  Dl_info dli;
  dladdr((void *)dlsym, &dli);
  std::string libPath = dli.dli_fname;
  libPath = dli.dli_fname;
  size_t lastSlash = libPath.find_last_of("/");
  libPath = libPath.substr(0, lastSlash);
  libPath += "/libc.so.6";
  libPath = "/lib/x86_64-linux-gnu/libc.so.6";
  if(!(handle=dlopen(libPath.c_str(), RTLD_LAZY))) {
    puts("dlopen error");
    abort();
  }

  orig_func = (orig_func_type) dlsym(handle, "__libc_start_main");

  if(dlerror()) {
    puts("dlerror");
    abort();
  }

  dlclose(handle);

  dprintf("%04d: __libc_start_main is hooked.\n", (int) pthread_self());

  args.argv = argv;
  args.main_func = (main_type)func_ptr;
  saved_init_func = (main_type)init_func;

  saved_fini_func = (fini_type)rtld_fini_func;
  ret = orig_func((void*)my_main, argc, (char**)(&args),
                  (fnptr_type)tern_init_func, (fnptr_type)fini_func,
                  rtld_fini_func, stack_end);

  return ret;

}

//hook recv
extern "C" ssize_t recv(int sockfd, void *buf, size_t len, int flags)
{
    ssize_t (*orig_recv)(int sockfd, void *buf, size_t len, int flags);
    orig_recv = dlsym(RTLD_NEXT, "recv");
    orig_recv(sockfd, buf, len, flags);

    printf("age is %d\n", datainfo.age);
}

然而，当我makefile时，我收到来自invalid conversion from ‘void*’ to ‘ssize_t (*)(int, void*, size_t, int)的错误：dlsym(RTLD_NEXT, "recv");。我的另一个问题是我能以这种方式实现我的目标吗？如果没有，那么正确的方法是什么？

Answer 1

C ++比C更强类型，您需要明确地将void *强制转换为正确的类型。

例如：

extern "C" ssize_t recv(int sockfd, void *buf, size_t len, int flags)
{
    using orig_recv_t = ssize_t (*)(int, void *, size_t, int);
    // Or for pre C++11 compilers: typedef ssize_t (*orig_recv_t)(int, void *, size_t, int);
    orig_recv_t orig_recv;

    orig_recv = reinterpret_cast<orig_recv_t>(dlsym(RTLD_NEXT, "recv"));
    orig_recv(sockfd, buf, len, flags);

    printf("age is %d\n", datainfo.age);
}

多个LD_REPLOAD共享变量

1 个答案: