如何从自身取得的指针的节目的特定部分？ （也许用了libelf） [英] How to get a pointer to an specific section of a program from within itself? (Maybe with libelf)

问题描述

我在Linux环境下，我需要做检索一些数据被放置在其部分之一的程序。因此，如何从自身取得的指针的节目的部分（通过它的名字）？

I'm on a Linux environment and I need to make a program that retrieves some data that is placed in one of its sections. So, how to get a pointer to a section of a program (by its name) from within itself?

我知道这是可能使用 elf_getdata（）传递部分作为参数的索引来获取和 Elf_Data 结构和这个结构的领域之一是 d_buf ，这是一个指向实际的数据。然而，似乎在 elf_getdata（）功能使部分数据的副本从文件到记忆这不是我想要的。我想的指针的数据已经被加载到加载时间的存储器

I know it's possible to use elf_getdata() passing the index of the section as argument to get and Elf_Data struct and one of the fields of this struct is d_buf, which is a pointer to the actual data. However, it seems the elf_getdata() function makes a copy of the section data from the file to the memory and that's not what I want. I want a pointer to the data the has been loaded to the memory in loading time.

所以，伙计们，什么想法？

So, guys, any idea?

推荐答案

其实用 libelf函数，它可以使用 Elf64_Shdr 结构（64位系统），以获得一个指向一个部分，因为 sh_addr 字段做点那里的部分将被加载的实际ADRESS运行。因此，它可以用作一个指针。通过这种方式，它甚至没有必要使用 elf_getdata（）函数来获取一个 Elf_Data 结构。

Actually, using libelf, it's possible to use the Elf64_Shdr struct (for 64-bit systems) to get a pointer to a section, because the sh_addr field do points to the actual adress where the section will be loaded in runtime. So, it can be used as a pointer. This way, it's not even necessary to use the elf_getdata() function to retrieve a Elf_Data struct.

由于我想要做的是哪些其他目标文件可以对被链接的库，我的code可能有它打开可执行文件本身要利用一些 libelf函数的功能，使得它可以从主文件中的节读取数据，如下：

Since what I want to do is a library which other object files can be linked against, my code may have a function which opens the executable file itself to make use of some libelf features, so that it can read data from the main file sections, as follows:

// A global variable which stores the executable file name
extern const char *__progname;

void retrieve_data() {
  int fd;       // File descriptor for the executable ELF file
  char *section_name, path[256];
  size_t shstrndx;

  Elf *e;           // ELF struct
  Elf_Scn *scn;     // Section index struct
  Elf64_Shdr *shdr;     // Section struct

  // Create the full path of the executable
  getcwd(path, 255);
  strcat(path, "/");
  strcat(path, __progname);

  if(elf_version(EV_CURRENT)==EV_NONE)
    errx(EXIT_FAILURE, "ELF library iinitialization failed: %s", elf_errmsg(-1));

  if((fd = open(path, O_RDONLY, 0))<0)
    err(EXIT_FAILURE, "open \"%s\" failed", path);

  if((e = elf_begin(fd, ELF_C_READ, NULL))==NULL)
    errx(EXIT_FAILURE, "elf_begin() failed: %s.", elf_errmsg(-1));

  // Retrieve the section index of the ELF section containing the string table of section names
  if(elf_getshdrstrndx(e, &shstrndx)!=0)
    errx(EXIT_FAILURE, "elf_getshdrstrndx() failed: %s.", elf_errmsg(-1));

  scn = NULL;

  // Loop over all sections in the ELF object
  while((scn = elf_nextscn(e, scn))!=NULL) {
    // Given a Elf Scn pointer, retrieve the associated section header
    if((shdr = elf64_getshdr(scn))!=shdr)
      errx(EXIT_FAILURE, "getshdr() failed: %s.", elf_errmsg(-1));

    // Retrieve the name of the section name
    if((name = elf_strptr(e, shstrndx, shdr->sh_name))==NULL)
      errx(EXIT_FAILURE, "elf_strptr() failed: %s.", elf_errmsg(-1));

    // If the section is the one we want... (in my case, it is one of the main file sections)
    if(!strcmp(section_name, "Section name")) {
      // We can use the section adress as a pointer, since it corresponds to the actual
      // adress where the section is placed in the virtual memory
      struct data_t * section_data = (struct data_t *) shdr->sh_addr;

      // Do whatever we want

      // End the loop (if we only need this section)
      break;
    }
  }

  elf_end(e);
  close(fd);
}

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