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分类: LINUX

2013-06-05 09:01:06

原文地址:insmod源码分析 作者:galaxylancer

    问题的提出是前一阵和lgx聊天发现,一个被strip的module也可以被成功的insmod,当时知道一些insmod 的原理觉得不太可能,因为一个正常的module文件其实就是标准的ELF格式object文件,如果将他的 symtab strip掉的话,那些printk这类的symbol将不能被正常的解析,理论上是不可能加载成功的,于是 做了一个简单的module在turbo7上测试了一把,modutils的版本是2.4.6,出人意料的是竟然成功的被加 载,真是觉得真是不可思议,因此觉得有必要研究一下insmod的具体实现,最好的方法当然是go to the source
先说说关于module的几个系统调用,主要有 sys_create_module,sys_init_module,sys_query_module,sys_delete_module。我们简单的分析一 下module的创建过程,有一个重要地数据结构必然要提到,那就是struct module,定义如下。
struct module
{
        unsigned long size_of_struct;  /* == sizeof(module) */
        struct module *next;
        const char *name;
        unsigned long size;
        union
        {
                atomic_t usecount;
                long pad;
        } uc;                          /* Needs to keep its size - so says rth */
        unsigned long flags;            /* AUTOCLEAN et al */
        unsigned nsyms;        symbol的个数
        unsigned ndeps;
        struct module_symbol *syms;        此module实现的对外输出的所有symbol
        struct module_ref *deps;
        struct module_ref *refs;
        int (*init)(void);
        void (*cleanup)(void);
        const struct exception_table_entry *ex_table_start;
        const struct exception_table_entry *ex_table_end;
#ifdef __alpha__
        unsigned long gp;
#endif
        /* Members past this point are extensions to the basic
            module support and are optional.  Use mod_opt_member()
            to examine them.  */
        const struct module_persist *persist_start;
        const struct module_persist *persist_end;
        int (*can_unload)(void);
};
而kernel中有一个全局变量module_list为所有的module的list,在系统初始化是module_list只有一项为
struct module *module_list = &kernel_module;
kernel_module中的syms为__start___ksymtab即为kernel对外输出的symbol的所有项。
sys_create_module负责分配空间生成一个module结构,加入module_list中。而sys_init_module是将insmod 在用户空间地module结构复制到由sys_create_module创建地空间中。
好了,我们现在分析一下insmod的主要工作流程(modtuils-2.4.6版)
1.get_kernel_info函数负责取得kernel中先以注册的modules,放入module_stat中,并将kernel实现的各个 symbol放入ksyms中,个数为ksyms。
最终调用new_get_kernel_info
//type的三种类型,影响get_kernle_info的流程。
#define K_SYMBOLS 1 /* Want info about symbols */
#define K_INFO 2 /* Want extended module info */
#define K_REFS 4 /* Want info about references */
static int new_get_kernel_info(int type)
{
    struct module_stat *modules;
    struct module_stat *m;
    struct module_symbol *syms;
    struct module_symbol *s;
    size_t ret;
    size_t bufsize;
    size_t nmod;
    size_t nsyms;
    size_t i;
    size_t j;
    char *module_names;
    char *mn;
    drop();
    /*
    * Collect the loaded modules
    */
    module_names = xmalloc(bufsize = 256);
    //取得现有module的名称,ret返回个数,module_names返回各个module名称,字符0分割
    while (query_module(NULL, QM_MODULES, module_names, bufsize, &ret)) {
        if (errno != ENOSPC) {
            error("QM_MODULES: %m\n");
            return 0;
        }
        module_names = xrealloc(module_names, bufsize = ret);
    }
    module_name_list = module_names;
    l_module_name_list = bufsize;
    n_module_stat = nmod = ret;
    module_stat = modules = xmalloc(nmod * sizeof(struct module_stat));
    memset(modules, 0, nmod * sizeof(struct module_stat));
    /* Collect the info from the modules */
    //循环取得各个module的信息,QM_INFO的使用。
    for (i = 0, mn = module_names, m = modules;
        i < nmod;
        ++i, ++m, mn += strlen(mn) + 1) {
        struct module_info info;
        //info包括module的地址,大小,flag和使用计数器。
        m->;name = mn;
        if (query_module(mn, QM_INFO, &info, sizeof(info), &ret)) {
            if (errno == ENOENT) {
            /* The module was removed out from underneath us. */
                m->;flags = NEW_MOD_DELETED;
                continue;
            }
            /* else oops */
            error("module %s: QM_INFO: %m", mn);
            return 0;
        }
        m->;addr = info.addr;
        if (type & K_INFO) {//取得module的信息
            m->;size = info.size;
            m->;flags = info.flags;
            m->;usecount = info.usecount;
            m->;modstruct = info.addr;
        }//将info值传给module_stat结构
        if (type & K_REFS) {//取得module的引用关系
            int mm;
            char *mrefs;
            char *mr;
            mrefs = xmalloc(bufsize = 64);
            while (query_module(mn, QM_REFS, mrefs, bufsize, &ret)) {
                if (errno != ENOSPC) {
                    error("QM_REFS: %m");
                    return 1;
                }
                mrefs = xrealloc(mrefs, bufsize = ret);
            }
            for (j = 0, mr = mrefs;
                j < ret;
                ++j, mr += strlen(mr) + 1) {
                for (mm = 0; mm < i; ++mm) {
                    if (strcmp(mr, module_stat[mm].name) == 0) {
                        m->;nrefs += 1;
                        m->;refs = xrealloc(m->;refs, m->;nrefs * sizeof(struct module_stat **));
                        m->;refs[m->;nrefs - 1] = module_stat + mm;
                        break;
                    }
                }
            }
            free(mrefs);
        }
        if (type & K_SYMBOLS) { /* 取得symbol信息,正是我们要得*/
            syms = xmalloc(bufsize = 1024);
            while (query_module(mn, QM_SYMBOLS, syms, bufsize, &ret)) {
                if (errno == ENOSPC) {
                    syms = xrealloc(syms, bufsize = ret);
                    continue;
                }
                if (errno == ENOENT) {
                    /*
                    * The module was removed out
                    * from underneath us.
                    */
                    m->;flags = NEW_MOD_DELETED;
                    free(syms);
                    goto next;
                } else {
                    error("module %s: QM_SYMBOLS: %m", mn);
                    return 0;
                }
            }
            nsyms = ret;
            //syms是module_symbol结构,ret返回symbol个数
            m->;nsyms = nsyms;
            m->;syms = syms;
            /* Convert string offsets to string pointers */
            for (j = 0, s = syms; j < nsyms; ++j, ++s)
                s->;name += (unsigned long) syms;
        }
        next:
    }
    if (type & K_SYMBOLS) { /* Want info about symbols */
        /* Collect the kernel's symbols.  */
        syms = xmalloc(bufsize = 16 * 1024);
        //name为NULL,返回kernel_module的symbol。
        while (query_module(NULL, QM_SYMBOLS, syms, bufsize, &ret)) {
            if (errno != ENOSPC) {
                error("kernel: QM_SYMBOLS: %m");
                return 0;
            }
            syms = xrealloc(syms, bufsize = ret);
        }
        //将值返回给nksyms和ksyms两个全局变量存储。
        nksyms = nsyms = ret;
        ksyms = syms;
        /* name原来只是一个结构内的偏移,加上结构地址为真正的字符串地址 */
        for (j = 0, s = syms; j < nsyms; ++j, ++s)
            s->;name += (unsigned long) syms;
    }
    return 1;
}

2.set_ncv_prefix(NULL);判断symbolname中是否有前缀,象_smp之类,这里没有。

3.检查是否已有同名的module。
...
    for (i = 0; i < n_module_stat; ++i) {
        if (strcmp(module_stat.name, m_name) == 0) {
            error("a module named %s already exists", m_name);
            goto out;
        }
    }//判断是否已经存在。
...
 
4.obj_load,将。o文件读入到struct obj_file结构f中。
 
struct obj_file *obj_load (int fp, Elf32_Half e_type, const char *filename)
{
  struct obj_file *f;
  ElfW(Shdr) *section_headers;
  int shnum, i;
  char *shstrtab;
  /* Read the file header.  */
  f = arch_new_file();//生成新的obj_file结构
  memset(f, 0, sizeof(*f));
  f->;symbol_cmp = strcmp;//设置symbol名的比较函数就是strcmp
  f->;symbol_hash = obj_elf_hash;//设置计算symbol hash值的函数
  f->;load_order_search_start = &f->;load_order;//??
  gzf_lseek(fp, 0, SEEK_SET);
  if (gzf_read(fp, &f->;header, sizeof(f->;header)) != sizeof(f->;header))
    {//取得object文件的ELF头结构。
      error("cannot read ELF header from %s", filename);
      return NULL;
    }
  if (f->;header.e_ident[EI_MAG0] != ELFMAG0
      || f->;header.e_ident[EI_MAG1] != ELFMAG1
      || f->;header.e_ident[EI_MAG2] != ELFMAG2
      || f->;header.e_ident[EI_MAG3] != ELFMAG3)
    {//判断ELF的magic,是否是ELF文件
      error("%s is not an ELF file", filename);
      return NULL;
    }
  if (f->;header.e_ident[EI_CLASS] != ELFCLASSM//i386的机器上为ELFCLASS32,表示32bit
      || f->;header.e_ident[EI_DATA] != ELFDATAM//此处值为ELFDATA2LSB,表示编码方式
      || f->;header.e_ident[EI_VERSION] != EV_CURRENT//此值固定,表示版本
      || !MATCH_MACHINE(f->;header.e_machine))//机器类型
    {
      error("ELF file %s not for this architecture", filename);
      return NULL;
    }
  if (f->;header.e_type != e_type && e_type != ET_NONE)//类型必为ET_REL
    {
      switch (e_type) {
      case ET_REL:
    error("ELF file %s not a relocatable object", filename);
    break;
      case ET_EXEC:
    error("ELF file %s not an executable object", filename);
    break;
      default:
    error("ELF file %s has wrong type, expecting %d got %d",
        filename, e_type, f->;header.e_type);
    break;
      }
      return NULL;
    }
  /* Read the section headers.  */
  if (f->;header.e_shentsize != sizeof(ElfW(Shdr)))
    {
      error("section header size mismatch %s: %lu != %lu",
        filename,
        (unsigned long)f->;header.e_shentsize,
        (unsigned long)sizeof(ElfW(Shdr)));
      return NULL;
    }
  shnum = f->;header.e_shnum;//section个数
  f->;sections = xmalloc(sizeof(struct obj_section *) * shnum);
  memset(f->;sections, 0, sizeof(struct obj_section *) * shnum);
  section_headers = alloca(sizeof(ElfW(Shdr)) * shnum);//section表的大小
  gzf_lseek(fp, f->;header.e_shoff, SEEK_SET);
  if (gzf_read(fp, section_headers, sizeof(ElfW(Shdr))*shnum) != sizeof(ElfW(Shdr))*shnum)
    {//获得section表内容
      error("error reading ELF section headers %s: %m", filename);
      return NULL;
    }
  /* Read the section data.  */
  for (i = 0; i < shnum; ++i)
    {
      struct obj_section *sec;
      f->;sections = sec = arch_new_section();//分配内存给每个section
      memset(sec, 0, sizeof(*sec));
      sec->;header = section_headers;//设置section表项地址
      sec->;idx = i;//section表中第几个
      switch (sec->;header.sh_type)//section的类型
    {
    case SHT_NULL:
    case SHT_NOTE:
    case SHT_NOBITS:
      /* ignore */
      break;
    case SHT_PROGBITS:
    case SHT_SYMTAB:
    case SHT_STRTAB:
    case SHT_RELM://将以上各种类型的section内容读到结构中。
      if (sec->;header.sh_size >; 0)
        {
          sec->;contents = xmalloc(sec->;header.sh_size);
          gzf_lseek(fp, sec->;header.sh_offset, SEEK_SET);
          if (gzf_read(fp, sec->;contents, sec->;header.sh_size) != sec->;header.sh_size)
        {
          error("error reading ELF section data %s: %m", filename);
          return NULL;
        }
        }
      else
        sec->;contents = NULL;
      break;
    //描述relocation的section
#if SHT_RELM == SHT_REL
    case SHT_RELA:
      if (sec->;header.sh_size) {
        error("RELA relocations not supported on this architecture %s", filename);
        return NULL;
      }
      break;
#else
    case SHT_REL:
      if (sec->;header.sh_size) {
        error("REL relocations not supported on this architecture %s", filename);
        return NULL;
      }
      break;
#endif
    default:
      if (sec->;header.sh_type >;= SHT_LOPROC)
        {
          if (arch_load_proc_section(sec, fp) < 0)
        return NULL;
          break;
        }
      error("can't handle sections of type %ld %s",
        (long)sec->;header.sh_type, filename);
      return NULL;
    }
    }
  /* Do what sort of interpretation as needed by each section.  */
//shstrndx存的是section字符串表的索引值,就是第几个section
//shstrtab就是那个section了。
  shstrtab = f->;sections[f->;header.e_shstrndx]->;contents;
  for (i = 0; i < shnum; ++i)
    {
      struct obj_section *sec = f->;sections;
      sec->;name = shstrtab + sec->;header.sh_name;
    }//根据strtab,取得每个section的名字
  for (i = 0; i < shnum; ++i)
    {
      struct obj_section *sec = f->;sections;
      /* .modinfo and .modstring should be contents only but gcc has no
      *  attribute for that.  The kernel may have marked these sections as
      *  ALLOC, ignore the allocate bit.
      */也就是说即使modinfo和modstring有此标志位,也去掉。
      if (strcmp(sec->;name, ".modinfo") == 0 ||
      strcmp(sec->;name, ".modstring") == 0)
    sec->;header.sh_flags &= ~SHF_ALLOC;//ALLOC表示此section是否占用内存
      if (sec->;header.sh_flags & SHF_ALLOC)
    obj_insert_section_load_order(f, sec);//确定section load的顺序
            //根据的是flag的类型加权得到优先级
           
      switch (sec->;header.sh_type)
    {
    case SHT_SYMTAB://符号表
      {
        unsigned long nsym, j;
        char *strtab;
        ElfW(Sym) *sym;
        if (sec->;header.sh_entsize != sizeof(ElfW(Sym)))
          {
        error("symbol size mismatch %s: %lu != %lu",
              filename,
              (unsigned long)sec->;header.sh_entsize,
              (unsigned long)sizeof(ElfW(Sym)));
        return NULL;
          }
        //计算符号表表项个数,nsym也就是symbol个数
        nsym = sec->;header.sh_size / sizeof(ElfW(Sym));
        //sh_link是符号字符串表的索引值
        strtab = f->;sections[sec->;header.sh_link]->;contents;
        sym = (ElfW(Sym) *) sec->;contents;
        /* Allocate space for a table of local symbols.  */
        //为所有符号分配空间
        j = f->;local_symtab_size = sec->;header.sh_info;
        f->;local_symtab = xmalloc(j *= sizeof(struct obj_symbol *));
        memset(f->;local_symtab, 0, j);
        /* Insert all symbols into the hash table.  */
        for (j = 1, ++sym; j < nsym; ++j, ++sym)
          {
        const char *name;
        if (sym->;st_name)//有值就是strtab的索引值
          name = strtab+sym->;st_name;
        else//如果为零,此symbolname是一个section的name,比如.rodata之类的
          name = f->;sections[sym->;st_shndx]->;name;
        //obj_add_symbol将符号加入到f->;symbab这个hash表中
        obj_add_symbol(f, name, j, sym->;st_info, sym->;st_shndx,
                  sym->;st_value, sym->;st_size);
          }
      }
    break;
    }
    }
  /* second pass to add relocation data to symbols */
  for (i = 0; i < shnum; ++i)
    {
      struct obj_section *sec = f->;sections;
      switch (sec->;header.sh_type)
    {
    case SHT_RELM:
      {//找到描述重定位的section
        unsigned long nrel, j;
        ElfW(RelM) *rel;
        struct obj_section *symtab;
        char *strtab;
        if (sec->;header.sh_entsize != sizeof(ElfW(RelM)))
          {
        error("relocation entry size mismatch %s: %lu != %lu",
              filename,
              (unsigned long)sec->;header.sh_entsize,
              (unsigned long)sizeof(ElfW(RelM)));
        return NULL;
          }
    //算出rel有几项,存到nrel中
        nrel = sec->;header.sh_size / sizeof(ElfW(RelM));
        rel = (ElfW(RelM) *) sec->;contents;
        //rel的section中sh_link相关值是符号section的索引号
        symtab = f->;sections[sec->;header.sh_link];
        //而符号section中sh_link是符号字符串section的索引号
        strtab = f->;sections[symtab->;header.sh_link]->;contents;
        /* Save the relocate type in each symbol entry.  */
        //存储需要relocate的符号的rel的类型
        for (j = 0; j < nrel; ++j, ++rel)
          {
        ElfW(Sym) *extsym;
        struct obj_symbol *intsym;
        unsigned long symndx;//取得这个需relocate的符号索引值
        symndx = ELFW(R_SYM)(rel->;r_info);
        if (symndx)
          {
            extsym = ((ElfW(Sym) *) symtab->;contents) + symndx;
            if (ELFW(ST_BIND)(extsym->;st_info) == STB_LOCAL)
              {//local类型
            /* Local symbols we look up in the local table to be sure
              we get the one that is really intended.  */
            intsym = f->;local_symtab[symndx];
              }
            else
              {//其他类型,从hash表中取
            /* Others we look up in the hash table.  */
            const char *name;
            if (extsym->;st_name)
              name = strtab + extsym->;st_name;
            else
              name = f->;sections[extsym->;st_shndx]->;name;
            intsym = obj_find_symbol(f, name);
              }
            intsym->;r_type = ELFW(R_TYPE)(rel->;r_info);
          }
          }
      }
      break;
    }
    }
  f->;filename = xstrdup(filename);
  return f;
}

...
struct obj_symbol *
obj_add_symbol (struct obj_file *f, const char *name, unsigned long symidx,
        int info, int secidx, ElfW(Addr) value, unsigned long size)
{
  struct obj_symbol *sym;//计算符号的hash值
  unsigned long hash = f->;symbol_hash(name) % HASH_BUCKETS;
  int n_type = ELFW(ST_TYPE)(info);
  int n_binding = ELFW(ST_BIND)(info);
    //开始symtab为空的所以肯定找不到,一项一项向里加。
  for (sym = f->;symtab[hash]; sym; sym = sym->;next)
    if (f->;symbol_cmp(sym->;name, name) == 0)
      {//找到符号对应的值,保存原有的值。
    int o_secidx = sym->;secidx;
    int o_info = sym->;info;
    int o_type = ELFW(ST_TYPE)(o_info);
    int o_binding = ELFW(ST_BIND)(o_info);
    /* A redefinition!  Is it legal?  */
    if (secidx == SHN_UNDEF)
      return sym;
    else if (o_secidx == SHN_UNDEF)
      goto found;
    else if (n_binding == STB_GLOBAL && o_binding == STB_LOCAL)
      {
        /* Cope with local and global symbols of the same name
          in the same object file, as might have been created
          by ld -r.  The only reason locals are now seen at this
          level at all is so that we can do semi-sensible things
          with parameters.  */
        struct obj_symbol *nsym, **p;
        nsym = arch_new_symbol();//生成一个新的sym加到hash表中
        nsym->;next = sym->;next;
        nsym->;ksymidx = -1;
        /* Excise the old (local) symbol from the hash chain.  */
        for (p = &f->;symtab[hash]; *p != sym; p = &(*p)->;next)
          continue;
        *p = sym = nsym;
        goto found;
      }
    else if (n_binding == STB_LOCAL)
      {
        /* Another symbol of the same name has already been defined.
          Just add this to the local table.  */
        sym = arch_new_symbol();
        sym->;next = NULL;
        sym->;ksymidx = -1;//加到本地结构中
        f->;local_symtab[symidx] = sym;
        goto found;
      }
    else if (n_binding == STB_WEAK)
      return sym;
    else if (o_binding == STB_WEAK)
      goto found;
    /* Don't unify COMMON symbols with object types the programmer
      doesn't expect.  */
    else if (secidx == SHN_COMMON
        && (o_type == STT_NOTYPE || o_type == STT_OBJECT))
      return sym;
    else if (o_secidx == SHN_COMMON
        && (n_type == STT_NOTYPE || n_type == STT_OBJECT))
      goto found;
    else
      {
        /* Don't report an error if the symbol is coming from
          the kernel or some external module.  */
        if (secidx <= SHN_HIRESERVE)
          error("%s multiply defined", name);
        return sym;
      }
      }
  /* Completely new symbol.  */
  //开始的时候都会走到这里来
  sym = arch_new_symbol();//创建新的sym结构加入到hash表中
  sym->;next = f->;symtab[hash];
  f->;symtab[hash] = sym;
  sym->;ksymidx = -1;
  if (ELFW(ST_BIND)(info) == STB_LOCAL && symidx != -1) {
    if (symidx >;= f->;local_symtab_size)
      error("local symbol %s with index %ld exceeds local_symtab_size %ld",
        name, (long) symidx, (long) f->;local_symtab_size);
    else
      f->;local_symtab[symidx] = sym;//如果是文件内使用的加入到此结构中
  }
found://以后用kernel中的symbol解析时会走到此处,value会添上正确的值
  sym->;name = name;
  sym->;value = value;
  sym->;size = size;
  sym->;secidx = secidx;
  sym->;info = info;
  sym->;r_type = 0;    /* should be R_arch_NONE for all arch */
  return sym;
}
...
 
5.比较kernel版本和module的版本。
...
    /* Version correspondence?  */
    k_version = get_kernel_version(k_strversion);
    m_version = get_module_version(f, m_strversion);
    if (m_version == -1) {
        error("couldn't find the kernel version the module was compiled for");
        goto out;
    }
    k_crcs = is_kernel_checksummed();//kernel的symbol是否含有RXXXXXXX的校验
    m_crcs = is_module_checksummed(f);//module的symbol是否含有RXXXXXX的校验
    if ((m_crcs == 0 || k_crcs == 0) &&//如果都含有,就不必比较version了
        strncmp(k_strversion, m_strversion, STRVERSIONLEN) != 0) {
        if (flag_force_load) {//-f选项会强行加载
            lprintf("Warning: kernel-module version mismatch\n"
                  "\t%s was compiled for kernel version %s\n"
                "\twhile this kernel is version %s",
                filename, m_strversion, k_strversion);
        } else {
            if (!quiet)
                error("kernel-module version mismatch\n"
                      "\t%s was compiled for kernel version %s\n"
                      "\twhile this kernel is version %s.",
                      filename, m_strversion, k_strversion);
            goto out;
        }
    }
    if (m_crcs != k_crcs)//两者比一样
    //就不使用strcmp比较符号名字,而用ncv_strcmp比较使得printk和pirntk_RXXXXX
    //是相同的。还有重新创建hash表
        obj_set_symbol_compare(f, ncv_strcmp, ncv_symbol_hash);
...

6.add_kernel_symbols替换。o中的symbol为ksyms中的符号值
...
static void add_kernel_symbols(struct obj_file *f)
{
    struct module_stat *m;
    size_t i, nused = 0;
    /* 使用已有的module中的symbol */
    for (i = 0, m = module_stat; i < n_module_stat; ++i, ++m)
        if (m->;nsyms &&
            add_symbols_from(f, SHN_HIRESERVE + 2 + i, m->;syms, m->;nsyms))
            m->;status = 1 /* used */, ++nused;
    n_ext_modules_used = nused;
    /* 使用kernel export的symbol  */
    if (nksyms)
        add_symbols_from(f, SHN_HIRESERVE + 1, ksyms, nksyms);
}

/*
* Conditionally add the symbols from the given symbol set
* to the new module.
*/
static int add_symbols_from(struct obj_file *f, int idx,
                struct module_symbol *syms, size_t nsyms)
{
    struct module_symbol *s;
    size_t i;
    int used = 0;
    for (i = 0, s = syms; i < nsyms; ++i, ++s) {
        /*
        * Only add symbols that are already marked external.
        * If we override locals we may cause problems for
        * argument initialization.
        * We will also create a false dependency on the module.
        */
        struct obj_symbol *sym;
        //表中是否有需要此名字的的symbol
        sym = obj_find_symbol(f, (char *) s->;name);
        if (sym && !ELFW(ST_BIND) (sym->;info) == STB_LOCAL) {
            sym = obj_add_symbol(f, (char *) s->;name, -1,
                  ELFW(ST_INFO) (STB_GLOBAL, STT_NOTYPE),
                        idx, s->;value, 0);
            /*将hash表中的待解析的symbol的value添成正确的值
            * Did our symbol just get installed?
            * If so, mark the module as "used".
            */
            if (sym->;secidx == idx)
                used = 1;
        }
    }
    return used;
}
...
7.create_this_module(f, m_name)生成module结构,加入module_list中。
...
static int create_this_module(struct obj_file *f, const char *m_name)
{
    struct obj_section *sec;
    //创建一个.this节,节大小为struct module的大小
    sec = obj_create_alloced_section_first(f, ".this", tgt_sizeof_long,
                          sizeof(struct module));
    memset(sec->;contents, 0, sizeof(struct module));
    //向hash表中添加一个__this_module的符号
    obj_add_symbol(f, "__this_module", -1, ELFW(ST_INFO) (STB_LOCAL, STT_OBJECT),
              sec->;idx, 0, sizeof(struct module));
    //创建.kstrtab节
    obj_string_patch(f, sec->;idx, offsetof(struct module, name), m_name);
    return 1;
}
...

8.obj_check_undefineds检查是否还有un_def的symbol
...
obj_check_undefineds(struct obj_file *f, int quiet)
{
  unsigned long i;
  int ret = 1;
  for (i = 0; i < HASH_BUCKETS; ++i)
    {
      struct obj_symbol *sym;
      for (sym = f->;symtab; sym ; sym = sym->;next)
    if (sym->;secidx == SHN_UNDEF)
      {
        if (ELFW(ST_BIND)(sym->;info) == STB_WEAK)
          {
        sym->;secidx = SHN_ABS;
        sym->;value = 0;
          }
        else if (sym->;r_type) /* assumes R_arch_NONE is 0 on all arch */
          {
        if (!quiet)
            error("unresolved symbol %s", sym->;name);
        ret = 0;
          }
      }
    }
  return ret;
}
...
9.add_archdata添加结构相关的section,不过i386没什么用。
10.add_kallsyms如果symbol使用的都是kernel提供的,就添加一个.kallsyms section
11.obj_load_size计算module的大小
...
/* Module has now finished growing; find its size and install it.  */
m_size = obj_load_size(f);    /* DEPMOD */
...
obj_load_size (struct obj_file *f)
{
  unsigned long dot = 0;
  struct obj_section *sec;
  /* Finalize the positions of the sections relative to one another.  */
  for (sec = f->;load_order; sec ; sec = sec->;load_next)
    {//按照装载的顺序,计算module的大小。
      ElfW(Addr) align;
      align = sec->;header.sh_addralign;
      if (align && (dot & (align - 1)))
    dot = (dot | (align - 1)) + 1;
      sec->;header.sh_addr = dot;
      dot += sec->;header.sh_size;
    }
  return dot;
}
...
12.create_module调用sys_create_module系统调用创建模块,分配module的空间
13.obj_relocate
...
int
obj_relocate (struct obj_file *f, ElfW(Addr) base)
{//base就是create_module时分配的地址m_addr
  int i, n = f->;header.e_shnum;
  int ret = 1;
  /* Finalize the addresses of the sections.  */
//将各个section的sh_addr(原来是相对地址)加上此基地址
  arch_finalize_section_address(f, base);
  /* And iterate over all of the relocations.  */
  for (i = 0; i < n; ++i)
    {
      struct obj_section *relsec, *symsec, *targsec, *strsec;
      ElfW(RelM) *rel, *relend;
      ElfW(Sym) *symtab;
      const char *strtab;
      relsec = f->;sections;//找到重定位section
      if (relsec->;header.sh_type != SHT_RELM)
    continue;
    //relse的sh_link指向.symtab节,sh_info指向.text节
      symsec = f->;sections[relsec->;header.sh_link];
      targsec = f->;sections[relsec->;header.sh_info];
      //symtab节的sh_link指向符号字符串那节。
      strsec = f->;sections[symsec->;header.sh_link];
    //获得相应section的内容
      rel = (ElfW(RelM) *)relsec->;contents;
      relend = rel + (relsec->;header.sh_size / sizeof(ElfW(RelM)));
      symtab = (ElfW(Sym) *)symsec->;contents;
      strtab = (const char *)strsec->;contents;
      for (; rel < relend; ++rel)
    {
      ElfW(Addr) value = 0;
      struct obj_symbol *intsym = NULL;
      unsigned long symndx;
      ElfW(Sym) *extsym = 0;
      const char *errmsg;
      /* Attempt to find a value to use for this relocation.  */
    //根据rel的描述找到需要重定位的符号索引值
      symndx = ELFW(R_SYM)(rel->;r_info);
      if (symndx)
        {//同上,找到符号的描述,存入intsym变量中。
          /* Note we've already checked for undefined symbols.  */
          extsym = &symtab[symndx];
          if (ELFW(ST_BIND)(extsym->;st_info) == STB_LOCAL)
        {
          /* Local symbols we look up in the local table to be sure
            we get the one that is really intended.  */
          intsym = f->;local_symtab[symndx];
        }
          else
        {
          /* Others we look up in the hash table.  */
          const char *name;
          if (extsym->;st_name)
            name = strtab + extsym->;st_name;
          else
            name = f->;sections[extsym->;st_shndx]->;name;
          intsym = obj_find_symbol(f, name);
        }
//虽然有些函数的symbol的value已经被填写过了,但是rel中还有象.rodata节这样的需要
//relocate的符号,因此他的value是0+section[.rodata]->;header->;sh_addr的值
          value = obj_symbol_final_value(f, intsym);
        }
#if SHT_RELM == SHT_RELA
#if defined(__alpha__) && defined(AXP_BROKEN_GAS)
      /* Work around a nasty GAS bug, that is fixed as of 2.7.0.9.  */
      if (!extsym || !extsym->;st_name ||
          ELFW(ST_BIND)(extsym->;st_info) != STB_LOCAL)
#endif
      value += rel->;r_addend;
#endif
      /* Do it! *///此函数将.text中需要relocate的地方都填写正确value
      //非常重要的一个函数
      switch (arch_apply_relocation(f,targsec,symsec,intsym,rel,value))
        {//f:objfile结构,targsec:.text节,symsec:.symtab节,rel:.rel结构,value:绝对地址
        case obj_reloc_ok:
          break;
        case obj_reloc_overflow:
          errmsg = "Relocation overflow";
          goto bad_reloc;
        case obj_reloc_dangerous:
          errmsg = "Dangerous relocation";
          goto bad_reloc;
        case obj_reloc_unhandled:
          errmsg = "Unhandled relocation";
          goto bad_reloc;
        case obj_reloc_constant_gp:
          errmsg = "Modules compiled with -mconstant-gp cannot be loaded";
          goto bad_reloc;
        bad_reloc:
          if (extsym)
        {
          error("%s of type %ld for %s", errmsg,
            (long)ELFW(R_TYPE)(rel->;r_info),
            strtab + extsym->;st_name);
        }
          else
        {
          error("%s of type %ld", errmsg,
            (long)ELFW(R_TYPE)(rel->;r_info));
        }
          ret = 0;
          break;
        }
    }
    }
  /* Finally, take care of the patches.  */
  if (f->;string_patches)
    {
      struct obj_string_patch_struct *p;
      struct obj_section *strsec;
      ElfW(Addr) strsec_base;
      strsec = obj_find_section(f, ".kstrtab");
      strsec_base = strsec->;header.sh_addr;
      for (p = f->;string_patches; p ; p = p->;next)
    {
      struct obj_section *targsec = f->;sections[p->;reloc_secidx];
      *(ElfW(Addr) *)(targsec->;contents + p->;reloc_offset)
        = strsec_base + p->;string_offset;
    }
    }
  if (f->;symbol_patches)
    {
      struct obj_symbol_patch_struct *p;
      for (p = f->;symbol_patches; p; p = p->;next)
    {
      struct obj_section *targsec = f->;sections[p->;reloc_secidx];
      *(ElfW(Addr) *)(targsec->;contents + p->;reloc_offset)
        = obj_symbol_final_value(f, p->;sym);
    }
    }
  return ret;
}
...
14.init_module初始化有create_module生成的module,是由sys_init_module系统调用实现的。
...
static int init_module(const char *m_name, struct obj_file *f,
              unsigned long m_size, const char *blob_name,
              unsigned int noload, unsigned int flag_load_map)
{//一般情况下后三个参数都是0
    struct module *module;
    struct obj_section *sec;
    void *image;
    int ret = 0;
    tgt_long m_addr;
//找到原来用create_this_module生成的.this节
    sec = obj_find_section(f, ".this");
    module = (struct module *) sec->;contents;
    m_addr = sec->;header.sh_addr;//base基地址
    module->;size_of_struct = sizeof(*module);//module结构大小
    module->;size = m_size;//module总共的大小
    module->;flags = flag_autoclean ? NEW_MOD_AUTOCLEAN : 0;
    sec = obj_find_section(f, "__ksymtab");
    if (sec && sec->;header.sh_size) {//模块自己export的symbol
        module->;syms = sec->;header.sh_addr;
        module->;nsyms = sec->;header.sh_size / (2 * tgt_sizeof_char_p);
    }
    if (n_ext_modules_used) {//填写此module依靠的模块
        sec = obj_find_section(f, ".kmodtab");
        module->;deps = sec->;header.sh_addr;
        module->;ndeps = n_ext_modules_used;
    }
    //填写init_module,cleanup_module的地址
    module->;init = obj_symbol_final_value(f, obj_find_symbol(f, "init_module"));
    module->;cleanup = obj_symbol_final_value(f,
        obj_find_symbol(f, "cleanup_module"));
    //exception_table_entry的地址,一般没有
    sec = obj_find_section(f, "__ex_table");
    if (sec) {
        module->;ex_table_start = sec->;header.sh_addr;
        module->;ex_table_end = sec->;header.sh_addr + sec->;header.sh_size;
    }
    sec = obj_find_section(f, ".text.init");
    if (sec) {//这个runsize不知道是什么东西,一般没有
        module->;runsize = sec->;header.sh_addr - m_addr;
    }
    sec = obj_find_section(f, ".data.init");
    if (sec) {
        if (!module->;runsize ||
            module->;runsize >; sec->;header.sh_addr - m_addr)
            module->;runsize = sec->;header.sh_addr - m_addr;
    }
    sec = obj_find_section(f, ARCHDATA_SEC_NAME);
    if (sec && sec->;header.sh_size) {
        module->;archdata_start = sec->;header.sh_addr;
        module->;archdata_end = module->;archdata_start + sec->;header.sh_size;
    }
    sec = obj_find_section(f, KALLSYMS_SEC_NAME);
    if (sec && sec->;header.sh_size) {
        module->;kallsyms_start = sec->;header.sh_addr;
        module->;kallsyms_end = module->;kallsyms_start + sec->;header.sh_size;
    }
    if (!arch_init_module(f, module))//i386此函数直接返回
        return 0;
    /*
    * Whew!  All of the initialization is complete.
    * Collect the final module image and give it to the kernel.
    */
    image = xmalloc(m_size);
    obj_create_image(f, image);//现在用户空间分配module的image的内存,然后将
    //各个section的内容拷入image中,包括原文件中的section和后来构造的section
    if (flag_load_map)
        print_load_map(f);
    if (blob_name) {
        int fd, l;
        fd = open(blob_name, O_WRONLY|O_CREAT|O_TRUNC, S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH);
        if (fd < 0) {
            error("open %s failed %m", blob_name);
            ret = -1;
        }
        else {
            if ((l = write(fd, image, m_size)) != m_size) {
                error("write %s failed %m", blob_name);
                ret = -1;
            }
            close(fd);
        }
    }
    if (ret == 0 && !noload) {//调用系统调用完成最后的insmod工作。
        fflush(stdout);        /* Flush any debugging output */
        ret = sys_init_module(m_name, (struct module *) image);
        if (ret) {
            error("init_module: %m");
            lprintf("Hint: insmod errors can be caused by incorrect module parameters, "
                "including invalid IO or IRQ parameters");
        }
    }
    free(image);
    return ret == 0;
}
...

最后清理一下思路。
1.get_kernel_info函数负责取得kernel中先以注册的modules,放入module_stat中.
2.set_ncv_prefix(NULL);判断symbolname中是否有前缀,象_smp之类,一般没有。
3.检查是否已有同名的module。
4.obj_load,将。o文件读入到struct obj_file结构f中。
5.比较kernel版本和module的版本,在版本的判断中,不是想象中的那样简单,还有是否有checksum的逻 辑关系。
6.add_kernel_symbols替换.o中的symbol为ksyms中的符号值
7.create_this_module(f, m_name)生成module结构,加入module_list中。
8.obj_check_undefineds检查是否还有un_def的symbol
9.add_archdata添加结构相关的section,不过i386没什么用。
10.add_kallsyms如果symbol使用的都是kernel提供的,就添加一个.kallsyms section
11.obj_load_size计算module的大小
12.create_module调用sys_create_module系统调用创建模块,分配module的空间
13.obj_relocate重定位module文件中.text中的地址
14.init_module先在用户空间创建module结构的image影响,是由sys_init_module系统调用实现向kernel的 copy。
经过对insmod主要流程的分析,发现原来的理解没有偏差,那问什么会被成功加载呢?后来仔细用gdb跟 了一把,发现所有对symtab的操作都被调过,因为它一直为NULL嘛,可是发现在没有symtab的情况 下,2.4.6竟然一个判断都没有,一路平稳的跑了下来,然后在module结构init和cleanup指针都为NULL的情 况下被加载成功。
而2.4.10的modutils中的insmod对strip后的module加载后有除零的异常.在obj_load函数实现中有
278                nsyms = symtab->;header.sh_size / symtab->;header.sh_entsize;
由于module中根本没有symtab所以这个结构中的所有值都为0。
后话:
    其实将这篇文章加到backdoor研究中有些牵强,但是前一阵和backend讨论内核方式后门实现时讨论 了kernel image的方式,但只是限于理论上的讨论,考虑到以后有这方面实现的可能,那时有可能要自己 实现一个简单的insmod流程完成对symbol的解析和重定位,关于那三个module方面的系统调用在《情景 分析》中有较详细的讨论,就不班门弄斧了。
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