ltrace的源代码分析以及修改。

时间：2007-04-02 来源：loughsky

ltrace其可以打印出程序运行过程中的库函数调用，其原理是通过解析elf文件，找到需要解析的符号，及该符号所对应的地址。在程序运行起来后，首先使用ptrace技术，将该符号对应的位置替换为软件中断，这样在程序运行到该中断时，会通知ltrace，ltrace从而能打印出当时所调用的函数。在ltrace中，只时能够列出库函数的调用，那么如何来列出自身的函数调用呢？在elf文件中，我们可以找到symbol的section，该section中包含了程序中所用到的所有符号。其格式如下 typedef struct {
      Elf32_Word st_name;
      Elf32_Addr st_value;
      Elf32_Word st_size;
      unsigned char st_info;
      unsigned char st_other;
      Elf32_Half st_shndx;
} Elf32_Sym; st_name为symbol section所连接的string section的偏移量。 * st_value This member gives the value of the associated symbol. Depending on
the context, this may be an absolute value, an address, etc.;
details appear below. * st_info This member specifies the symbol's type and binding attributes. A
list of the values and meanings appears below. The following code
shows how to manipulate the values.     #define ELF32_ST_BIND(i) ((i)>>4)
    #define ELF32_ST_TYPE(i) ((i)&0xf)
    #define ELF32_ST_INFO(b, t) (((b)<<4)+((t)&0xf)) + Figure 1-18: Symbol Types, ELF32_ST_TYPE Name         Value
====         =====
STT_NOTYPE       0
STT_OBJECT       1
STT_FUNC         2
STT_SECTION      3
STT_FILE         4
STT_LOPROC      13
STT_HIPROC      15 所以我们找到symbol表中，type为STT_FUNC，其st_value就是该函数所对应的起始位置。在这里我们只要修改ltrace中的elf.c文件中的do_init_elf (struct ltelf *lte, const char *filename)；增加如下部分，来读取SYMTAB表。 else if (shdr.sh_type == SHT_SYMTAB)
{
   Elf_Data *data;
   size_t j;    lte->symbol = elf_getdata (scn, NULL);
   if (lte->symbol == NULL || elf_getdata (scn, lte->symbol) != NULL)
     error (EXIT_FAILURE, 0, "Couldn't get .symbol data from \"%s\"",
     filename);

      lte->symbol_count=shdr.sh_size/shdr.sh_entsize;
   scn = elf_getscn (lte->elf, shdr.sh_link);
     if (scn == NULL || gelf_getshdr (scn, &shdr) == NULL)
       error (EXIT_FAILURE, 0, "Couldn't get section header from \"%s\"",
         filename);     data = elf_getdata (scn, NULL);
   if (data == NULL || elf_getdata (scn, data) != NULL
       || shdr.sh_size != data->d_size || data->d_off)
     error (EXIT_FAILURE, 0, "Couldn't get .symbolstr data from \"%s\"",
     filename);     lte->symbolstr = data->d_buf;
    } 修改read_elf (const char *filename) for (i = 0; i < lte->symbol_count; ++i)   //读取每个符号
{
     GElf_Sym sym;
     const char *name;
     GElf_Addr addr;
     if(gelf_getsym(lte->symbol,i,&sym) == NULL)
         error (EXIT_FAILURE, 0, "Couldn't get symbol data from \"%s\"",filename);
     if (ELF32_ST_TYPE(sym.st_info) != STT_FUNC) //找出符号类型为STT_FUNC
         continue;

     name=lte->symbolstr+sym.st_name;        //将符号名称从偏移量转换为具体的值。
     if (in_load_libraries (name, lte))
     {
        addr = sym.st_value;
        if (addr != 0)
           add_library_symbol (addr, name, &library_symbols);  //将符号加入到全局的hash表中。
     }
} 其他代码就不用修改了，可见ltrace代码的结构性与层次性非常的清晰。