如何查看ELF文件

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#readelf #查找最大的函数 #查找最大的变量

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这篇博客介绍了如何利用readelf工具查看ELF文件的详细信息，包括文件头、Sections和符号表。通过结合awk、sort和grep命令，可以进行更深入的分析，如找出最大的函数和变量。示例中展示了查找并排序ELF文件中最大函数和变量的方法。

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查看elf文件

以一个简单的C程序为例：

#include <stdio.h>
#include <string.h>

int main(int argc, char**argv)
{
    printf("build date: %s %s\n",  __DATE__, __TIME__);
    return 0;
}

编译、执行

➜  ~ gcc test.c -o test
➜  ~ ./test 
build date: Nov 12 2017 22:07:06
➜  ~

查看elf文件头信息–readelf -h

➜  ~ readelf -h test
ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x400430
  Start of program headers:          64 (bytes into file)
  Start of section headers:          6624 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         9
  Size of section headers:           64 (bytes)
  Number of section headers:         31
  Section header string table index: 28

查看Sections—readelf -S

➜  ~ readelf -S test
There are 31 section headers, starting at offset 0x19e0:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .interp           PROGBITS         0000000000400238  00000238
       000000000000001c  0000000000000000   A       0     0     1
  [ 2] .note.ABI-tag     NOTE             0000000000400254  00000254
       0000000000000020  0000000000000000   A       0     0     4
  [ 3] .note.gnu.build-i NOTE             0000000000400274  00000274
       0000000000000024  0000000000000000   A       0     0     4
  [ 4] .gnu.hash         GNU_HASH         0000000000400298  00000298
       000000000000001c  0000000000000000   A       5     0     8
  [ 5] .dynsym           DYNSYM           00000000004002b8  000002b8
       0000000000000060  0000000000000018   A       6     1     8
  [ 6] .dynstr           STRTAB           0000000000400318  00000318
       000000000000003f  0000000000000000   A       0     0     1
  [ 7] .gnu.version      VERSYM           0000000000400358  00000358
       0000000000000008  0000000000000002   A       5     0     2
  [ 8] .gnu.version_r    VERNEED          0000000000400360  00000360
       0000000000000020  0000000000000000   A       6     1     8
  [ 9] .rela.dyn         RELA             0000000000400380  00000380
       0000000000000018  0000000000000018   A       5     0     8
  [10] .rela.plt         RELA             0000000000400398  00000398
       0000000000000030  0000000000000018  AI       5    24     8
  [11] .init             PROGBITS         00000000004003c8  000003c8
       000000000000001a  0000000000000000  AX       0     0     4
  [12] .plt              PROGBITS         00000000004003f0  000003f0
       0000000000000030  0000000000000010  AX       0     0     16
  [13] .plt.got          PROGBITS         0000000000400420  00000420
       0000000000000008  0000000000000000  AX       0     0     8
  [14] .text             PROGBITS         0000000000400430  00000430
       00000000000001a2  0000000000000000  AX       0     0     16
  [15] .fini             PROGBITS         00000000004005d4  000005d4
       0000000000000009  0000000000000000  AX       0     0     4
  [16] .rodata           PROGBITS         00000000004005e0  000005e0
       000000000000002c  0000000000000000   A       0     0     4
  [17] .eh_frame_hdr     PROGBITS         000000000040060c  0000060c
       0000000000000034  0000000000000000   A       0     0     4
  [18] .eh_frame         PROGBITS         0000000000400640  00000640
       00000000000000f4  0000000000000000   A       0     0     8
  [19] .init_array       INIT_ARRAY       0000000000600e10  00000e10
       0000000000000008  0000000000000000  WA       0     0     8
  [20] .fini_array       FINI_ARRAY       0000000000600e18  00000e18
       0000000000000008  0000000000000000  WA       0     0     8
  [21] .jcr              PROGBITS         0000000000600e20  00000e20
       0000000000000008  0000000000000000  WA       0     0     8
  [22] .dynamic          DYNAMIC          0000000000600e28  00000e28
       00000000000001d0  0000000000000010  WA       6     0     8
  [23] .got              PROGBITS         0000000000600ff8  00000ff8
       0000000000000008  0000000000000008  WA       0     0     8
  [24] .got.plt          PROGBITS         0000000000601000  00001000
       0000000000000028  0000000000000008  WA       0     0     8
  [25] .data             PROGBITS         0000000000601028  00001028
       0000000000000010  0000000000000000  WA       0     0     8
  [26] .bss              NOBITS           0000000000601038  00001038
       0000000000000008  0000000000000000  WA       0     0     1
  [27] .comment          PROGBITS         0000000000000000  00001038
       0000000000000034  0000000000000001  MS       0     0     1
  [28] .shstrtab         STRTAB           0000000000000000  000018cd
       000000000000010c  0000000000000000           0     0     1
  [29] .symtab           SYMTAB           0000000000000000  00001070
       0000000000000648  0000000000000018          30    47     8
  [30] .strtab           STRTAB           0000000000000000  000016b8
       0000000000000215  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
  I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)
➜  ~

查看符号表，并排序 – readelf -s

通过readelf -s命令可以查看符号表。

通过awk命令可以提取出类型、符号名、大小等信息。
通过sort可以对符号表进行排序。
通过grep命令可以提取出OBJ或者FUNCTION。
这些命令的组合，可以实现很多强大的功能。
如下图查看test里面最大的函数：

➜  ~ readelf -s test |awk '{print $3"\t"$4"\t"$8}' |grep FUNC |sort -rh |head -n 5
101 FUNC    __libc_csu_init
47  FUNC    main
42  FUNC    _start
2   FUNC    __libc_csu_fini
0   FUNC    register_tm_clones

如下图查看test里面最大的变量：

➜  ~ readelf -s test |awk '{print $3"\t"$4"\t"$8}' |grep OBJ |sort -rh |head -n 1
4   OBJECT  _IO_stdin_used