ELF文件格式,是一个开放的可执行文件和链接文件格式,其主要工作在Linux系统上,是一种用于二进制文件、可执行文件、目标代码、共享库和核心转储格式文件,ELF文件格式类似于PE格式,但比起PE结构来ELF结构显得更加的简单,Linux文件结构相比于Windows结构来说简单一些.
读取ELF头： 首先需要先来编译一个简单的ELF文件,然后将文件编译并连接.
[root@localhost ~]# cat lyshark.c
#include <stdio.h>
int main()
{
printf("hello lyshark");
return 0;
}
[root@localhost ~]# gcc -c lyshark.c
[root@localhost ~]# gcc -o lyshark lyshark.o
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Linux系统中有一个默认命令readelf -h可以解析指定文件的头结构.
[root@localhost ~]
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64 64位程序
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V 调用约定
ABI Version: 0
Type: EXEC (Executable file) 可执行文件
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x400430 #程序的入口地址
Start of program headers: 64 (bytes into file)
Start of section headers: 6464 (bytes into file)
Flags: 0x0 #标志
Size of this header: 64 (bytes) #本头大小
Size of program headers: 56 (bytes) #程序头大小
Number of program headers: 9
Size of section headers: 64 (bytes) #节头大小
Number of section headers: 31 #节表数量
Section header string table index: 30 #字符串表索引节头
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通过hexdump工具查看文件16进制文件头hexdump -s 0 -n 64 -C lyshark
[root@localhost ~]
00000000 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 |.ELF............|
00000010 02 00 3e 00 01 00 00 00 30 04 40 00 00 00 00 00 |..>.....0.@.....|
00000020 40 00 00 00 00 00 00 00 40 19 00 00 00 00 00 00 |@.......@.......|
00000030 00 00 00 00 40 00 38 00 09 00 40 00 1f 00 1e 00 |....@.8...@.....|
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linux系统中的节头文件保存在/usr/include/elf.h我通过查找找到了ELF64所对应的结构数据
typedef uint16_t Elf64_Half; 16
typedef uint32_t Elf64_Word; 32
typedef uint64_t Elf64_Addr; 64
typedef uint64_t Elf64_Off; 64
#define EI_NIDENT (16)
typedef struct
{
unsigned char e_ident[EI_NIDENT];
Elf64_Half e_type;
Elf64_Half e_machine;
Elf64_Word e_version;
Elf64_Addr e_entry;
Elf64_Off e_phoff;
Elf64_Off e_shoff;
Elf64_Word e_flags;
Elf64_Half e_ehsize;
Elf64_Half e_phentsize;
Elf64_Half e_phnum;
Elf64_Half e_shentsize;
Elf64_Half e_shnum;
Elf64_Half e_shstrndx;
} Elf64_Ehdr;
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通过编程实现Magic的读取,或者说实现的是文件头e_ident[16]文件的读取,通过定义可得知文件头大小是16字节
#include <stdio.h>
#include <stdlib.h>
#include <elf.h>
int main(int argc,char* argv[])
{
if(argc < 2){ exit(0); }
FILE *fp;
Elf64_Ehdr elf_header;
fp = fopen(argv[1],"r");
if(fp == NULL) { exit(0); }
int readfile;
readfile = fread(&elf_header,sizeof(Elf64_Ehdr),1,fp);
if(readfile == 0){ exit(0); }
if(elf_header.e_ident[0] == 0x7F || elf_header.e_ident[1] == 'E')
{
printf("头标志: ");
for(int x =0;x<16;x++)
{
printf("%x ",elf_header.e_ident[x]);
}
printf("\n");
}
return 0;
}
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编译并运行即可读取出文件头部的前16个字节的字节数组,我们最需要关注的就是开头前4个字节,其标志着PE文件的开始
[root@localhost ~]
[root@localhost ~]
头标志: 7f 45 4c 46 2 1 1 0 0 0 0 0 0 0 0 0
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除此之外,读取其他头结构数据,代码与上方类似,只需要稍微改动一下就好.
if(elf_header.e_ident[0] == 0x7F || elf_header.e_ident[1] == 'E')
{
printf("文件类型: %hx\n",elf_header.e_type);
printf("运行平台: %hx\n",elf_header.e_machine);
printf("入口虚拟RVA: 0x%x\n",elf_header.e_entry);
printf("程序头文件偏移: %d(bytes)\n",elf_header.e_phoff);
printf("节头表文件偏移: %d(bytes)\n",elf_header.e_shoff);
printf("ELF文件头大小: %d\n",elf_header.e_ehsize);
printf("ELF程序头大小: %d\n",elf_header.e_phentsize);
printf("ELF程序头表计数: %d\n",elf_header.e_phnum);
printf("ELF节头表大小: %d\n",elf_header.e_shentsize);
printf("ELF节头表计数: %d\n",elf_header.e_shnum);
printf("字符串表索引节头: %d\n",elf_header.e_shstrndx);
}
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运行后,就可以读取到所有的节头数据.
[root@localhost ~]
文件类型: 2
运行平台: 3e
入口虚拟RVA: 0x400430
程序头文件偏移: 64(bytes)
节头表文件偏移: 6464(bytes)
ELF文件头大小: 64
ELF程序头大小: 56
ELF程序头表计数: 9
ELF节头表大小: 64
ELF节头表计数: 31
字符串表索引节头: 30
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读取ELF节表： 首先打开elf.h头文件,找到这个声明处Elf64_Shdr.
typedef uint32_t Elf64_Word; 32
typedef uint64_t Elf64_Addr; 64
typedef uint64_t Elf64_Off; 64
typedef uint64_t Elf64_Xword; 64
typedef struct
{
Elf64_Word sh_name; /* 节区名称 */
Elf64_Word sh_type; /* 节区类型 */
Elf64_Xword sh_flags; /* 节区标志 */
Elf64_Addr sh_addr; /* 如果在内存中运行,此处存放数据的内存地址 */
Elf64_Off sh_offset; /* 节区数据相对于文件的实际偏移量 */
Elf64_Xword sh_size; /* 节区大小 */
Elf64_Word sh_link; /* 节头表索引链接,其解释依赖于节区类型 */
Elf64_Word sh_info; /* 额外信息 */
Elf64_Xword sh_addralign; /* 节地址对其约束 */
Elf64_Xword sh_entsize; /* 固定大小项的表 */
} Elf64_Shdr;
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通过使用hexdump -s 144从偏移为144的位置开始读取,向后读取100个字节,就是节表所在位置.
[root@localhost ~]
00000090 38 02 40 00 00 00 00 00 1c 00 00 00 00 00 00 00 |8.@.............|
000000a0 1c 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 |................|
000000b0 01 00 00 00 05 00 00 00 00 00 00 00 00 00 00 00 |................|
000000c0 00 00 40 00 00 00 00 00 00 00 40 00 00 00 00 00 |..@.......@.....|
000000d0 0c 07 00 00 00 00 00 00 0c 07 00 00 00 00 00 00 |................|
000000e0 00 00 20 00 00 00 00 00 01 00 00 00 06 00 00 00 |.. .............|
000000f0 10 0e 00 00 |....|
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编程实现简单的节表读取,只需要在上方代码基础上进行修改即可.
if(elf_header.e_ident[0] == 0x7F || elf_header.e_ident[1] == 'E')
{
int shnum, x;
Elf64_Shdr *shdr = (Elf64_Shdr*)malloc(sizeof(Elf64_Shdr) * elf_header.e_shnum);
temp = fseek(fp, elf_header.e_shoff, SEEK_SET);
temp = fread(shdr, sizeof(Elf64_Shdr) * elf_header.e_shnum, 1, fp);
rewind(fp);
fseek(fp, shdr[elf_header.e_shstrndx].sh_offset, SEEK_SET);
char shstrtab[shdr[elf_header.e_shstrndx].sh_size];
char *names = shstrtab;
temp = fread(shstrtab, shdr[elf_header.e_shstrndx].sh_size, 1, fp);
printf("节类型\t节地址\t节偏移\t节大小\t节名称\n");
for(shnum = 0; shnum < elf_header.e_shnum; shnum++)
{
names = shstrtab;
names=names+shdr[shnum].sh_name;
printf("%x\t%x\t%x\t%x\t%s \n",shdr[shnum].sh_type,shdr[shnum].sh_addr,shdr[shnum].sh_offset,shdr[shnum].sh_size,names);
}
}
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Linux系统中也可以使用objdump命令读取程序的节表信息.
[root@localhost ~]
lyshark: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .interp 0000001c 0000000000400238 0000000000400238 00000238 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
1 .note.ABI-tag 00000020 0000000000400254 0000000000400254 00000254 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .note.gnu.build-id 00000024 0000000000400274 0000000000400274 00000274 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 .gnu.hash 0000001c 0000000000400298 0000000000400298 00000298 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .dynsym 00000060 00000000004002b8 00000000004002b8 000002b8 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
5 .dynstr 0000003f 0000000000400318 0000000000400318 00000318 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 .gnu.version 00000008 0000000000400358 0000000000400358 00000358 2**1
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当然objdump命令,还可以排查文件的SO加载情况.
[root@localhost ~]
NEEDED libpcre.so.1
NEEDED libz.so.1
NEEDED libpthread.so.0
NEEDED libc.so.6
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