教你用php讀取elf結構

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#前提知識

1. UNIX系統的可執行文件都採用ELF格式，類型分為目標檔案、執行檔和共用函式庫
2. ELF格式探析之三：sections
3. 本範例基於64位元的小端序linux機器

以讀取目標檔案hello.o為例

#include <stdio.h>
void say_hello(char *who) {
    printf("hello, %s!\n", who);
}
char *my_name = "wb";
int man() {
    say_hello(my_name);
    return 0;
}
// 执行gcc -c hello.c生成hello.o

目標檔案elf結構主要有：

1. ELF header，位於檔案的0~64字節，儲存檔案的描述訊息，Section header table的起始位置
2. N個Section
3. Section header table，每個條目64位元組，對應一個Section的資訊
4. Program header table，可執行檔需要，本例的hello.o沒有

進一步分析elf結構

1. 首先用readelf指令讀取elf訊息：readelf -h hello.o。總結如下：
2. ELF header佔用64位元組
3. N個Section佔用6488-64-1472=4952位元組
4. Section header table佔用23*64=1472位元組

Section header table佔用23*64=1472位元組節

readelf -h hello.o
ELF Header:
Class:                             ELF64
Data:                              2's complement, little endian
OS/ABI:                            UNIX - System V
Type:                              REL (Relocatable file)
Machine:                           Advanced Micro Devices X86-64
Start of program headers:          0 (bytes into file)
Start of section headers:          5016 (bytes into file) //Section header table的起始位置
Size of this header:               64 (bytes) //ELF header的占用大小
Size of program headers:           0 (bytes)  //hello.o没有program header table
Number of program headers:         0          //hello.o没有program header table
Size of section headers:           64 (bytes) //Section header table每个条目占用大小
Number of section headers:         23         //Section header table条目个数
Section header string table index: 22         //.shstrtab Section位于Section header table第22个条目

用php讀取.shstrtab Section內容

.shstrtab Section其實是儲存的所有Section的名字######

<?php
$fp = fopen("hello.o", "rb");

fseek($fp, 40, SEEK_SET);
$sh_off = fread($fp, 8);
$sh_off = unpack("P", $sh_off);
var_dump("section header offset in file: ". $sh_off[1]);

fseek($fp, 10, SEEK_CUR);
$sh_ent_size = fread($fp, 2);
$sh_ent_size = unpack("v", $sh_ent_size);
var_dump("section header entry size: ". $sh_ent_size[1]);

$sh_num = fread($fp, 2);
$sh_num = unpack("v", $sh_num);
var_dump("section header number: ". $sh_num[1]);

$sh_strtab_index = fread($fp, 2);
$sh_strtab_index = unpack("v", $sh_strtab_index);
var_dump("section header string table index: ". $sh_strtab_index[1]);

fseek($fp, $sh_off[1] + $sh_strtab_index[1] * $sh_ent_size[1], SEEK_SET);
fseek($fp, 24, SEEK_CUR); //sh_name(4) + sh_type(4) + sh_flags(8) + sh_addr(8) = 24
$str_table_off = fread($fp, 8);
$str_table_off = unpack("P", $str_table_off);
var_dump("section name string table offset: ". $str_table_off[1]);

$str_table_size = fread($fp, 8);
$str_table_size = unpack("P", $str_table_size);
var_dump("section name string table size: ". $str_table_size[1]);

fseek($fp, $str_table_off[1], SEEK_SET);
$str = fread($fp, $str_table_size[1]);
print_r(explode("\x00", trim($str, "\x00")));

// 读取所有Section条目信息
for ($i =0;$i < $sh_num[1]; $i ++) {
    fseek($fp, $sh_off[1] + $i * $sh_ent_size[1], SEEK_SET);
    $sh_name = fread($fp, 4);
    $sh_name = unpack("V", $sh_name);
    fseek($fp, 20, SEEK_CUR); //sh_type(4) + sh_flags(8) + sh_addr(8) = 20
    $sh_offset = fread($fp, 8);
    $sh_offset = unpack("P", $sh_offset);

    $sh_size = fread($fp, 8);
    $sh_size = unpack("P", $sh_size);

    printf("section: %2s name: %-24s offset: %12s size: %12s\n", $i, get_section_name($sh_name[1]), $sh_offset[1], $sh_size[1]);
}

function get_section_name($start) {
    global $str;
    $name = substr($str, $start);
    return strstr($name, "\x00", true);
}

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