Recommended: "PHP Video Tutorial"
Prerequisite knowledge
- UNIX system executable The files are all in ELF format, and the types are divided into target files, executable files and shared libraries
- Analysis of ELF format three: sections
- This example is based on a 64-bit little-endian Linux machine
Take reading the target file hello.o as an example
#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
The elf structure of the target file mainly includes:
- ELF header, located 0~64 bytes of the file store the description information of the file, the starting position of the Section header table
- N Section
- Section header table, each entry is 64 bytes, corresponding to a Section Information
- Program header table, executable file needs, hello.o in this example does not
Further analysis of the elf structure
- First use the readelf command to read the elf information: readelf -h hello.o. The summary is as follows:
- ELF header occupies 64 bytes
- N Sections occupy 6488-64-1472=4952 bytes
- Section header table occupies 23*64=1472 words Section
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
Use php to read the .shstrtab Section content
- .shstrtab Section is actually the name of all stored Sections
<?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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