Linux系统编程练习 ELF文件头读取和修改
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};
//Check if the file is an elf format file
static int is_elf(elf_header *ehdr)
{
??????? if(strncmp(ehdr->magic,MAGIC,strlen(MAGIC)) == 0)
??????????????? return 1;
??????? else
??????????????? return 0;
}
int read_elf_header(elf_header *ehdr,char *filename)
{
??????? FILE *fp;
??????? fp = fopen(filename,"r");
??????? if(fp == NULL)
??????????????? return FOPEN_FAILED;
??????? bzero(ehdr,sizeof(elf_header));
??????? fread(ehdr,sizeof(elf_header),1,fp);
??????? if(strncmp(ehdr->magic,MAGIC,strlen(MAGIC)) != 0)
??????? {
??????????????? fclose(fp);
??????????????? return FORMAT_ERROR;
??????? }
??????? fclose(fp);
??????? return SUCCESS;
}
static void put_tip(char *tip)
{
??????? printf(" %s",tip);
??????? printf("\033[%dC",INFO_POS - strlen(tip)); //每次输出第二列信息，即读出的数据的时候，先向右移动光固定个单位，保证对齐。详见ANSI控制码表
}
int print_elfhdr_info(elf_header *ehdr)
{
??????? if(strncmp(ehdr->magic,MAGIC,strlen(MAGIC)) != 0)
??????? {
??????????????? puts("Not an elf format file.");
??????????????? return FORMAT_ERROR;
??????? }
??????? puts("ELF header:");
??????? put_tip("Magic:");
??????? printf("%2x %2x %2x %2x (%c%c%c%c)\n"
??????????????????????? ,ehdr->magic[0],ehdr->magic[1],ehdr->magic[2],ehdr->magic[3]
??????????????????????? ,ehdr->magic[0],ehdr->magic[1],ehdr->magic[2],ehdr->magic[3]);
??????? put_tip("Class:");
??????? if(ehdr->addr_width == 1)
??????????????? printf("ELF32\n");
??????? if(ehdr->addr_width == 2)
??????????????? printf("ELF64\n");
??????? put_tip("Byte order:");
??????? if(ehdr->byteorder == 1)
??????????????? printf("Little-endian\n");
??????? else
??????????????? printf("Big-endian\n");
??????? put_tip("Version:");
??????? printf("0x%02x\n",ehdr->hversion);
??????? put_tip("File type:");
??????? if(ehdr->filetype > CUR_FTYPE_MAX)
??????????????? ehdr->filetype = 0;
??????? puts(file_type[ehdr->filetype]);
??????? put_tip("Machine:");
??????? if(ehdr->archtype > CUR_ARCH_MAX)
??????? {
??????????????? ehdr->archtype = 0;
??????? }
??????? puts(arch_type[ehdr->archtype]);
??????? put_tip("File version");
??????? printf("0x%x\n",ehdr->fversion);
??????? put_tip("Entry point address:");
??????? printf("0x%x\n%",ehdr->entry);
??????? put_tip("Start of program headers:");
??????? printf("%d (bytes into file)\n",ehdr->phdrpos);
??????? put_tip("Start of section headers:");
??????? printf("%d (bytes into file)\n",ehdr->shdrpos);
??????? put_tip("flags:");
??????? printf("%d(0x%x)\n",ehdr->flags,ehdr->flags);
??????? put_tip("Size of this header:");
??????? printf("%d bytes\n",ehdr->hdrsize);
??????? put_tip("Size of program headers:");
??????? printf("%d bytes\n",ehdr->phdrent);
??????? put_tip("Number of program headers:");
??????? printf("%d\n",ehdr->phdrcnt);
??????? put_tip("Size of section headers:");
??????? printf("%d bytes\n",ehdr->shdrent);
??????? put_tip("Number of section headers:");
??????? printf("%d\n",ehdr->shdrcnt);
??????? put_tip("Section header string table index:");
??????? printf("%d(0x%x)\n",ehdr->strsec,ehdr->strsec);
??????? return SUCCESS;
}
static void mod_help()
{
??????? puts("Select the part you want to modify:");
??????? puts("1 -- Magic number.");
??????? puts("2 -- Class(1:ELF32 2:ELF64).");
??????? puts("3 -- Byteorder(1:Little-endian 2:Big-endian).");
??????? puts("4 -- File type(1:relocatable 2:executable 3:shared object 4:core-image)");
??????? puts("s -- save");
??????? puts("q -- quit without saving");
??????? puts("x -- save and quit");
}
static int atoh(char *buf)//将16进制形式的字符串转换成数值
{
??????? int tmp = 0;
??????? while(*buf != '\0')
??????? {
??????????????? if(*buf >= '0' && *buf <= '9')
??????????????????????? tmp = tmp * 16 + *buf - '0';
??????????????? else
??????????????? {
??????????????????????? if((*buf >= 'a' && *buf <= 'f'))
??????????????????????????????? tmp = tmp * 16 + *buf - 'a' + 10;
??????????????????????? else
??????????????????????? {
??????????????????????????????? if((*buf >= 'A' && *buf <= 'F'))
??????????????????????????????????????? tmp = tmp * 16 + *buf - 'A' + 10;
??????????????????????????????? else
??????????????????????????????????????? return INVAL;
??????????????????????? }
??????????????? }
??????????????? buf++;
??????? }
??????? return tmp;
}
//以下是修改ELF文件头的函数，只为了测试，如果随意修改会使文件无法使用
static void mod_magic(elf_header *ehdr)//修改MAGIC号
{
??????? int i;
??????? int magic;
??????? int cur_val;
??????? char buf[20];
??????? char *tmp;
??????? printf("\tOld magic number: %x %x %x %x\n",ehdr->magic[0],ehdr->magic[1],ehdr->magic[2],ehdr->magic[3]);
??????? MODTIP();
??????? puts("\te.g: 0a0b0c0d or 0A0B0C0D");
??????? puts("\tEnter new value");
??????? bzero(buf,20);
??????? putchar('\t');
??????? fgets(buf,19,stdin);
??????? tmp = buf;
??????? while(*tmp != '\0')
??????? {
??????????????? if((*tmp >= '0' && *tmp <= '9') || (*tmp >= 'a' && *tmp <= 'f') || (*tmp >= 'A' && *tmp <= 'Z'))
??????????????? {
??????????????????????? *(tmp + 8) = 0;
??????????????????????? magic = atoh(tmp);
??????????????????????? break;
??????????????? }
??????????????? tmp++;
??????? }
?????? //如果输入的是0x11223344，那么存在文件上就是0x44332211(小端模式)，而访问magic字段的时候是以char型来访问，所以访问顺序正好相反，这里要交换字节序存入文件。
??????? tmp = (char *)(&magic) + 3;
??????? for(i = 0;i < 4;i++)
??????????????? ehdr->magic[i] = *(tmp - i);
}
static void mod_class(elf_header *ehdr) //修改地址宽度，32位或是64位
{
??????? char opt[2];
??????? printf("\tOld class: %d \n\t1:ELF32 \n\t2:ELF64\n",ehdr->addr_width);
??????? MODTIP();
??????? while(1)
??????? {
??????????????? opt[0] = getchar();
??????????????? opt[1] = getchar();
??????????????? if(opt[0] > '2' || opt[0] < '1')
??????????????????????? puts("\tWrong choice.Value should be 1 or 2");
??????????????? else
??????????????????????? break;
??????? }
??????? ehdr->addr_width = opt[0] - '0';
}
static void mod_byteorder(elf_header *ehdr) //修改字节序
{
??????? char opt[2];
??????? printf("\tOld byte order: %d \n\t1:Little-endian \n\t2:Big-endian\n",ehdr->byteorder);
??????? MODTIP();
??????? while(1)
??????? {
??????????????? opt[0] = getchar();
??????????????? opt[1] = getchar();
??????????????? if(opt[0] > '2' || opt[0] < '1')
??????????????????????? puts("\tWrong choice.Value should be 1 or 2");
??????????????? else
??????????????????????? break;
??????? }
??????? ehdr->byteorder = opt[0] - '0';
}
static void mod_filetype(elf_header *ehdr)//修改文件类型
{
??????? char opt[2];
??????? printf("\tOld file type: %d \n\t1:Relocatable \n\t2:Executable \n\t3:Shared object \n\t4:Core image\n)",ehdr->filetype);
??????? MODTIP();
??????? while(1)
??????? {
??????????????? opt[0] = getchar();
??????????????? opt[1] = getchar();
??????????????? if(opt[0] > '4' || opt[0] < '1')
??????????????????????? puts("\tWrong choice.Value should be 1 or 2");
??????????????? else
??????????????????????? break;
??????? }
??????? ehdr->filetype = opt[0] - '0';
}
int modify_elfhdr(elf_header *ehdr,char *filename)
{
??????? char opt[2];
??????? FILE *fp;
//????? elf_header ehdr_backup;
??????? fp = fopen(filename,"r+");
??????? if(fp == NULL)
??????????????? return FOPEN_FAILED;
??????? fread(ehdr,sizeof(elf_header),1,fp);
??????? if(!is_elf(ehdr))
??????? {
??????????????? puts("Not an elf format file.");
??????????????? return FORMAT_ERROR;
??????? }
//????? memcpy(&ehdr_backup,ehdr,sizeof(elf_header));
??????? while(1)
??????? {
??????????????? mod_help();
??????????????? opt[0] = getchar();
??????????????? opt[1] = getchar();
??????????????? switch(opt[0])
??????????????? {
??????????????????????? case '1':
??????????????????????????????? mod_magic(ehdr);
??????????????????????????????? break;
??????????????????????? case '2':
??????????????????????????????? mod_class(ehdr);
??????????????????????????????? break;
??????????????????????? case '3':
??????????????????????????????? mod_byteorder(ehdr);
??????????????????????????????? break;
??????????????????????? case '4':
??????????????????????????????? mod_filetype(ehdr);
??????????????????????????????? break;
??????????????????????? case 's':
??????????????????????????????? fseek(fp,0,SEEK_SET);
??????????????????????????????? fwrite(ehdr,sizeof(elf_header),1,fp);
??????????????????????????????? break;
??????????????????????? case 'q':
??????????????????????????????? goto out;
??????????????????????????????? break;
??????????????????????? case 'x':
??????????????????????????????? fseek(fp,0,SEEK_SET);
??????????????????????????????? fwrite(ehdr,sizeof(elf_header),1,fp);
??????????????????????????????? goto out;
??????????????????????????????? break;
??????????????????????? default:
??????????????????????????????? puts("Wrong choice.");
??????????????? }
??????? }
out:
??????? fclose(fp);
??????? return SUCCESS;
}
//main.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include "elf_head.h"
static void help()
{
??????? puts("******************************************");
??????? puts("*???? ELF header reader and modifier???? *");
??????? puts("*???? 0 - exit?????????????????????????? *");
??????? puts("*???? 1 - print header info????????????? *");
??????? puts("*???? 2 - modify elf header????????????? *");
??????? puts("******************************************");
}
int main(int argc,char **argv)
{
??????? char cmd;
??????? elf_header ehdr;
??????? if(argc != 2)
??????? {
???????????????? printf("Usage: %s [filename]\n",strchr(argv[0],'/') + 1);
??????????????? return 0;
??????? }
??????? switch(read_elf_header(&ehdr,argv[1]))
??????? {
??????????????? case FOPEN_FAILED:
??????????????????????? printf("Can not open %s.\n",argv[1]);
??????????????????????? goto out;
??????????????? case FORMAT_ERROR:
??????????????????????? printf("%s is not an elf format file.\n",argv[1]);
??????????????????????? goto out;
??????? }
??????? while(cmd != '0')
??????? {
??????????????? help();
??????????????? cmd = getchar();
??????????????? switch(cmd)
??????????????? {
??????????????????????? case '1':
??????????????????????????????? getchar();//由于终端不是非标准模式，所以输入字符后都要使用回车，回车符本身会当作输入的字符，这里使用getchar()目的就是使下次获得输入的字符时不为回车符
??????????????????????????????? print_elfhdr_info(&ehdr);
??????????????????????????????? break;
??????????????????????? case '2':
??????????????????????????????? getchar();
??????????????????????????????? modify_elfhdr(&ehdr,argv[1]);
??????????????????????????????? break;
??????????????????????? default:
??????????????????????????????? break;
??????????????? }
??????? }
out:
??????? return 0;
}
