可以从虚拟地址空间内的任意地址加载和运行位置无关的 ELF 可执行文件。
我尝试构建以下简单程序:
pie.c
#include <stdio.h>
void main(void)
{
printf("hello, pie!\n");
}
构建命令:gcc pie.c -o pie -pie
可执行文件的 ELF header 是:派 :
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: DYN (Shared object file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x530 <================= FIXED entry point
Start of program headers: 64 (bytes into file)
Start of section headers: 6440 (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: 29
Section header string table index: 28
所以入口点地址是固定的。它是符号
_start
那里:0000000000000530 T _start
所以这意味着 _start
必须放在 0x530
.这与位置独立不矛盾吗?
添加 1 - 10:44 上午 8/16/2021
我试图在没有
-pie
的情况下构建相同的程序旗帜:gcc pie.c -o pie_not
生成的 ELF 头是:pie_not :
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: DYN (Shared object file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x530 <============ Still the same value as with -pie flag
Start of program headers: 64 (bytes into file)
Start of section headers: 6440 (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: 29
Section header string table index: 28
我进一步比较了构建结果pie
和 pie_not
.它们的二进制文件是相同的。那么操作系统如何判断哪一个应该被视为位置无关代码呢?
添加 2 - 2021 年 8 月 16 日上午 10:56
我想我想通了。
似乎gcc使用
-pie
默认情况下。为了避免它,我必须添加 -no-pie
明确地标记。gcc pie.c -o pie_not -no-pie
而生成的 ELF 头是这样的:pie_not :
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) <==== File type also changed!!!
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x400400 <===== Entry point address changed!
Start of program headers: 64 (bytes into file)
Start of section headers: 6376 (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: 29
Section header string table index: 28
我相信加载器依赖于 ELF header 中的 Type 来决定如何处理二进制文件。
最佳答案
对于位置无关的可执行文件,ELF header 中的入口点地址不用作入口点的绝对地址。相反,它被用作随机选择的基地址的偏移量。
所以在你的例子中,如果基地址被随机选择为 0x567812340000
,那么执行将从绝对地址 0x567812340530
开始.
关于gcc - 位置无关的可执行文件和固定入口点地址,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/68792831/