使用RIP相对的OSX的x64汇编处理 [英] Using RIP-relative addressing in OSX x64 assembly

问题描述

我试图做一个基本的的printf 例如，在x86-64的组装$ C $下OSX，这是我的第一个版本：

I was trying to make a basic printf example in x86-64 assembly code for OSX, here's my first version:

section .data
msg db 'hello', 0Ah

section .text
extern _printf

global _main
_main:
  sub rsp, 8

  mov rdi, msg
  mov rax, 0
  call _printf

  add rsp, 8

  ret

所以这个code为移动信息的绝对地址为 RDI 的第一个参数 _printf ，和gcc再抱怨缺少位置无关code。二进制仍然有效，但：

So this code is moving the absolute address of msg into rdi for the first argument to _printf, and gcc then complains about the lack of position-independent code. The binary still works though:

→ nasm -f macho64 new.asm && gcc -m64 -o new new.o && ./new
ld: warning: PIE disabled. Absolute addressing (perhaps -mdynamic-no-pic) not allowed in code signed PIE, but used in _main from new.o. To fix this warning, don't compile with -mdynamic-no-pic or link with -Wl,-no_pie
hello

所以，当我改变code使用RIP相对寻址，使用 [REL ...] NASM语法，报警消失，但可执行现在赛格故障：

So when I change the code to use RIP-relative addressing, using the [rel ...] nasm syntax, the warning disappears but the executable now seg faults:

section .data
msg db 'hello', 0Ah

section .text
extern _printf

global _main
_main:
  sub rsp, 8

  mov rdi, [rel msg]
  mov rax, 0
  call _printf

  add rsp, 8

  ret

当我编译并运行它：

And when I compile and run it:

→ nasm -f macho64 new.asm && gcc -m64 -o new new.o && ./new
zsh: segmentation fault  ./new

有谁知道是怎么回事了？

Does anyone know what's going wrong?

推荐答案

现在的问题是，原来的 MOV偏下，味精加载信息到 RDI 的组装时间。

The problem is that the original mov rdi, msg loaded the memory address of msg into rdi at assemble time.

当它被更改为 MOV偏下，[REL味精] ，由此产生它用的值code 信息作为相对地址，在调试的时候看到的：

When it was changed to mov rdi, [rel msg], this produced code which used the value in msg as the relative address, as seen when debugging:

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x00000a6f6c6c6568

请注意如何地址包含字节从信息， 0x00000a＆LT; olleh＆GT;

正确的解决办法是使用 LEA 指令在运行时加载信息的有效RIP-相对地址，像这样：

The correct solution is to use the lea instruction to load the effective RIP-relative address of msg at runtime, like so:

lea rdi, [rel msg]

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