从[esp]以64位代码加载时出现段错误 [英] Segfault when loading from [esp] in 64-bit code

问题描述

我对x86汇编很陌生，并且我正在尝试构建一个hello world程序.我正在尝试创建一个子例程，该子例程将单个字节写入stdout，但是我遇到了问题.

I'm quite new to x86 assembly, and I'm trying to build off a hello world program. I'm trying to make a subroutine, that writes a single byte to stdout, but i've hit a problem.

mov ebx, [esp+1]行(加载我调用子例程时传递的字节)会导致段错误.

The line mov ebx, [esp+1] (to load the byte passed, when I call the subroutine) causes a segfault.

我尝试将ebx寄存器与其自身进行异或，以确保它为空，以确保它不会与syscall混淆

I've tried xoring the ebx register with itself, to make sure that it is empty, to make sure, that it doesn't mess with the syscall

_start:
    push 32h
    call _writeByte

    ; This just jumps to an exit routine
    jmp  _exit

_writeByte:
    ; This line causes the problem. If I remove it the program works fine
    mov  ebx, [esp+1]
    xor  ebx, ebx

    mov  eax, 1
    mov  edi, 1
    mov  esi, tmp
    mov  edx, 1
    syscall

    ret

为什么程序会出现段错误?

Why is the program segfaulting?

推荐答案

我处于x64模式，就像一群人在使用mov ebx, [rsp+8]的注释中建议的那样起作用，因为esp只是低4个字节的寄存器.堆栈位于虚拟地址空间的低4 GiB之外，因此ESP！= RSP和[esp]将是未映射的页面.

I'm in x64 mode, and like a bunch of people suggested in the comments using mov ebx, [rsp+8] worked, because esp are just the 4 lower bytes of the register. The stack is outside the low 4 GiB of virtual address space, so ESP != RSP and [esp] will be an unmapped page.

请注意，x86-64调用约定在寄存器中而不是在堆栈中传递前几个arg，因此您通常根本不想这样做(除非您的函数有很多arg).

Note that x86-64 calling conventions pass the first few args in register, not on the stack, so you normally don't want to do this at all (unless your function has lots of args).

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