主栈对齐 [英] main and stack alignment

问题描述

我有一个打印文本和浮点数的功能.这是一个不使用main的版本

I have a function which prints text and a floating point number. Here is a version which does not use main

extern printf
extern _exit

section .data
    hello:     db 'Hello world! %f',10,0
    pi:        dq  3.14159
section .text
    global _start
_start:
    xor eax, eax
    lea rdi, [rel hello]
    movsd xmm0, [rel pi]
    mov eax, 1
    call printf
    mov rax, 0
    jmp _exit

我这样组装并链接

nasm -felf64 hello.asm
ld hello.o -dynamic-linker /lib64/ld-linux-x86-64.so.2 -lc -melf_x86_64

运行正常.但是，现在我想使用main来做到这一点.

This runs fine. However, now I want to do this using main.

global main
extern printf

section .data
    hello:     db 'Hello world! %f',10,0
    pi:        dq  3.14159
section .text
    main:
    sub rsp, 8
    xor eax, eax
    lea rdi, [rel hello]
    movsd xmm0, [rel pi]
    mov eax, 1
    call printf
    mov rax, 0
    add rsp, 8
    ret

我这样组装和链接

nasm -felf64 hello_main.asm
gcc hello_main.o

这也很好.但是，在调用printf之前，我必须从堆栈指针中减去8个字节，然后在出现分段错误之后，再向堆栈指针中添加8个字节.

This runs fine as well. However, I had to subtract eight bytes from the stack pointer before calling printf and then add eight bytes to the stack pointer after otherwise I get a segmentation fault.

看看堆栈指针，我看到不使用main时它是16字节对齐的，但是使用main时它只是8字节对齐的.必须减去和增加八个字节的事实表明，它始终是8字节对齐的，而从来没有16字节对齐的(除非我误解了).为什么是这样?我以为x86_64代码可以假定堆栈是16字节对齐的(至少对于我认为包括main的标准库函数调用而言).

Looking at the stack pointer I see that without using main it's 16-byte aligned but with main it's only eight byte aligned. The fact that eight bytes has to be subtracted and added says that it's always 8-byte aligned and never 16-byte aligned (unless I misunderstand something). Why is this? I thought with x86_64 code we could assume that the stack is 16-byte aligned (at least for standard library function calls which I would think includes main).

推荐答案

根据ABI，堆栈指针+ 8在进入函数时应保持16字节对齐.您必须减去8的原因是call本身在堆栈上放置了8个字节的返回地址，从而违反了此约束.基本上，您必须确保堆栈指针的总移动量是16的倍数，包括返回地址.因此，堆栈指针需要移动16 + 8的倍数，以便为返回地址留出空间.

According to the ABI, the stack pointer + 8 should be kept 16 byte aligned upon entry to functions. The reason you have to subtract 8 is that call itself places 8 bytes of return address on the stack, thereby violating this constraint. Basically you have to make sure the total stack pointer movement is a multiple of 16, including the return address. Thus the stack pointer needs to be moved by multiple of 16 + 8 to leave room for the return address.

至于_start，我认为您也可以依靠它而无需手动对齐.碰巧的是，由于堆栈中已经有东西，因此在您的情况下它可以正常工作.

As for _start, I don't think you can rely on it working without manual alignment either. It just so happens that in your case it works due to the things already on the stack.

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