从程序集（x86/64）调用读取系统调用会产生分段错误（编译器构造）

Question

我正在为类 C 语言构建编译器，并且我正在尝试将在程序集中实现的基本“void readString(int, char*)”函数与我的编译器生成的程序集链接起来。

编译后的类c文件是

void main () {
    char t[20];     
    readString(7,t); // Read 7 bytes and place them in t buffer
}

编译后，生成的文件是：out.s文件（注意调用约定是：通过堆栈传递参数。在这种语言中，整数也有2字节大小）：

.$0:
.globl main
main:
pushq %rbp
movq %rsp,%rbp
subq $20,%rsp

.$1:
movw $7,%ax  # Push first argument in the stack
pushw %ax

.$2:
leaq -20(%rbp),%rax # Push address of the second arg in the stack
pushq %rax

.$3:
subq $8,%rsp   # this is not important, needed for the convention being followed
pushq 16(%rbp) # pushing "access link",
call _readString 
addq $26,%rsp # caller clears the "leftovers" 

.$4:
.$main_0_11:
movq %rbp,%rsp
popq %rbp
ret

库函数的 reads.asm 中的代码：

.intel_syntax noprefix
            .global _readString

_readString     push    rbp
            mov     rbp, rsp
            push    rdi
            push    rsi

            mov     rdi, [rbp+32]           # First argument 
            mov     rsi, [rbp+34]           # Second Argument

            mov     rdx, rdi

doRead:
            mov     byte ptr [rsi], 0x00
            xor     rax, rax
            mov     rdi, rax
            syscall                         # read syscall, reads up to $rdx bytes
            or      rax, rax                # nothing read
            jz      finish                  #
            add     rsi, rax                #
            cmp     byte ptr [rsi-1], 0x0a  # check if last character read was '\n'
            jne     addZero                 #
            sub     rsi, 1                  # if so, replace with '\0'
addZero:
            mov     byte ptr [rsi], 0x00
finish:
            pop     rsi
            pop     rdi
            pop     rbp
            ret

链接/运行如下

$ gcc -c out.s
$ gcc -c reads.s 
$ gcc out.o reads.o 
$ ./a.out
[2] Segmentation fault  ./a.out

描述调用约定的图片

Answer 1

好的，正如@David Hoelzer 建议的那样，我再次检查了“readString”从堆栈中获取参数的方式，并且它们按顺序颠倒了！

所以 reads.asm 的第一行会变成

_readString:
            push    rbp
            mov     rbp, rsp
            push    rdi
            push    rsi

            movzx   edi, word ptr [rbp+40] # Get first argument, check comments for more info
            mov     rsi, [rbp+32]  # Second Argument


            mov     rdx, rdi