Asm x64 segfault /简单程序

Question

我正在用x64 asm编写一些小程序，但我在完成这个程序时遇到了麻烦。 我在scasb指令上遇到了段错误。我猜这个问题来自strLen块中的mov eax, my_str ; address of buffer passed to ... ?行。我试过不同的寄存器，当然eax不是正确的（32位）使用。我在FreeBSD上，因此是系统调用。 有什么想法吗？

以下是代码：

 ; 
 ; StrLen.asm : Read a string on stdin and return its length using a strLen replacement
 ; 

 section .data 
 userMsg : 
 db "Please enter a string (max 1024 bytes): ", 0 
 lenUserMsg equ $-userMsg 

 dispMsg : 
 db "Length: ", 0 
 lenDispMsg equ $-dispMsg 

 section .bss ;Uninitialized data 
 my_strLength equ 1024 ; Input buffer size 
 my_str : 
 resb my_strLength ; Input buffer 


 section .text 
 global _start 

 _start: 

 ;;; User Prompt ;;; 
 mov rax, 4 ; write 
 mov rdi, 1 ; stdout 
 mov rsi, userMsg ; msg text 
 mov rdx, lenUserMsg ; msg length 
 syscall 

 ;;; Read and store the string ;;; 
 mov rax, 3            ; read 
 mov rdi, 0            ; stdin 
 mov rsi, my_str       ; address of my_str passed to rsi 
 mov edx, my_strLength ; read n bytes in one go 
 syscall 
 ;mov r9, rax          ; keep sys_read return value (nb of bytes read) 

 ;;; strLen() ;;; 
 mov eax, my_str       ; address of buffer passed to ... ? 
 sub al, al            ; zeroes al 
 not rcx               ; 1/ecx = ecx highest value 
 cld                   ; clear direction flag for byte search 
 repne scasb           ; repeat byte search until match 
 not rcx               ; ecx contains 1/length +1 
 dec rcx               ; -1 & ok we have the length 

 ;;; Write back the length ;;; 
 mov rax, rdi          ; write 
 mov rdi, 1            ; stdout 
 ; mov rcx, rcx        ; nb bytes read 
 mov rdx, my_strLength 
 syscall 

 ;;; Exit ;;; 
 mov rax, 1            ; exit 
 xor rdi, rdi          ; return value 
 syscall