大會 - Segfault與子程序

Question

我想調試爲什麼我在我的子程序中得到一個段錯誤。

它發生在子程序末尾的ret線上 - 就像在句子末尾0x00字節一樣。

主營：

   .data 
string:   .string "aaaaaaaaaaa" 
endofstring: .space 8 
msg:   .string "%c occurs %d times \n" 

       .text 
       .global main 

main: 

    mov  $string,%rsi   #rsi = string storage 
    mov  $0x61, %ah    #storage of a 
    mov  $0x65, %al    #storage of e 
    mov  $0x69, %bh    #storage of i 
    mov  $0x6F, %bl    #storage of o 
    mov  $0x75, %ch    #storage of u 


#Case A 
    mov  %ah,%cl     #1 byte register for cmp later on. 
    mov  $0, %rax    #initialize count to 0 
    call FREQ     #Generate %rax value for 


    mov  %rax, %rdx    #count for printf (2nd argument) 
    mov  $msg, %rdi    #format for printf(1st argument) 
    mov  %r8, %rsi    #ch for printf (3rd argument) 

    xor  %rax, %rax    #reset %rax for printf output 

    call printf     #print the frequency value of the ch in string 


#Case E 
    mov  %al,%cl 
    mov  $0, %rax    #initialize count to 0 
    call FREQ 

    mov  %rax, %rdx    #count for printf (2nd argument) 
    mov  $msg, %rdi    #format for printf(1st argument) 
    mov  %r8, %rsi    #ch for printf (3rd argument) 

    xor  %rax, %rax    #reset %rax for printf output 

    call printf     #print the frequency value of the ch in string 

#Case O 
    mov  %bh,%cl 
    mov  $0, %rax    #initialize count to 0 
    call FREQ 

    mov  %rax, %rdx    #count for printf (2nd argument) 
    mov  $msg, %rdi    #format for printf(1st argument) 
    mov  %r8, %rsi    #ch for printf (3rd argument) 

    xor  %rax, %rax    #reset %rax for printf output 

    call printf     #print the frequency value of the ch in string 

#Case I 
    mov  %bl,%cl 
    mov  $0, %rax    #initialize count to 0 
    call FREQ 

    mov  %rax, %rdx    #count for printf (2nd argument) 
    mov  $msg, %rdi    #format for printf(1st argument) 
    mov  %r8, %rsi    #ch for printf (3rd argument) 

    xor  %rax, %rax    #reset %rax for printf output 

    call printf     #print the frequency value of the ch in string 
#Case U 
    mov  %ch,%cl 
    mov  $0, %rax    #initialize count to 0 
    call FREQ 

    mov  %rax, %rdx    #count for printf (2nd argument) 
    mov  $msg, %rdi    #format for printf(1st argument) 
    mov  %r8, %rsi    #ch for printf (3rd argument) 

    xor  %rax, %rax    #reset %rax for printf output 

    call printf     #print the frequency value of the ch in string 

    jmp done 


done: 

    ret 

子程序：

.text 
    .globl FREQ 



FREQ: 
    #subprogram body 
Start: 
    cmpb $0,8(%rsi)    #check for end of the string 
    je  donefreq 

loopfreq: 
    cmp  %cl, 8(%rsi)   #compare first string char with vowel 
    je  incrementstring   #if equal - jump to increment_string 
    add  $1, %rsi    #if not - increment string 
    jmp  Start     #jump to loop to check for end of string status/next char 

incrementstring: 
    add  $1, %rsi    #increment to next string character 
    add  $1, %rax    #add 1 to frequency of character 
    jmp  Start 

donefreq: 
    ret 

不知道爲什麼會這樣。 - 我希望調試給了一些更多的信息:(

有沒有人有一個想法，爲什麼會發生這種情況？我完全按照我的筆記的大綱爲被調用函數，所以我不知道問題在哪裏在被叫

Answer 1

你不應該使用%rsp爲指針，以你的字符串，你是在棧破壞返回地址，以便您ret指令試圖跳轉到一些假地址。使用%rsi或其他通用寄存器。該堆棧指針並不是一個你可以隨意使用的寄存器