汇编语言修改输入字符串错误

Question

我正在尝试在 Linux 上编写一个 32 位 x86 NASM 程序，它从文本文件中获取一个字符串，编码将字符串中的所有字符向上移动 1 和 printf 它输出到控制台。除了用于进行移位的循环 .rot132 和 printf 之外，几乎所有的东西都能正常工作。每当调用 .rot132 时，它都会给出一个 segmentation_fault（核心转储）。我想当我执行 mov ecx, [eax] 时它会导致某种错误。我怎样才能克服这部分？

;;sddddddddddddddddgsd

[SECTION .data] ; Section containing initialized data

WriteCode db "w",0
OpenCode db "r",0
Filename db "text.txt",0
fileFmt: dd "%c\n",10,0
fileFmt1: dd "%s",10,0

[SECTION .bss] ; Section containing uninitialized data
TextLenght EQU 72 ; Define length of a line of  text data
Text resb TextLenght ; Reserve space for disk-based help text line
BUFSIZE EQU 64 ; Define length of text line buffer 
Buff resb BUFSIZE+5 ; Reserve space for a line of text

[SECTION .text] ; Section containing code
;; These externals are all from the glibc standard C library:
extern fopen
extern fclose
extern fgets
extern fprintf
extern printf
extern sscanf
extern time

global main ; Required so linker can find entry point

main:

diskhelp:
        mov  ebx, Filename ; push file name to ebx
        push OpenCode ; Push pointer to open-for-read code “r“
        push ebx ; Pointer to name of help file is passed in ebx
        call fopen ; Attempt to open the file for reading
        add esp,8 ; Clean up the stack
        cmp eax,0 ; fopen returns null if attempted open failed
        jne .disk ; Read help info from disk file...
        ret

.disk:  mov ebx,eax ; Save handle of opened file in ebx

.rdln:  push ebx ; Push file handle on the stack
        push dword TextLenght ; Limit line length of text read
        push Text ; Push address of text line buffer
        call fgets ; Read a line of text from the file
        add esp,12 ; Clean up the stack
        ;cmp eax,0 ; A returned null indicates error or EOF
        ;jle .rot13 ; If we get 0 in eax, close up & return

        ;push Text ; Push address of help line on the stack
        ;call printf ; Call printf to display help line
        ;add esp,4 ; Clean up the stack

.rot131: ; initial shift and test, this work
        mov edx, 0    ; our counter
        mov eax, Text ; move string into eax
        mov ecx, [eax]; move first char in string into ecx
        add ecx, 1    ; shift the char up by 1 
        push ecx      ; push to print
        push fileFmt
        call printf
        add esp, 8   ; clear the stack
        inc edx      ; increase the counter

.rot132: 
        inc eax     ; shift address of eax into next char
        mov ecx, [eax] ; move the char into ecx, replace old char; error ??
        add ecx, 1     ; shift the char by 1
        push ecx        ; print
        push fileFmt
        call printf
        add esp, 8      ; clear the stack
        inc edx         ;incrase counter
        cmp edx,4       ; stop loop after edx = 4 
        jne .rot132


        push ebx ; Push the handle of the file to be closed
        call fclose ; Closes the file whose handle is on the stack
        add esp,4 ; Clean up the stack

        ret ; Go home

gdb 调试：反汇编 .rot132:

   0x08048559 <+0>:     inc    %eax
   0x0804855a <+1>:     mov    (%eax),%ecx
   0x0804855c <+3>:     add    [=11=]x1,%ecx
   0x0804855f <+6>:     push   %ecx
   0x08048560 <+7>:     push   [=11=]x804a035
   0x08048565 <+12>:    call   0x80483b0 <printf@plt>
   0x0804856a <+17>:    add    [=11=]x8,%esp
   0x0804856d <+20>:    inc    %edx
   0x0804856e <+21>:    cmp    [=11=]x4,%edx
   0x08048571 <+24>:    jne    0x8048559 <rot132>
   0x08048573 <+26>:    push   %ebx
   0x08048574 <+27>:    call   0x80483d0 <fclose@plt>
   0x08048579 <+32>:    add    [=11=]x4,%esp
   0x0804857c <+35>:    ret
   0x0804857d <+36>:    xchg   %ax,%ax
   0x0804857f <+38>:    nop

Answer 1

fileFmt 需要 db，而不是 dd。您正在定义双字字符。

请注意，此代码不如编译器生成的代码好。只需为配置文件顶部明显 CPU 瓶颈的关键循环使用汇编语言，并且您已经在使用一个很好的算法。

一个例子是矩阵乘法。它是 embarassingly parallel，因此强制编译器使用最有效的方法来执行操作通常是有益的，例如使用矢量扩展，如 SSE。

编译器这么好，你通常写 "assembly language" 内在函数，比如 SSE 或 AVX 内在函数，这让编译器不用担心数据流和寄存器分配，让你专注于指令本身.

另请注意 Michael Petch 的好评：允许调用破坏 eax、ecx、edx。您可以相信他们会保留 ebx、esi、edi、ebp、esp。这意味着您需要停止对变量使用 ecx 和 edx 并使用保留的内容，例如 ebx、edi、esi、ebp.

汇编语言修改输入字符串错误

assembly language modify input string error

string

x86

assembly

nasm