Question

我正在尝试编写为字符串分配一些内存并用随机字符填充该内存的代码。现在我决定采用mmap-syscalls的方式，而不使用malloc函数。

这是我的代码：

.code32
.data
    random: .ascii "/dev/random\0"

.section .data
    c: .ascii "t"   #just a test static character, for test
    n: .ascii "\n"  #end of string
.text
    .global _start
    _start:


pushl $0    # offset of 0
pushl $-1 # the file handle of the open file
pushl $33     # MAP_SHARED flag set to write changed data back to file
pushl $3     # PROT_READ and PROT_WRITE permissions
pushl $42   
pushl $0     # Allow the system to select the location in memory to start
movl %esp,%ebx   # copy the parameters location to EBX
movl $90,%eax    # set the system call value
int $0x80
movl %eax,%edi  #the adress of allocated memory is stored in edi
pushl %eax  #save the adress on stack 
movl $42, %ecx  #now i want to generate 42 random symbols and print them
loop:
    dec %ecx        
    pushl %ecx                              #generate a random number, using the kernel Entropy Collector
    movl $5,%eax                            # sys_open
    movl $random,%ebx                       # Filename string

    movl $0,%ecx      # O_RDONLY flag
    int $0x80
                                    # Read one random number
    movl %eax,%ebx     # The result of sys_open
    movl $3,%eax       # sys_read
    movl (%esp),%ecx     # The stack is our buffer
    movl $1,%edx       # 
    int $0x80       #random number on stack
    popl %eax       #eax represents the random number
    movl $100,%ebx
    divl %ebx       #modulo 223
    leal 34(%edx),%eax
    stosb           #load a random char into allocated memory           
    popl %ecx
    cmpl $0,%ecx
    jne loop

movl $4,%eax        #syscall write
movl $1,%ebx
movl $43,%edx
popl %ecx       #pop the saved adress of string
int  $0x80

movl $1,%eax        #exit
int $0x80

`

导致MOVSB - 操作中的段错误。

所以我不确定几件事情：

我只是在ESI之前将33到255之间的整数推送到MOVSB的方式，希望整数被识别为ascii符号，它就是这个符号。
我分配内存的方式
也不太确定LEAL - 操作的正确性，它的目的是在每次跳转到一个新字节时移动分配的（42 ??）字节的内存。虽然这个错误导致错误导致MOVSB。

汇编程序中的mmap和字符串操作

0 个答案: