我编写了一个x86（IA-32）汇编程序，该程序应该从标准输入读取字符串，但无法理解为什么会导致SEGFAULT。
我用GNU汇编器汇编了这个程序，使用了以下标志：

$ gcc (flags used) (file_name)

下面是该程序的代码：

.text

.globl _start

MAX_CHAR=30

_start:

    ## Start message ##
    movl $4, %eax
    movl $1, %ebx
    movl $msg, %ecx
    movl $len, %edx
    int $0x80

    ## READ ##
    movl $3, %eax       #sys_read (number 3)
    movl $0, %ebx       #stdin (number 0)
    movl %esp, %ecx     #starting point
    movl $MAX_CHAR, %edx    #max input
    int $0x80       #call

    ## Need the cycle to count input length ##  
    movl $1, %ecx       #counter
end_input:
    xor %ebx, %ebx
    mov (%esp), %ebx
    add $1, %esp        #get next char to compare 
    add $1, %ecx        #counter+=1
    cmp $0xa, %ebx      #compare with "\n" 
    jne end_input       #if not, continue 

    ## WRITE ##
    sub %ecx, %esp      #start from the first input char
    movl $4, %eax       #sys_write (number 4)
    movl $1, %ebx       #stdout (number 1)
    movl %ecx, %edx     #start pointer
    movl %esp, %ecx     #length
    int $0x80       #call
     

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

.data

msg: .ascii "Insert an input:\n"
len =.-msg

什么原因导致SEGFAULT？
任何帮助都是受欢迎的。