在 GAS 中进行系统调用并使用 .data 部分中的变量并在另一个子程序中访问它们以进行系统调用 [英] Making a system call in GAS and using variables in .data section and accessing them for a system call inside another sub-routine

问题描述

这是我使用 GAS 语法为 64 位英特尔程序集编写的代码示例.当代码运行时，期望打印出字符串:在 _print 子例程中.

Here is the code example that I have written using GAS syntax for 64 bit intel assembly. When the code is run the expectation is to print out the string: Inside the _print subroutine.

#This example is a an example to call a subroutine 
.global _start

.section .text

_start:

    call _print

_exit:   
    #exit call
    mov $60, %rax
    xor %rdi, %rdi

    syscall

_print:
    #set up the stack frame
    push %rbp
    mov %rsp, %rbp

    # write syscall's parameter set up
    mov std_out_fd, %rdi
    mov $message, %rsi
    movq message_size, %rdx
    mov write_syscall_no, %rax

    syscall

    #Restore callers registers value
    #pop %rdx
    #pop %rsi
    #pop %rdi 

    #Destroy the stack frame:
    mov %rbp, %rsp
    pop %rbp

    ret


.section .data


std_out_fd: .int 0x02

message: .ascii "Inside the _print subroutine.\n"

message_size: .byte 30

write_syscall_no: .int 0x01

==========================================

=========================================

当我尝试使用声明的变量message_size"作为 write 系统调用的第三个参数时，我在屏幕上打印消息后打印了一些奇怪的额外字符:

When I try to use the declared variable 'message_size' as the third parameter for the write system call I get a few weird extra characters printed after the message is printed on the screen:

ali@alix2:~/Programming/asm/GAS-Syntax/SubRoutine$ as -o subroutine.o subroutine.s
ali@alix2:~/Programming/asm/GAS-Syntax/SubRoutine$ ld -o subroutine subroutine.o
ali@alix2:~/Programming/asm/GAS-Syntax/SubRoutine$ ./subroutine
Inside the _print subroutine.
`;`Qali@alix2:~/Programming/asm/GAS-Syntax/SubRoutine$

但是当我没有使用变量时，我将其更改为移动 $30，%rdx

but when instead of using the variable I change it to mov $30, %rdx

然后它完美地工作并且没有额外的字符(;Q)将被写入标准输出.

then it works perfectly and none of the extra characters(;Q) will be written to stdout anymore.

ali@alix2:~/Programming/asm/GAS-Syntax/SubRoutine$ ./subroutine
Inside the _print subroutine.
ali@alix2:~/Programming/asm/GAS-Syntax/SubRoutine$ 

谁能解释一下这背后的原因?谢谢.

Could anyone explain the reason behind this? Thanks.

推荐答案

movq message_size, %rdx 是 64 位 (qword) 加载，包括 .byte 30 和 .int 0x1，以及 3 个字节.使用调试器(例如 GDB)查看寄存器中的值.并使用 strace ./subroutine 来跟踪系统调用，并表明您正在向 write 传递一个巨大的长度.

movq message_size, %rdx is a 64-bit (qword) load that include the .byte 30, and the .int 0x1, and 3 bytes past that. Use a debugger (GDB for example) to look at values in registers. And use strace ./subroutine to trace system calls and show that you're passing a huge length to write.

您可以使用 movzbl message_size(%rip), %edx 加载将该字节零扩展到 RDX.

You could use a movzbl message_size(%rip), %edx load to zero-extend that byte into RDX.

或者更好的是，让汇编程序为您计算大小作为汇编时间常数
.equ message_size, .- message 以便您可以使用 $message_size 作为即时消息.无需单独硬编码大小或将其存储在数据存储器中.

Or better, let the assembler calculate sizes for you as assemble-time constants with
.equ message_size, . - message so you can use $message_size as an immediate. No need to hard-code the size separately or to store it in data memory.

电话号码相同；不要为它做 64 位加载，尤其不要从 32 位 .int 加载！如果在那之后 .data 中有任何其他内容，它会将垃圾加载到 RAX 的高字节中.你可以只是 mov 到 EAX，它隐式地零扩展到 RAX.

Same for the call number; don't do a 64-bit load for it, especially not from a 32-bit .int! If you had any other stuff in .data after that, it would load garbage into the high bytes of RAX. You could just mov into EAX which zero-extends into RAX implicitly.

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