问题描述
我找不到在 MacOS 上编写 64 位程序集的好文档.
I'm having trouble finding the good documentation for writing 64-bit assembly on MacOS.
64 位 SysV ABI 在 A 部分中说明了以下内容.2.1 和 这篇SO帖子引用了它:
The 64-bit SysV ABI says the following in section A.2.1 and this SO post quotes it:
系统调用是通过 syscall 指令完成的.内核破坏注册 %rcx 和 %r11.
从系统调用返回,寄存器 %rax 包含系统调用.-4095 和 -1 之间的值表示错误,它是 -errno.
Returning from the syscall, register %rax contains the result of the system-call. A value in the range between -4095 and -1 indicates an error, it is -errno.
这两句话在 Linux 上没问题,但在 macOS Sierra 上是错误的,代码如下:
Those two sentences are ok on Linux but are wrong on macOS Sierra with the following code:
global _start
extern _exit
section .text
_start:
; Align stack to 16 bytes for libc
and rsp, 0xFFFFFFFFFFFFFFF0
; Call write
mov rdx, 12 ; size
mov rsi, hello ; buf
mov edi, 1 ; fd
mov rax, 0x2000004 ; write ; replace to mov rax, 0x1 on linux
syscall
jc .err ; Jumps on error on macOS, but why?
jnc .ok
.err:
mov rdi, -1
call _exit ; exit(-1)
.ok:
; Expect rdx to be 12, but it isn't on macOS!
mov rdi, rdx
call _exit ; exit(rdx)
; String for write
section .data
hello:
.str db `Hello world
`
.len equ $-hello.str
使用 NASM 编译:
Compile with NASM:
; MacOS: nasm -f macho64 syscall.asm && ld syscall.o -lc -macosx_version_min 10.12 -e _start -o syscall
; Linux: nasm -f elf64 syscall.asm -o syscall.o && ld syscall.o -lc -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o syscall
在 macOS 上运行:
Run on macOS:
./syscall # Return value 0
./syscall >&- # Return value 255 (-1)
我发现:
- 系统调用返回
errno
并在出错时设置进位标志,而不是在rax
中返回 rdx
寄存器被syscall
破坏- 在 Linux 上,一切正常
-errno
- A syscall return
errno
an sets the carry flag on error, instead of returning-errno
inrax
rdx
register is clobbered bysyscall
- On Linux, everything works as expected
为什么 rdx
被破坏了?为什么系统调用不返回 -errno
?在哪里可以找到真正的文档?
Why is rdx
clobbered? Why doesn't a syscall return -errno
? Where can I find the real documentation?
我发现有人谈论系统调用错误的进位标志的唯一地方是 这里
The only place I found where someone talks about the carry flag for syscall errors is here
推荐答案
我用过这个:
# as hello.asm -o hello.o
# ld hello.o -macosx_version_min 10.13 -e _main -o hello -lSystem
.section __DATA,__data
str:
.asciz "Hello world!
"
.section __TEXT,__text
.globl _main
_main:
movl $0x2000004, %eax # preparing system call 4
movl $1, %edi # STDOUT file descriptor is 1
movq str@GOTPCREL(%rip), %rsi # The value to print
movq $13, %rdx # the size of the value to print
syscall
movl %eax, %edi
movl $0x2000001, %eax # exit (return value of the call to write())
syscall
并且能够将返回值捕获到 eax
中.这里的返回值是 write
系统调用实际写入的字节数.是的,MacOS 是 BSD 变体,它是进位标志,告诉您系统调用是否错误(errno 只是一个外部链接变量).
and was able to catch return value into eax
. Here return value is the number of bytes actually written by write
system call. And yes MacOS being a BSD variant it is the carry flag that tells you if the syscall was wrong or not (errno is just an external linkage variable).
# hello_asm.s
# as hello_asm.s -o hello_asm.o
# ld hello_asm.o -e _main -o hello_asm
.section __DATA,__data
str:
.asciz "Hello world!
"
good:
.asciz "OK
"
.section __TEXT,__text
.globl _main
_main:
movl $0x2000004, %eax # preparing system call 4
movl $5, %edi # STDOUT file descriptor is 5
movq str@GOTPCREL(%rip), %rsi # The value to print
movq $13, %rdx # the size of the value to print
syscall
jc err
movl $0x2000004, %eax # preparing system call 4
movl $1, %edi # STDOUT file descriptor is 1
movq good@GOTPCREL(%rip), %rsi # The value to print
movq $3, %rdx # the size of the value to print
syscall
movl $0, %edi
movl $0x2000001, %eax # exit 0
syscall
err:
movl $1, %edi
movl $0x2000001, %eax # exit 1
syscall
这会以错误代码 1 退出,因为使用了描述符 5,如果您尝试使用描述符 1,那么它将打印另一条消息并以 0 退出.
This will exits with error code one because descriptor 5 was used, if you try descriptor 1 then it will work printing another message and exiting with 0.
这篇关于MacOS 程序集的 64 位系统调用文档的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!