FreeBSD/amd64汇编-如何阅读"ARGC"？

Question

有时返回正确的值，有时返回0，有时(看似)随机数……都来自同一个可执行文件。

.section .text
.global _start
_start:
    movq    $1, %rax
    popq    %rdi
    syscall

例如：

%as -o this.o this.s ; ld -o this this.o

%./this; echo $?
1

%./this 1; echo $?
0

%./this 1 2; echo $?
3

%./this 1 2 a; echo $?
4

%./this 1 2 a f; echo $?
0

%_

我还是个汇编语言新手，但我很有信心，获取参数计数就像从堆栈中弹出一样简单。是我做错了什么，还是真的搞砸了？

Answer 1

我在FreeBSD 9.0/amd64上也遇到了同样的问题。我所做的是(我使用nasm进行汇编)：

$ cat foo.asm
global _start
_start:
        mov     rax, 4          ; write
        mov     rdi, 1          ; stdout
        mov     rsi, rsp        ; address
        mov     rdx, 16         ; 16bytes
        syscall

        mov     rax, 1          ; exit
        syscall
$ nasm -f elf64 foo.asm && ld -o foo foo.o
$ ./foo | hd
00000000  00 00 00 00 00 00 00 00  01 00 00 00 00 00 00 00  |................|
00000010
$ ./foo 2 | hd
00000000  02 00 00 00 00 00 00 00  b8 dc ff ff ff 7f 00 00  |................|
00000010
$ ./foo 2 3 | hd
00000000  00 00 00 00 00 00 00 00  03 00 00 00 00 00 00 00  |................|
00000010
$ ./foo 2 3 4 | hd
00000000  00 00 00 00 00 00 00 00  04 00 00 00 00 00 00 00  |................|
00000010
$ ./foo 2 3 4 5 | hd
00000000  05 00 00 00 00 00 00 00  b0 dc ff ff ff 7f 00 00  |................|
00000010

我本以为argc在rsp，但事实并非如此。

我猜是内核(映像激活器)设置了寄存器。我搜索了源代码树，在/usr/src/sys/amd64/amd64/machdep.c (exec_setregs)中找到了以下代码。

        regs->tf_rsp = ((stack - 8) & ~0xFul) + 8;
        regs->tf_rdi = stack;           /* argv */

这些行看起来表明rsp是对齐的，实际数据在rdi。我修改了代码，得到了预期的结果。

$ cat foo.asm
global _start
_start:
        push    rdi
        mov     rax, 4          ; write
        mov     rdi, 1          ; stdout
        pop     rsi
        mov     rdx, 16         ; 16bytes
        syscall

        mov     rax, 1          ; exit
        syscall
$ nasm -f elf64 foo.asm && ld -o foo foo.o
$ ./foo | hd
00000000  01 00 00 00 00 00 00 00  b0 dc ff ff ff 7f 00 00  |................|
00000010
$ ./foo 2 | hd
00000000  02 00 00 00 00 00 00 00  a8 dc ff ff ff 7f 00 00  |................|
00000010
$ ./foo 2 3 | hd
00000000  03 00 00 00 00 00 00 00  a8 dc ff ff ff 7f 00 00  |................|
00000010
$ ./foo 2 3 4 | hd
00000000  04 00 00 00 00 00 00 00  a8 dc ff ff ff 7f 00 00  |................|
00000010
$ ./foo 2 3 4 5 | hd
00000000  05 00 00 00 00 00 00 00  a8 dc ff ff ff 7f 00 00  |................|
00000010

你能试试rdi吗？

Answer 2

对于FreeBSD的标准调用约定，您需要：

movl %edi, %eax

返回状态等于传递的参数数量的最短完整程序为：

movl    %edi, %eax
ret