x86 Linux汇编程序从_start获取程序参数

./Program "text"

[esp+4]

在Linux上,来自C的熟悉的argc和argv变量总是由内核传递到堆栈中,甚至可用于完全独立的汇编程序,也不与C库中的启动代码链接.这在 i386 System V ABI中有记录,以及进程启动环境的其他详细信息(寄存器值,堆栈对齐).

在x86 Linux可执行文件的ELF入口点(a.k.a._start)：

> ESP指向argc
> ESP 4指向argv [0],数组的开始.即你应该传递给main的值为char ** argv是lea eax,[esp 4],而不是mov eax,[esp 4])

最小装配程序如何获得argc和argv

我将展示如何在GDB中读取argv和argc [0].

CMDLINE-x86.S

#include <sys/syscall.h>

    .global _start
_start:
    /* Cause a breakpoint trap */
    int $0x03

    /* exit_group(0) */
    mov $SYS_exit_group,%eax
    mov $0,%ebx
    int $0x80

CMDLINE-x86.gdb

set confirm off
file cmdline-x86
run
# We'll regain control here after the breakpoint trap
printf "argc: %dn",*(int*)$esp
printf "argv[0]: %sn",((char**)($esp + 4))[0]
quit

示例会话

$cc -nostdlib -g3 -m32 cmdline-x86.S -o cmdline-x86
$gdb -q -x cmdline-x86.gdb cmdline-x86
<...>  
Program received signal SIGTRAP,Trace/breakpoint trap.
_start () at cmdline-x86.S:8
8   mov $SYS_exit_group,%eax
argc: 1
argv[0]: /home/scottt/Dropbox/stackoverflow/cmdline-x86

说明

>我放置了一个软件断点(int $0x03),使程序在ELF入口点(_start)之后立即陷入调试器.
>然后我在GDB脚本中使用printf进行打印

>带有表达式*(int *)$esp的argc
>带表达式的argv((char **)($esp 4))[0]

x86-64版本

差异很小：

>用RSP替换ESP
>将地址大小从4更改为8
>当我们调用exit_group(0)来正确终止进程时,遵守不同的Linux系统调用约定

cmdline.S

#include <sys/syscall.h>

    .global _start
_start:
    /* Cause a breakpoint trap */
    int $0x03

    /* exit_group(0) */
    mov $SYS_exit_group,%rax
    mov $0,%rdi
    syscall

cmdline.gdb

set confirm off
file cmdline
run
printf "argc: %dn",*(int*)$rsp
printf "argv[0]: %sn",((char**)($rsp + 8))[0]
quit

常规C程序如何获得argc和argv

您可以从常规C程序中反汇编_start,以查看它是如何从堆栈中获取argc和argv并在调用__libc_start_main时传递它们的.以我的x86-64机器上的/ bin / true程序为例：

$gdb -q /bin/true
Reading symbols from /usr/bin/true...Reading symbols from /usr/lib/debug/usr/bin/true.debug...done.
done.
(gdb) disassemble _start
Dump of assembler code for function _start:
   0x0000000000401580 <+0>: xor    %ebp,%ebp
   0x0000000000401582 <+2>: mov    %rdx,%r9
   0x0000000000401585 <+5>: pop    %rsi
   0x0000000000401586 <+6>: mov    %rsp,%rdx
   0x0000000000401589 <+9>: and    $0xfffffffffffffff0,%rsp
   0x000000000040158d <+13>:    push   %rax
   0x000000000040158e <+14>:    push   %rsp
   0x000000000040158f <+15>:    mov    $0x404040,%r8
   0x0000000000401596 <+22>:    mov    $0x403fb0,%rcx
   0x000000000040159d <+29>:    mov    $0x4014c0,%rdi
   0x00000000004015a4 <+36>:    callq  0x401310 <__libc_start_main@plt>
   0x00000000004015a9 <+41>:    hlt    
   0x00000000004015aa <+42>:    xchg   %ax,%ax
   0x00000000004015ac <+44>:    nopl   0x0(%rax)

__libc_start_main()的前三个参数是：

> RDI：指向main()的指针
> RSI：argc,你可以看到它是如何从堆栈弹出的第一件事
> RDX：argv,弹出argc后RSP的值. (GLIBC源代码中的ubp_av)

x86 _start非常相似：

Dump of assembler code for function _start:
   0x0804842c <+0>: xor    %ebp,%ebp
   0x0804842e <+2>: pop    %esi
   0x0804842f <+3>: mov    %esp,%ecx
   0x08048431 <+5>: and    $0xfffffff0,%esp
   0x08048434 <+8>: push   %eax
   0x08048435 <+9>: push   %esp
   0x08048436 <+10>:    push   %edx
   0x08048437 <+11>:    push   $0x80485e0
   0x0804843c <+16>:    push   $0x8048570
   0x08048441 <+21>:    push   %ecx
   0x08048442 <+22>:    push   %esi
   0x08048443 <+23>:    push   $0x80483d0
   0x08048448 <+28>:    call   0x80483b0 <__libc_start_main@plt>
   0x0804844d <+33>:    hlt    
   0x0804844e <+34>:    xchg   %ax,%ax
End of assembler dump.