为什么除了被orig_eax提供EAX？ [英] Why is orig_eax provided in addition to eax?

问题描述

为什么是包含在 SYS / user.h 的结构的 orig_eax 成员user_regs_struct ？

Why is the orig_eax member included in sys/user.h's struct user_regs_struct?

推荐答案

由于它是在结构pt_regs ，这是.... <一个href=\"http://tomoyo.sourceforge.jp/cgi-bin/lxr/source/arch/x86/include/asm/user_32.h#L77\">http://tomoyo.sourceforge.jp/cgi-bin/lxr/source/arch/x86/include/asm/user_32.h#L77

Because it was in struct pt_regs, which is .... http://tomoyo.sourceforge.jp/cgi-bin/lxr/source/arch/x86/include/asm/user_32.h#L77

 73  * is still the layout used by user mode (the new
 74  * pt_regs doesn't have all registers as the kernel
 75  * doesn't use the extra segment registers)

所以，很多的用户空间的实用程序期待一个 orig_eax 字段在这里，所以它被包含在 user_regs_struct 太（要与旧调试器和 ptrace的 RS兼容）

So, a lot of user-space utilities expect an orig_eax field here, so it is included in user_regs_struct too (to be compatible with older debuggers and ptracers)

接下来的问题是：为什么是包含在结构pt_regs 的 orig_eax 会员？。

Next question is "Why is the orig_eax member included in struct pt_regs?".

这是在Linux中0.95 <一加href=\"http://lxr.linux.no/#linux-old+v0.95/include/sys/ptrace.h#L44\">http://lxr.linux.no/#linux-old+v0.95/include/sys/ptrace.h#L44.
我认为这是一些其他UNIX之后完成与 pt_regs 结构。在0.95评论说：

It was added in linux 0.95 http://lxr.linux.no/#linux-old+v0.95/include/sys/ptrace.h#L44. I suggest this was done after some other unix with pt_regs struct. Comment in 0.95 says

  29 * this struct defines the way the registers are stored on the 
  30 * stack during a system call.

因此​​， orig_eax 的地方是由系统调用接口中定义。这是<一个href=\"http://lxr.linux.no/#linux-old+v0.95/kernel/sys_call.s\">http://lxr.linux.no/#linux-old+v0.95/kernel/sys_call.s

So, the place of orig_eax is defined by syscall interface. Here it is http://lxr.linux.no/#linux-old+v0.95/kernel/sys_call.s

  17 * Stack layout in 'ret_from_system_call':
  18 *      ptrace needs to have all regs on the stack.
  19 *      if the order here is changed, it needs to be 
  20 *      updated in fork.c:copy_process, signal.c:do_signal,
  21 *      ptrace.c ptrace.h
  22 *
  23 *       0(%esp) - %ebx
 ...
  29 *      18(%esp) - %eax
 ...
  34 *      2C(%esp) - orig_eax

为什么我们需要保存旧的 EAX 两次？因为 EAX 将用于系统调用的返回值（同一个文件，略低于）：

Why do we need to save old eax twice? Because eax will be used for the return value of syscall (same file, a bit below):

  96_system_call:
  97        cld
  98        pushl %eax              # save orig_eax
  99        push %gs
...
 102        push %ds
 103        pushl %eax              # save eax.  The return value will be put here.
 104        pushl %ebp
...
 117        call _sys_call_table(,%eax,4)

ptrace的需要能够同时读取系统调用之前，所有的寄存器状态和系统调用的返回值;但返回值写入％EAX 。然后，原始的 EAX ，使用系统调用将丢失之前。要保存它，有一个 orig_eax 字段。

Ptrace needs to be able to read both all registers state before syscall and the return value of syscall; but the return value is written to %eax. Then original eax, used before syscall will be lost. To save it, there is a orig_eax field.

更新：感谢于R ..和伟大的LXR，我没有在linux 0.95全搜索 orig_eax 的

UPDATE: Thanks to R.. and great LXR, I did a full search of orig_eax in linux 0.95.

它不仅用于ptrace的，而且在 do_signal 重新启动系统调用时（如果有一个系统调用，以结束 ERESTARTSYS ）

It is used not only in ptrace, but also in do_signal when restarting a syscall (if there is a syscall, ended with ERESTARTSYS)

 158                        *(&eax) = orig_eax;

UPDATE2：莱纳斯说一些有趣的事情吧：

这一点很重要ORIG_EAX被设置为某个值是的不的一
  有效的系统呼叫号码，因此，系统调用启动逻辑（见
  信号处理code）不会触发。

It's important that ORIG_EAX be set to some value that is not a valid system call number, so that the system call restart logic (see the signal handling code) doesn't trigger.

UPDATE3： ptrace的研究应用（调试器）可以改变 orig_eax 来改变系统调用号被称为：< A HREF =htt​​p://lkml.org/lkml/1999/10/30/82> http://lkml.org/lkml/1999/10/30/82 （内核的某些版本，则是在EIO ptrace的改变一个ORIG_EAX）

UPDATE3: ptracer app (debugger) can change orig_eax to change system call number to be called: http://lkml.org/lkml/1999/10/30/82 (in some versions of kernel, is was EIO to change in ptrace an ORIG_EAX)

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