Re: [PATCH] i386/audit: stop scribbling on the stack frame

Andy Lutomirski
2014-10-25T00:00:38Z

On Fri, Oct 24, 2014 at 1:19 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 10/23/2014 12:38 PM, Eric Paris wrote:
>>>
>>> After the call __audit_syscall_entry aren't they already polluted?
>>> Isn't that the reason we need to reload EAX?
>>
>> Well, I guess EAX is special...
>>
>
> Because system calls are "asmlinkage", all the parameters are on the
> stack, but %eax is used as the index into the system call table. This
> should thus be fine until we get rid of regparm(0) entirely, if that
> ever happens.
>

...and because __audit_syscall_entry *isn't* asmlinkage, it uses the
other convention, which is where the confusion comes from. And, by
the time you get to sysenter_do_call, nothing cares about ecx, so you
can freely clobber it while popping from the stack. I get it now.

--Andy

> -hpa
>
>

--
Andy Lutomirski
AMA Capital Management, LLC