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

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Date:	Fri, 24 Oct 2014 17:00:38 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Eric Paris <eparis@...hat.com>,
	Richard Guy Briggs <rgb@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-audit@...hat.com
Subject: Re: [PATCH] i386/audit: stop scribbling on the stack frame

On Fri, Oct 24, 2014 at 1:19 PM, H. Peter Anvin <hpa@...or.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
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