linux-kernel - Re: [PATCH] x86: introduce push/pop macros which generate CFI_REL_OFFSET and CFI

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Date:	Mon, 12 Jan 2015 12:26:38 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	X86 ML <x86@...nel.org>, Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86: introduce push/pop macros which generate
 CFI_REL_OFFSET and CFI_RESTORE

On Mon, Jan 12, 2015 at 12:22 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 01/12/2015 12:14 PM, Andy Lutomirski wrote:
>> On Mon, Jan 12, 2015 at 12:11 PM, Borislav Petkov <bp@...en8.de> wrote:
>>> On Mon, Jan 12, 2015 at 11:46:53AM -0800, Andy Lutomirski wrote:
>>>> Dumb example:
>>>>
>>>>     pushq_cfi $__KERNEL_DS /* ss */
>>>>
>>>> This doesn't save anything that the unwinder would care about.
>>>
>>> And? The unwinder or whatever looks at that info simply ignores stuff it
>>> is not interested in, no?
>>
>> But CFI_REL_OFFSET $__KERNEL_DS, 0 probably isn't even well-formed and
>> won't build.
>>
>
> I think this is relatively easy to deal with at the expense of a large
> .ifeq statement in the macro.
>
>

Is the usage you have in mind something like:

pushq_cfi %r11 /* saves r11 */
pushq_cfi $0 /* saves nothing */
pushq_cfi %r11, savereg=rip /* saves rip */

I think I prefer the somewhat less magical:

pushq_reg_cfi %r11 /* saves r11 */
pushq_reg_cfi %r11, savereg=rip /* saves rip */
pushq_nounwind_cfi $0

Or something like that.

--Andy

         -hpa
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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