linux-kernel - Re: [PATCH, RFC] x86: also CFI-annotate certain inline asm()s

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Date:	Wed, 05 Nov 2014 17:13:08 +0000
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<luto@...capital.net>, <mingo@...e.hu>, <tglx@...utronix.de>,
	<hpa@...or.com>
Cc:	<tonyj@...e.de>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH, RFC] x86: also CFI-annotate certain inline asm()s

>>> Andy Lutomirski <luto@...capital.net> 11/04/14 8:40 PM >>>
>On 11/04/2014 01:24 AM, Jan Beulich wrote:
>> The main obstacle to having done this long ago was the need to
>> determine whether annotations are needed in the first place: They need
>> to be avoided when a frame pointer got set up. Since I can't see a way
>> to determine this before the compilation phase, this is being achieved
>> by inspecting the memory address generated by the compiler in an
>> interposed assembler macro. Of course this isn't really nice code, and
>> this the main reason I'm posting this as RFC only at this point (with
>> the hope that maybe someone has an idea of how to achieve the same
>> thing in a more elegant way).
>
>Ask binutils for help?

Binutils know as little about the code the compiler generated as we do.

>Is the issue that the CFI annotation you need is different depending on
>whether there's a frame pointer or not?

No - as said above, they need to be avoided altogether when there's a
frame pointer.

> If so, can you add some
>comments so that mere asm mortals have some prayer of understanding how
>your magic works and what the desired output annotations are in the
>various cases?

Honestly I have a hard time seeing where comments would help here. Plus
the difficult part isn't how the annotations look like, but (see above) simply
whether to emit them at all.

Jan


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