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 21:11:45 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	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>,
	"H. Peter Anvin" <hpa@...or.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 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?

> Better example:
> 
>     pushq_cfi \child_rip /* rip */
>     CFI_REL_OFFSET    rip,0
> 
> Doing this with a macro would need a fancier macro.

I'd ask first whether we really need this at all.

> Then there's crap like:
> 
>     pushq_cfi %rdi
>     SCHEDULE_USER
>     popq_cfi %rdi

I guess we can add a gas regname argument optional and if it is set, use
it and if not, use the reg itself... Or something like that in the best
effort type of approach.

> I would need to look a lot more carefully to figure out whether this
> would need CFI_REL_OFFSET.
> 
> If we actually had a DWARF unwinder in the kernel, maybe we could have
> real test cases :-/

I don't think that's ever going to happen.

I'd say we do the CFI annotation on a best effort basis but not
sacrifice readability in the process. If it can't be annotated, well,
tough luck.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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