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 12:14:32 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
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 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.

>
>> 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.
>

This stuff is at least useful (in theory) for debugging with gdb.  And
I wouldn't mind an optional DWARF unwinder to get higher quality
backtraces.  Obviously any such thing would need to be quite robust.
I think SuSE has one.

> --
> Regards/Gruss,
>     Boris.
>
> Sent from a fat crate under my desk. Formatting is fine.
> --



-- 
Andy Lutomirski
AMA Capital Management, LLC
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