linux-kernel - Re: [PATCH] x86/debug: Remove perpetually broken, unmaintainable dwarf annotations

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Message-ID: <CALCETrW5fQ_F_UoLBXbwN8z_+kLMpTiST-za_qdP-oBhaavqLQ@mail.gmail.com>
Date:	Mon, 1 Jun 2015 12:53:36 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Jan Beulich <JBeulich@...e.com>,
	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Brian Gerst <brgerst@...il.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [PATCH] x86/debug: Remove perpetually broken, unmaintainable
 dwarf annotations

On Mon, Jun 1, 2015 at 12:45 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Fri, May 29, 2015 at 10:47:31AM -0700, Andy Lutomirski wrote:
>> On Thu, May 28, 2015 at 6:17 AM, Ingo Molnar <mingo@...nel.org> wrote:
>> >
>> > * Jan Beulich <JBeulich@...e.com> wrote:
>> >
>> >> > and meanwhile you can keep a revert of this patch ported to SUSE kernels in
>> >> > whatever fashion you prefer.
>> >>
>> >> Funny suggestion - I don't think that's reasonable for us to do. Or if we were
>> >> to, we could as well invest in doing the re-work you're asking for; I don't
>> >> think anyone will have the time to do either.
>> >
>> > That's fair enough: if there's not enough resources to keep a feature maintainable
>> > upstream then it should not be upstream in that form.
>> >
>> > This isn't just some driver we can let bit-rot in peace until it finds a
>> > maintainer (or not), without affecting anyone but users of that driver.
>> >
>> > This is hundreds of usage sites of ugly code intermixed with critical pieces of
>> > assembly code that negatively affects the hackability of everything.
>> >
>> > Also, with the feature missing completely, maybe someone finds a method to
>> > introduce it in a maintainable fashion, while with the feature included upstream
>> > there's very little pressure to do that. As a bonus we'd also win a workable dwarf
>> > unwinder.
>>
>> Before doing something drastic like this, I think we should get Josh's
>> opinion, since I think he's working on a new (?) unwinder.
>>
>> FWIW, musl is considering some kind of automatic annotation scheme:
>>
>> http://www.openwall.com/lists/musl/2015/05/13/5
>
> Thanks for the link!  I found a newer version of it here:
>
>   http://www.openwall.com/lists/musl/2015/05/31/5
>
> Overall I think that script is a really good solution.
>
> From what I can tell, it tracks the CFA (stack pointer) perfectly.
> (Which is actually pretty straightfoward if you just hook into function
> entry/exit, push/pop, and add/sub to rsp).
>
> It also does a nice job at making a best effort at tracking the caller's
> register values (which are less important than CFA but still nice to
> have).

It might be nice to be able to reliably unwind out from an exception /
interrupt / syscall frame into userspace or into the kernel code that
trapped, complete with registers.

In any event, we'll almost certainly have to manually annotate these
weird types of entries.  I wonder if we could manage to annotate just
the entry parts and let a magic script do the rest.

--Andy
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