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

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: "Ingo Molnar" <mingo@kernel.org>,
	"Jan Beulich" <JBeulich@suse.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Peter Zijlstra" <a.p.zijlstra@chello.nl>,
	"Ingo Molnar" <mingo@elte.hu>, "Brian Gerst" <brgerst@gmail.com>,
	"Frédéric Weisbecker" <fweisbec@gmail.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Denys Vlasenko" <dvlasenk@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Frank Ch. Eigler" <fche@redhat.com>
Subject: Re: [PATCH] x86/debug: Remove perpetually broken, unmaintainable dwarf annotations
Date: Mon, 1 Jun 2015 15:19:38 -0500	[thread overview]
Message-ID: <20150601201938.GB6110@treble.redhat.com> (raw)
In-Reply-To: <CALCETrW5fQ_F_UoLBXbwN8z_+kLMpTiST-za_qdP-oBhaavqLQ@mail.gmail.com>

On Mon, Jun 01, 2015 at 12:53:36PM -0700, Andy Lutomirski wrote:
> On Mon, Jun 1, 2015 at 12:45 PM, Josh Poimboeuf <jpoimboe@redhat.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@kernel.org> wrote:
> >> >
> >> > * Jan Beulich <JBeulich@suse.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.

Yeah.  Any callable function can be annotated "magically".  We may have
to manually annotate the rest.

-- 
Josh