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

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Date:	Tue, 2 Jun 2015 10:00:24 -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 Tue, Jun 2, 2015 at 7:46 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Tue, Jun 02, 2015 at 07:57:06AM +0200, Ingo Molnar wrote:
>> * Andy Lutomirski <luto@...capital.net> wrote:
>> > 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:
>> > >> 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.
>>
>> Even the entry parts we could help without uglifying the code:
>>
>>  - either by adding a 'RET' instruction after IRET/SYSRET/SYSEXIT/etc. that the
>>    tooling can recognize as 'return from function'. That's much nicer than ugly
>>    annotations.
>>
>>  - enhancing the tooling script to also recognize these instructions as function
>>    returns - because they _are_ function returns.
>
> I think the problem with the entry code (and other non-function asm
> code) is that it's quite spaghetti-esque, with lots of jumps, returns,
> calls, etc to random places.  There aren't enough constraints which
> would help the tooling make sense of where execution begins and ends,
> when registers are saved or trashed, etc.
>
> Maybe over time we can figure out what constraints (and/or annotations)
> are needed there.
>

We can also try to reduce the spaghetti, which I'm working on.  (It's
even harder than I expected, and I doubt that any of my code will be
ready for 4.2.)

--Andy
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