linux-kernel - Re: [PATCH 00/19] x86/dumpstack: rewrite x86 stack dump code

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Message-ID: <CALCETrUvoUycLhRKHf6TQ9M3acqPxMCrk=UFmsBhvLPWw5JYtg@mail.gmail.com>
Date:	Fri, 22 Jul 2016 17:31:47 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH 00/19] x86/dumpstack: rewrite x86 stack dump code

On Fri, Jul 22, 2016 at 5:22 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> So without having yet looked at the code, I want people to understand
> that to a very real degree, the stack tracer that the *oopsing* code
> (ie what all the usual kernel fault handlers use) is very very special
> code and needs to be handled very carefully, and needs to be extra
> robust, even in the presence of stack corruption, and even in the
> presence of the dwarf info being totally corrupted. Because we've very
> much had both things happen.
>
> It is very possible that we should have two different stack tracers -
> the stupid "for oopses only" code that doesn't necessarily give the
> perfect trace, but is very anal and happily gives old stale addresses
> (which can be very useful for seeing what happened just before the
> "real" stack trace), and then a separate stack trace engine that is
> clever and gets things right, and if that one faults it can depend on
> the normal kernel fault handling picking up the pieces.

I think that Josh's code has the potential to be extremely robust
*and* give more correct results when possible.  One thing I intend to
review when v2 shows up is that it's as conservative as it needs to be
to avoid ever dereferencing an out-of-bounds pointer.  And Josh's oops
printer carefully walks and prints out all addresses on the stack
(complete with question marks) even if the unwinder doesn't find them.

>
> Yes, the current stack tracer is  crufty. No, it's not perfect. But it
> is very well tested, and has held up. That should not be dismissed.
>

I think you may be giving the current tracer slightly more credit than
it's due.  In my stack guard page patchset, I fixed two separate
issues, one of which caused recursive faults and one of which caused
it to output nothing at all.  So maybe *now* it's very robust :)  But
it's still an umaintainable mess IMO, and Josh's patchset helps a
*lot*.

--Andy