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

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Date:	Sat, 23 Jul 2016 07:53:50 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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 Sat, Jul 23, 2016 at 02:39:52PM +0900, Linus Torvalds wrote:
> On Sat, Jul 23, 2016 at 2:35 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > While doing the scanning and printing, it does call the frame pointer
> > unwinder in parallel, but like before, that's *only* used to determine
> > whether a found address should be printed without a question mark.  If
> > the unwinder goes off the rails, the scanning and printing of text
> > addresses goes on, undisturbed.
> >
> > The frame pointer unwinder code itself is quite careful not to
> > dereference anything it shouldn't (though of course I welcome any review
> > comments that find otherwise).
> 
> So this was the bug the last time around we did unwinders - the code
> would dereference the unwind tables, and the tables would be
> corrupted. End result: recursive oops.
> 
> And they were corrupted not even because of memory corruption, but
> simply because they contained incorrect data, due to compiler bugs and
> other issues.
> 
> I have really bad memories from that time. Several years after the
> fact. It took months to finally revert the crap, because the author
> continued to insist that "this was the last bug" for several passes
> through that thing.
> 
> As they say, "Once burned, twice shy". But in this case, it's more
> like "Four times burned, sixteen times as shy".

But that was DWARF, right?  This is still just simple frame pointers.

Don't think of it as a new unwinder.  Think of it instead as a "gentle
reshuffling of the existing code to vastly improve readability and
maintenance."

Yes, I would like to eventually propose a DWARF unwinder, which
hopefully learns from the mistakes of previous attempts.  But either
way, I think this patch set stands on its own as a big improvement.

-- 
Josh