linux-kernel - Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

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Message-ID: <20170713123539.t6az4pnrdjewjvms@treble>
Date:   Thu, 13 Jul 2017 07:35:39 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andres Freund <andres@...razel.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>, Jiri Slaby <jslaby@...e.cz>,
        "H. Peter Anvin" <hpa@...or.com>, Mike Galbraith <efault@....de>,
        Jiri Olsa <jolsa@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

On Thu, Jul 13, 2017 at 07:21:15AM -0500, Josh Poimboeuf wrote:
> On Thu, Jul 13, 2017 at 07:17:55AM -0500, Josh Poimboeuf wrote:
> > BTW, while we're throwing out ideas for this, here's another idea,
> > though it's almost certainly not a good one :-)
> > 
> > For user space stack unwinding, the kernel could emulate what the kernel
> > 'guess' unwinder does by scanning the user space stack and returning all
> > the text addresses it finds.

To clarify, text address would mean any address in a VMA with the
executable bit set.

> > The results wouldn't be 100% accurate, but they could end up being
> > useful over time.
> 
> And to expound further on the bad idea, maybe the "bad" addresses could
> be filtered out somehow in post-processing (insert lots of hand waving).

And some details on the post-processing: in most cases it should be
possible to determine which of the found stack addresses are valid by
looking at the call instructions immediately preceding the stack text
addresses, and making sure the call target points to the same function
as the previously found address.  But of course that wouldn't work for
indirect calls.

-- 
Josh