linux-kernel - Re: [PATCH] objtool: fix build of 64-bit kernel with 32-bit userspace

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Date:   Tue, 7 Nov 2017 11:55:39 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Jiri Slaby <jslaby@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] objtool: fix build of 64-bit kernel with 32-bit userspace

[ Adding Ingo to cc because I believe it was his suggestion to hide the
  guess unwinder behind CONFIG_EXPERT. ]

On Mon, Nov 06, 2017 at 06:27:53PM -0500, Mikulas Patocka wrote:
> This patch fixes building of 64-bit kernel on 32-bit userspace (I tested 
> it on RHEL-6-i686 and Debian-Sid-x32).

Thanks, I'll review the patch.

> There are still some more bugs - when building 64-bit kernel on 32-bit
> Debian 7 distribution, objtool corrupts object files so that the linker
> doesn't recognize them (ld doesn't recognize the format of the file at all
> and gold prints a lot of errors).

Hm, we should figure out what's going on there...

> Another problem - when building 32-bit kernel, why do you force frame
> pointers on and why did you hide CONFIG_GUESS_UNWINDER behind
> CONFIG_EXPERT? Frame pointers increase code size and register pressure,
> most users don't need precise stacktraces, so CONFIG_GUESS_UNWINDER should
> be default for non-expert users just like it was before.
> 
> Is there some technical reason why do you want to avoid 
> CONFIG_GUESS_UNWINDER?

The technical reason for avoiding the guess unwinder is that it's
sketchy: it gives false positive results.  Not only for oopses, but for
all the other users of the unwinder: /proc/<pid>/stack, perf, lockdep,
etc.  So it's a correctness issue.

I agree with you that the frame pointer unwinder has drawbacks, but if
somebody cares about those drawbacks, I would consider that person an
"expert" ;-)

-- 
Josh