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

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Date:   Mon, 27 Nov 2017 18:32:48 +0100
From:   Sven Joachim <svenjoac@....de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        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

On 2017-11-07 11:55 -0600, Josh Poimboeuf wrote:

> [ 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.

Any news on that?  After upgrading to 4.15-rc1 and running
"make oldconfig" I found out that the kernel would no longer build
unless I selected CONFIG_UNWINDER_FRAME_POINTER=y.

,----
| $ uname -m
| x86_64
| $ dpkg --print-architecture 
| i386
`----

>> 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" ;-)

Cheers,
       Sven