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

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Message-ID: <20171108080722.GS3165@worktop.lehotels.local>
Date:   Wed, 8 Nov 2017 09:07:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Josh Poimboeuf <jpoimboe@...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>,
        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 Tue, Nov 07, 2017 at 04:25:10PM -0500, Mikulas Patocka wrote:
> > The technical reason for avoiding the guess unwinder is that it's
> > sketchy: it gives false positive results.
> 
> I've always used kernels without frame pointer and I don't see any problem 
> with decoding stack traces with some phantom entries that were left in the 
> stack - it's easy to find out which functions could call which functions 
> and discard the phantom entries.
> 
> > 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.
> 
> Experts need these features, but casual users don't.
> 
> > 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" ;-)
> 
> The Kconfig entry says that frame pointers degrade performance by 5-10% - 
> so almost any user would care about it, not just experts.

You're running a 32bit kernel.... isn't that the same as not caring
about performance in any case?

I suppose the solution you're looking for is making ORC work for it; but
given hardly anybody still cares about 32bit x86 you'll probably have to
do it yourself.