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

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Date:   Tue, 7 Nov 2017 16:25:10 -0500 (EST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...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



On Tue, 7 Nov 2017, Josh Poimboeuf wrote:

> > 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.

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.

Mikulas