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

From: Mikulas Patocka <mpatocka@redhat.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Brian Gerst <brgerst@gmail.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Jiri Slaby <jslaby@suse.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	live-patching@vger.kernel.org, linux-kernel@vger.kernel.org,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] objtool: fix build of 64-bit kernel with 32-bit userspace
Date: Tue, 7 Nov 2017 16:25:10 -0500 (EST)	[thread overview]
Message-ID: <alpine.LRH.2.02.1711071607160.27454@file01.intranet.prod.int.rdu2.redhat.com> (raw)
In-Reply-To: <20171107175539.zkbzaapmok2b7hbb@treble>

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