Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}

From: Peter Zijlstra <peterz@infradead.org>
To: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Michael Matz <matz@suse.de>, Borislav Petkov <bp@alien8.de>,
	linux-toolchains@vger.kernel.org,
	Indu Bhagat <indu.bhagat@oracle.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org,
	"Jose E. Marchesi" <jemarch@gnu.org>,
	Miroslav Benes <mbenes@suse.cz>,
	Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will@kernel.org>,
	x86@kernel.org, linux-arm-kernel@lists.infradead.org,
	live-patching@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	Ard Biesheuvel <ardb@kernel.org>,
	Chen Zhongjin <chenzhongjin@huawei.com>,
	Sathvika Vasireddy <sv@linux.ibm.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Mark Brown <broonie@kernel.org>
Subject: Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}
Date: Wed, 14 Sep 2022 16:00:16 +0200	[thread overview]
Message-ID: <YyHecBM8D0i1lRu8@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20220914000416.daxbgccbxwpknn2q@treble>

On Wed, Sep 14, 2022 at 01:04:16AM +0100, Josh Poimboeuf wrote:

> > I will mention that objtool's existence is based on mistrust, of persons 
> > (not correctly annotating stuff) and of tools (not correctly heeding those 
> > annotations).  The mistrust in persons is understandable and can be dealt 
> > with by tools, but the mistrust in tools can't be fixed by making tools 
> > more complicated by emitting even more information; there's no good reason 
> > to assume that one piece of info can be trusted more than other pieces.  
> > So, if you mistrust the tools you have already lost.  That's somewhat 
> > philosophical, so I won't beat that horse much more either.
> 
> Maybe this is semantics, but I wouldn't characterize objtool's existence
> as being based on the mistrust of tools.  It's main motivation is to
> fill in the toolchain's blind spots in asm and inline-asm, which exist
> by design.

That and a fairly deep seated loathing for the regular CFI annotations
and DWARF in general. Linus was fairly firm he didn't want anything to
do with DWARF for in-kernel unwinding.

That left us in a spot that we needed unwind information in a 'better'
format than DWARF.

Objtool was born out of those contraints. ORC not needing the CFI
annotations and ORC being *much* faster at unwiding and generation
(debug builds are slow) were all good.