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

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Date:   Wed, 14 Sep 2022 16:00:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Michael Matz <matz@...e.de>, Borislav Petkov <bp@...en8.de>,
        linux-toolchains@...r.kernel.org,
        Indu Bhagat <indu.bhagat@...cle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, "Jose E. Marchesi" <jemarch@....org>,
        Miroslav Benes <mbenes@...e.cz>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Ard Biesheuvel <ardb@...nel.org>,
        Chen Zhongjin <chenzhongjin@...wei.com>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [RFC] Objtool toolchain proposal:
 -fannotate-{jump-table,noreturn}

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.