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Date:   Mon, 29 Aug 2022 21:46:27 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Segher Boessenkool <segher@kernel.crashing.org>,
        Borislav Petkov <bp@alien8.de>, X86 ML <x86@kernel.org>,
        Michael Matz <matz@suse.de>, linux-toolchains@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH] x86/sev: Mark snp_abort() noreturn
Message-ID: <20220830044627.t37gkqjdknu4wgz6@treble>
References: <20220824152420.20547-1-bp@alien8.de>
 <20220824172929.GA25951@gate.crashing.org>
 <YwaN2HtMyM0YEdSB@worktop.programming.kicks-ass.net>
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On Wed, Aug 24, 2022 at 10:45:12PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 24, 2022 at 12:29:29PM -0500, Segher Boessenkool wrote:
> > Hi!
> > 
> > On Wed, Aug 24, 2022 at 05:24:20PM +0200, Borislav Petkov wrote:
> > > Mark both the function prototype and definition as noreturn in order to
> > > prevent the compiler from doing transformations which confuse objtool
> > > like so:
> > > 
> > >   vmlinux.o: warning: objtool: sme_enable+0x71: unreachable instruction
> > 
> > Would -Wmissing-noreturn have caught this?  It sounds like you need this
> > (and then fix all resulting warnings) to not upset objtool?
> > 
> > It is nice to have this anyway (if there aren't a zillion false
> > positives), but it seems objtool is very fragile.
> 
> Well, just like gcc has noreturn heuristics so has objtool, it just
> turns into pain when they don't agree with one another.
> 
> Ideally noreturn would be reflected in the object file so we don't have
> to guess at it. STT_FUNC_NORETURN would do I suppose, except then all
> the tools will need to be taught how to deal with that, which is also
> very painful.
> 
> Another options is something like .symtab.noreturn which is another
> symbol table explicitly listing the noreturn functions. Since it's an
> extra section tools that don't know about it can freely ignore it and
> carry on as usual.

We're planning to talk about this at the LPC toolchains microconference.

My proposal is similar to yours except I called it .annotate.noreturn.
It would be enabled with a --annotate=noreturn compiler option.  It
would report both explicit noreturns (with the "noreturn" function
attribute) and implicit noreturns (static functions which only call
other noreturn functions).

There would also be an --annotate=jump_table option which creates an
.annotate.jump_table to describe switch statement jump table flow.

Those are the two biggest challenges for objtool.

-- 
Josh