linux-kernel - Re: odd objtool 'unreachable instruction' warning

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Message-ID: <20251029100533.GF3419281@noisy.programming.kicks-ass.net>
Date: Wed, 29 Oct 2025 11:05:33 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: David Laight <david.laight.linux@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: odd objtool 'unreachable instruction' warning

On Wed, Oct 29, 2025 at 09:56:38AM +0000, David Laight wrote:
> On Tue, 28 Oct 2025 12:29:11 -0700
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > Josh, Peter,
> >  due to another entirely unrelated discussion, I ended up resurrecting
> > my "make asm readable" patch that I have had in my local tree when I
> > want to look at the actual generated code for user accesses.
> > 
> > That is a local hack that just removes the alternative noise for the
> > common ops, so that I actually see the fences and clac/stac
> > instructions as such, instead of seeing them as nops in the object
> > file or as horrible noise in the assembler output.
> 
> I've toyed with using explicit nop sequences that would be identifiable
> as stac, clac and lfence.
> 
> At least that would tell you which is which.
> 
> Since the flags can be trashed there are plenty to choose from.
> (eg all the cmpb $n,%reg if you don't mind a false dependency.)

As long as you ensure that insn_is_nop() recognises it as such, they
won't actually ever get ran after alternative patching, since they'll be
rewritten in canonical nops by optimize_nops().

(be sure to use the tip/master branch, since I unified the various
is_nop implementations).