linux-kernel - Re: New objtool warning..

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Date:   Tue, 15 Dec 2020 22:49:18 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: New objtool warning..

On Tue, Dec 15, 2020 at 08:22:23PM -0800, Linus Torvalds wrote:
> I only see this on my laptop, but that's probably because my desktop
> is built using clang. So it's a gcc code generation interaction, I
> suspect..
> 
> Anyway, the new warning is
> 
>     drivers/gpu/drm/drm_edid.o: warning: objtool: do_cvt_mode() falls
> through to next function drm_mode_detailed.isra.0()
> 
> and googling around a bit I see that 0day ended up reporting it on the
> linux-next lists, and blames commit 991fcb77f490 ("drm/edid: Fix
> uninitialized variable in drm_cvt_modes()").
> 
> That presumably then makes gcc generate that odd code.
> 
> That "unreachable()" is because the compiler isn't smart enough to see
> that yes, there really are case statements for every single possible
> case. Oh well. Maybe the code should just make one of the possible
> cases also be the "default:" case, and that might fix it.
> 
> But maybe this is worth looking into for objtool too?
> 
> Anyway, I see it with gcc-10.2.1 as per current F32. Holler if you
> can't reproduce it, I can send the object file around.

I can't recreate with my compiler, but I think I've seen one like this
before.  I suspect s/unreachable()/BUG()/ would fix it?

But yeah, it _might_ be possible to make objtool a little smarter here.
Gimme the .o file and I can take a look tomorrow.

-- 
Josh