linux-kernel - Re: [PATCH] pinctrl: ingenic: Make unreachable path more robust

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Date:   Fri, 14 Feb 2020 14:37:38 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] pinctrl: ingenic: Make unreachable path more robust

On Fri, Feb 14, 2020 at 04:02:18PM -0300, Paul Cercueil wrote:
> Hi Josh,
> 
> 
> Le ven., févr. 14, 2020 at 10:37, Josh Poimboeuf <jpoimboe@...hat.com> a
> écrit :
> > In the second loop of ingenic_pinconf_set(), it annotates the switch
> > default case as unreachable().  The annotation is technically correct,
> > because that same case would have resulted in an early return in the
> > previous loop.
> > 
> > However, if a bug were to get introduced later, for example if an
> > additional case were added to the first loop without adjusting the
> > second loop, it would result in nasty undefined behavior: most likely
> > the function's generated code would fall through to the next function.
> > 
> > Another issue is that, while objtool normally understands unreachable()
> > annotations, there's one special case where it doesn't: when the
> > annotation occurs immediately after a 'ret' instruction.  That happens
> > to be the case here because unreachable() is immediately before the
> > return.
> > 
> > So change the unreachable() to BUG() so that the unreachable code, if
> > ever executed, would panic instead of introducing undefined behavior.
> > This also makes objtool happy.
> 
> I don't like the idea that you change this driver's code just to work around
> a bug in objtool, and I don't like the idea of working around a future bug
> that shouldn't be introduced in the first place.

It's not an objtool bug.  It's a byproduct of the fact that GCC's
undefined behavior is inscrutable, and there's no way to determine that
it actually *wants* to jump to a random function.

And anyway, regardless of objtool, the patch is meant to make the code
more robust.

Do you not agree that BUG (defined behavior) is more robust than
unreachable (undefined behavior)?

-- 
Josh