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

From: Paul Cercueil <paul@crapouillou.net>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Walleij <linus.walleij@linaro.org>,
	linux-gpio@vger.kernel.org, linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCH] pinctrl: ingenic: Make unreachable path more robust
Date: Fri, 14 Feb 2020 23:37:04 -0300	[thread overview]
Message-ID: <1581734224.3.14@crapouillou.net> (raw)
In-Reply-To: <20200214203738.af3y4gskukctvvum@treble>

Le ven., févr. 14, 2020 at 14:37, Josh Poimboeuf <jpoimboe@redhat.com> 
a écrit :
> 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@redhat.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)?

It's a dead code path. That would be an undefined behaviour, if it was 
taken, but it's not.

-Paul