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

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Date:   Fri, 14 Feb 2020 16:02:18 -0300
From:   Paul Cercueil <paul@...pouillou.net>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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

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.

-Paul

> 
> This fixes the following objtool warning:
> 
>   drivers/pinctrl/pinctrl-ingenic.o: warning: objtool: 
> ingenic_pinconf_set() falls through to next function 
> ingenic_pinconf_group_set()
> 
> Reported-by: Randy Dunlap <rdunlap@...radead.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  drivers/pinctrl/pinctrl-ingenic.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pinctrl/pinctrl-ingenic.c 
> b/drivers/pinctrl/pinctrl-ingenic.c
> index 96f04d121ebd..6b61ac6cd4d2 100644
> --- a/drivers/pinctrl/pinctrl-ingenic.c
> +++ b/drivers/pinctrl/pinctrl-ingenic.c
> @@ -2158,7 +2158,7 @@ static int ingenic_pinconf_set(struct 
> pinctrl_dev *pctldev, unsigned int pin,
>  			break;
> 
>  		default:
> -			unreachable();
> +			BUG();
>  		}
>  	}
> 
> --
> 2.21.1
>