linux-kernel - Re: [PATCH] x86/syscalls: Mark expected switch fall-throughs

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Message-ID: <20171128120512.Horde.1mz61Up1PsNtyHbrjWmK8L7@gator4166.hostgator.com>
Date:   Tue, 28 Nov 2017 12:05:12 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86/syscalls: Mark expected switch fall-throughs

Quoting Thomas Gleixner <tglx@...utronix.de>:

> On Mon, 27 Nov 2017, Gustavo A. R. Silva wrote:
>
>> In preparation to enabling -Wimplicit-fallthrough, mark switch cases
>> where we are expecting to fall through.
>
>>  		case 0:
>>  			if (!n--) break;
>>  			*args++ = regs->bx;
>> +			/* fall through */
>
> And these gazillions of pointless comments help enabling of
> -Wimplicit-fallthrough in which way?
>

The -Wimplicit-fallthrough option was added to GCC 7. We want to add  
that option to the top-level Makefile so we can have the compiler help  
us not make mistakes as missing "break"s or "continue"s. This also  
documents the intention for humans and provides a way for analyzers to  
report issues or ignore False Positives.

So prior to adding such option to the Makefile, we have to properly  
add a code comment wherever the code is intended to fall through.

During the process of placing these comments I have identified actual  
bugs (missing "break"s/"continue"s) in a variety of components in the  
kernel, so I think this effort is valuable. Lastly, such a simple  
comment in the code can save a person plenty of time during a code  
review.

Thanks
--
Gustavo A. R. Silva