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

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Date:   Tue, 28 Nov 2017 14:25:32 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        LKML <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 Tue, 28 Nov 2017, Alan Cox wrote:
>
>> > I have no idea who came up with that brilliant idea of parsing comments in
>> > the code. It's so simple to make this parser completely fail that it's not
>>
>> Stephen Johnson (author of the V7 portable C compiler), which is where
>> it's from (the lint tool). He also wrote yacc so he does know a bit about
>> parsers 8).
>
> I don't doubt that.
>
>> > even funny anymore.
>>
>> The notation in question has been standard in tools like lint since the
>> end of the 1970s
>
> Fair enough.
>
> Still that does not make the GCC implementation which defaults to take 'any
> comment' as valid any better and does not solve other parsing issues which
> have been pointed out in various GCC bugs. Using the macro annotation is
> distinct and has no ifs and buts.
>

The thing about taking 'any comment' as valid is false if you add the  
following to your Makefile:

KBUILD_CFLAGS  += $(call cc-option,-Wimplicit-fallthrough)

This option takes the following comments as valid:

/* fall through */
/* Fall through */
/* fall through - ... */
/* Fall through - ... */

Comments as fallthru, fallthrough, FALLTHRU are invalid.

And of course if you intentionally change the option to:

KBUILD_CFLAGS  += $(call cc-option,-Wimplicit-fallthrough=1)

it means that you obviously want to ignore any warning.

Thanks
--
Gustavo A. R. Silva