linux-kernel - Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

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Message-ID: <201907311301.EC1D84F@keescook>
Date:   Wed, 31 Jul 2019 13:02:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     hpa@...or.com, Joe Perches <joe@...ches.com>,
        Pavel Machek <pavel@....cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "Gustavo A . R . Silva" <gustavo@...eddedor.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Shawn Landden <shawn@....icu>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo
 keyword for switch/case use

On Wed, Jul 31, 2019 at 08:48:32PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 31, 2019 at 11:24:36AM -0700, hpa@...or.com wrote:
> > >> > +/*
> > >> > + * Add the pseudo keyword 'fallthrough' so case statement blocks
> > >> > + * must end with any of these keywords:
> > >> > + *   break;
> > >> > + *   fallthrough;
> > >> > + *   goto <label>;
> > >> > + *   return [expression];
> > >> > + *
> > >> > + *  gcc: >https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
> > >> > + */
> > >> > +#if __has_attribute(__fallthrough__)
> > >> > +# define fallthrough                   __attribute__((__fallthrough__))
> > >> > +#else
> > >> > +# define fallthrough                    do {} while (0)  /* fallthrough */
> > >> > +#endif
> > >> > +
> 
> > If the comments are stripped, how would the compiler see them to be
> > able to issue a warning? I would guess that it is retained or replaced
> > with some other magic token.
> 
> Everything that has the warning (GCC-7+/CLANG-9) has that attribute.

I'd like to make sure we don't regress Coverity most of all. If the
recent updates to the Coverity scanner include support for the attribute
now, then I'm all for it. :)

-- 
Kees Cook