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

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Message-ID: <CANiq72kMpP_PSBb0e5wMVNiuYvwSWdR+x_H_tPrEnQ_j1y7X+Q@mail.gmail.com>
Date:   Wed, 31 Jul 2019 22:59:01 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>, Joe Perches <joe@...ches.com>,
        Pavel Machek <pavel@....cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        linux-kernel <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 10:02 PM Kees Cook <keescook@...omium.org> wrote:
>
> 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. :)

We had this discussion a while ago (October) and the consensus was
that we would like to wait for a while until all tools were ready to
use the attribute and everyone's would be happy:

  https://lore.kernel.org/lkml/20181021171414.22674-1-miguel.ojeda.sandonis@gmail.com/

Is everyone happy this time around? :-)

Cheers,
Miguel