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Message-ID: <beb1762387a4132a2bb4c47397b737d77dd88742.camel@perches.com>
Subject: Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo
 keyword for switch/case use
From:   Joe Perches <joe@perches.com>
To:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Kees Cook <keescook@chromium.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Pavel Machek <pavel@ucw.cz>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Shawn Landden <shawn@git.icu>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
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Date:   Wed, 31 Jul 2019 15:07:50 -0700
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On Wed, 2019-07-31 at 22:59 +0200, Miguel Ojeda wrote:
> On Wed, Jul 31, 2019 at 10:02 PM Kees Cook <keescook@chromium.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@zytor.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? :-)

Note also that this doesn't actually _use_ fallthrough
it just reserves it.

Patches that convert /* fallthrough */ et al, to fallthrough
would only be published when everyone's happy enough.