linux-kernel - Re: [PATCH 2/4] compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

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Date:   Thu, 10 Oct 2019 13:37:18 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Joe Perches <joe@...ches.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Pavel Machek <pavel@....cz>,
        "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>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 2/4] compiler_attributes.h: Add 'fallthrough' pseudo
 keyword for switch/case use

On Sat, Oct 05, 2019 at 07:17:36PM +0200, Miguel Ojeda wrote:
> Hi Joe,
> 
> On Sat, Oct 5, 2019 at 6:46 PM Joe Perches <joe@...ches.com> wrote:
> >
> > Reserve the pseudo keyword 'fallthrough' for the ability to convert the
> > various case block /* fallthrough */ style comments to appear to be an
> > actual reserved word with the same gcc case block missing fallthrough
> > warning capability.
> >
> > All switch/case blocks now must end in one of:
> >
> >         break;
> >         fallthrough;
> >         goto <label>;
> >         return [expression];
> >         continue;
> >
> > fallthough is gcc's __attribute__((__fallthrough__)) which was introduced
> > in gcc version 7..
> 
> Nits: double period, missing "r" in fallthough.
> 
> > fallthrough devolves to an empty "do {} while (0)" if the compiler
> > version (any version less than gcc 7) does not support the attribute.
> 
> Perhaps add a short note why (empty statement warnings maybe? I don't
> remember them but it was months ago so maybe it changed).
> 
> > Signed-off-by: Joe Perches <joe@...ches.com>
> > Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> Please add Dan's Suggested-by and copy the things I wrote in the
> commit message when I proposed this:
> 
>   https://github.com/ojeda/linux/commit/668f011a2706ea555987e263f609a5deba9c7fc4

Perhaps just take this patch directly with Miguel's authorship into the
series and tweak the __fallthrough to fallthrough in subject and body.

-Kees

> 
> > ---
> >  include/linux/compiler_attributes.h | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> >
> > diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> > index 6b318efd8a74..cdf016596659 100644
> > --- a/include/linux/compiler_attributes.h
> > +++ b/include/linux/compiler_attributes.h
> > @@ -40,6 +40,7 @@
> >  # define __GCC4_has_attribute___noclone__             1
> >  # define __GCC4_has_attribute___nonstring__           0
> >  # define __GCC4_has_attribute___no_sanitize_address__ (__GNUC_MINOR__ >= 8)
> > +# define __GCC4_has_attribute___fallthrough__         0
> 
> This goes after __externally_visible__.
> 
> >  #endif
> >
> >  /*
> > @@ -185,6 +186,22 @@
> >  # define __noclone
> >  #endif
> >
> > +/*
> > + * 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
> 
> This also goes after __externally_visible__.
> 
> Please add:
> 
>   * Optional: only supported since gcc >= 7.1
>   * Optional: only supported since clang >= 10
>   * Optional: not supported by icc
> 
> As well as:
> 
>   clang: https://clang.llvm.org/docs/AttributeReference.html#fallthrough
> 
> See how I did it in the link above:
> 
>   https://github.com/ojeda/linux/commit/668f011a2706ea555987e263f609a5deba9c7fc4
> 
> > + */
> > +#if __has_attribute(__fallthrough__)
> > +# define fallthrough                    __attribute__((__fallthrough__))
> > +#else
> > +# define fallthrough                    do {} while (0)  /* fallthrough */
> > +#endif
> > +
> >  /*
> >   * Note the missing underscores.
> >   *
> > --
> > 2.15.0
> >
> 
> Cheers,
> Miguel

-- 
Kees Cook