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

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Message-ID: <c0669a7130645a20e99915385b7e712360c31ed9.camel@perches.com>
Date:   Sun, 04 Aug 2019 11:01:10 -0700
From:   Joe Perches <joe@...ches.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, 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>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Neil Horman <nhorman@...driver.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo
 keyword for switch/case use

On Tue, 2019-07-30 at 22:35 -0700, Joe Perches 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.

Linus?  Do you have an opinion about this RFC/patch?

> All switch/case blocks now must end in one of:
> 
> 	break;
> 	fallthrough;
> 	goto <label>;
> 	return [expression];
> 
> fallthough is gcc's __attribute__((__fallthrough__)) which was introduced
> in gcc version 7..
> 
> fallthrough devolves to an empty "do {} while (0)" if the compiler version
> (any version less than gcc 7) does not support the attribute.
> 
> Signed-off-by: Joe Perches <joe@...ches.com>
> ---
> 
> For allow compilation of a kernel that includes net/sctp, this patch
> also requires a rename of the use of fallthrough as a label in
> net/sctp/sm_make_chunk.c that was submitted as:
> 
> https://lore.kernel.org/lkml/e0dd3af448e38e342c1ac6e7c0c802696eb77fd6.1564549413.git.joe@perches.com/
> 
>  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
>  #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
> + */
> +#if __has_attribute(__fallthrough__)
> +# define fallthrough                    __attribute__((__fallthrough__))
> +#else
> +# define fallthrough                    do {} while (0)  /* fallthrough */
> +#endif
> +
>  /*
>   * Note the missing underscores.
>   *