linux-kernel - Re: [PATCH] compiler-clang: add build check for clang 10.0.1

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Date:   Wed, 26 Aug 2020 14:01:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] compiler-clang: add build check for clang 10.0.1

On Wed, Aug 26, 2020 at 01:14:19PM -0700, Nick Desaulniers wrote:
> During Plumbers 2020, we voted to just support the latest release of
> Clang for now.  Add a compile time check for this.
> 
> Older clang's may work, but we will likely drop workarounds for older
> versions.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/9
> Link: https://github.com/ClangBuiltLinux/linux/issues/941
> Suggested-by: Sedat Dilek <sedat.dilek@...il.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  include/linux/compiler-clang.h | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index cee0c728d39a..7338d3ffd240 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -3,6 +3,14 @@
>  #error "Please don't include <linux/compiler-clang.h> directly, include <linux/compiler.h> instead."
>  #endif
>  
> +#define CLANG_VERSION (__clang_major__ * 10000	\
> +		     + __clang_minor__ * 100	\
> +		     + __clang_patchlevel__)
> +
> +#if CLANG_VERSION < 100001
> +# error Sorry, your compiler is too old - please upgrade it.

Perhaps a bike-shed suggestion, but I think we should make this message
as specific (and helpful) as possible:

# error Sorry, your version of Clang is too old - please use 10.0.1 or newer.

Then anyone seeing this has several pieces of information:

- the kernel build was attempting to use Clang
	(maybe they accidentally poked the wrong configs in a CI)
- they need 10.0.1 or better
	("upgrade to what version?" doesn't need to be dug out of documentation,
         headers, etc)

With that, yes, let's do it. :)

Reviewed-by: Kees Cook <keescook@...omium.org>

(And likely we should improve the GCC message at the same time...)

-- 
Kees Cook