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

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Message-ID: <20200826214228.GB1005132@ubuntu-n2-xlarge-x86>
Date:   Wed, 26 Aug 2020 14:42:28 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Kees Cook <keescook@...omium.org>,
        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.

I think this part of the commit message is a little wishy-washy. If we
are breaking the build for clang < 10.0.1, we are not saying "may work",
we are saying "won't work". Because of this, we should take the
opportunity to clean up behind us and revert/remove parts of:

87e0d4f0f37f ("kbuild: disable clang's default use of -fmerge-all-constants")
b0fe66cf0950 ("ARM: 8905/1: Emit __gnu_mcount_nc when using Clang 10.0.0 or newer")
b9249cba25a5 ("arm64: bti: Require clang >= 10.0.1 for in-kernel BTI support")
3acf4be23528 ("arm64: vdso: Fix compilation with clang older than 8")

This could be a series or a part of this commit, I do not have a
strong preference. If we are not going to clean up behind us, this
should be a warning and not an error.

> 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>

Otherwise, I agree with Kees's comments. With them addressed, feel free
to add:

Acked-by: Nathan Chancellor <natechancellor@...il.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.
> +#endif
> +
>  /* Compiler specific definitions for Clang compiler */
>  
>  /* same as gcc, this was present in clang-2.6 so we can assume it works
> -- 
> 2.28.0.297.g1956fa8f8d-goog
>