linux-kernel - Re: [PATCH v2] arch: enable HAS_LTO

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Date:   Tue, 18 Jul 2023 10:50:21 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Jakob Koschel <jkl820.git@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v2] arch: enable HAS_LTO_CLANG with KASAN and KCOV

On Mon, Jul 17, 2023 at 11:49 PM Jakob Koschel <jkl820.git@...il.com> wrote:
>
> Both KASAN and KCOV had issues with LTO_CLANG if DEBUG_INFO is enabled.
> With LTO inlinable function calls are required to have debug info if
> they are inlined into a function that has debug info.
>
> Starting with LLVM 17 this will be fixed ([1],[2]) and enabling LTO with
> KASAN/KCOV and DEBUG_INFO doesn't cause linker errors anymore.
>
> Link: https://github.com/llvm/llvm-project/commit/913f7e93dac67ecff47bade862ba42f27cb68ca9
> Link: https://github.com/llvm/llvm-project/commit/4a8b1249306ff11f229320abdeadf0c215a00400
> Reviewed-by: Nathan Chancellor <nathan@...nel.org>
> Signed-off-by: Jakob Koschel <jkl820.git@...il.com>
> ---
> Changes in v2:
> - Added correct To:/Cc:
> - Added comment about required LLVM 17 version dependency
> - Link to v1: https://lore.kernel.org/r/20230717-enable-kasan-lto1-v1-1-471e706a5c4e@gmail.com
> ---
>  arch/Kconfig | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index aff2746c8af2..d722bcfcc870 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -745,7 +745,10 @@ config HAS_LTO_CLANG
>         depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)
>         depends on ARCH_SUPPORTS_LTO_CLANG
>         depends on !FTRACE_MCOUNT_USE_RECORDMCOUNT
> -       depends on !KASAN || KASAN_HW_TAGS
> +       # KASAN & KCOV were missing necessary debug information prior to LLVM 17
> +       # which are required with LTO.

Hi Jakob,
Thanks for follow up with a v2.  I think Nathan meant he was literally
looking for a link here in the comment, like:

# https://github.com/ClangBuiltLinux/linux/issues/1721

Mind sending a v3 with that? You can replace the comment you added to
arch/Kconfig in v2 outright with that link.

If folks need to know more they can follow that link or run git blame on it.

> +       depends on (!KASAN || KASAN_HW_TAGS || CLANG_VERSION >= 170000) || !DEBUG_INFO
> +       depends on (!KCOV || CLANG_VERSION >= 170000) || !DEBUG_INFO
>         depends on !GCOV_KERNEL
>         help
>           The compiler and Kconfig options support building with Clang's
>
> ---
> base-commit: fdf0eaf11452d72945af31804e2a1048ee1b574c
> change-id: 20230717-enable-kasan-lto1-656754c76241
>
> Best regards,
> --
> Jakob Koschel <jkl820.git@...il.com>
>


-- 
Thanks,
~Nick Desaulniers