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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 14 Jul 2021 11:13:57 -0700
Message-ID: <CAKwvOdnO1ZbM_FzY3qwokEkWDxsr37t_u57H_wEO6Pbu6CqFZw@mail.gmail.com>
Subject: Re: [PATCH 03/17] Makefile: generate `CLANG_FLAGS` even in GCC builds
To:     ojeda@kernel.org
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        rust-for-linux@vger.kernel.org, linux-kbuild@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Geoffrey Thomas <geofft@ldpreload.com>,
        Finn Behrens <me@kloenk.de>,
        Adam Bratschi-Kaye <ark.email@gmail.com>,
        Wedson Almeida Filho <wedsonaf@google.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Nathan Chancellor <nathan@kernel.org>
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On Sun, Jul 4, 2021 at 1:28 PM <ojeda@kernel.org> wrote:
>
> From: Miguel Ojeda <ojeda@kernel.org>
>
> To support Rust under GCC-built kernels, we need to save the flags that
> would have been passed if the kernel was being compiled with Clang.
>
> The reason is that `bindgen` -- the tool we use to generate Rust
> bindings to the C side of the kernel -- relies on `libclang` to
> parse C. Ideally:
>
>   - `bindgen` would support a GCC backend (requested at [1]),
>
>   - or the Clang driver would be perfectly compatible with GCC,
>     including plugins. Unlikely, of course, but perhaps a big
>     subset of configs may be possible to guarantee to be kept
>     compatible nevertheless.
>
> This is also the reason why GCC builds are very experimental and some
> configurations may not work (e.g. `GCC_PLUGIN_RANDSTRUCT`). However,
> we keep GCC builds working (for some example configs) in the CI
> to avoid diverging/regressing further, so that we are better prepared
> for the future when a solution might become available.
>
> [1] https://github.com/rust-lang/rust-bindgen/issues/1949
>
> Link: https://github.com/Rust-for-Linux/linux/issues/167
> Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
> Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
> Co-developed-by: Geoffrey Thomas <geofft@ldpreload.com>
> Signed-off-by: Geoffrey Thomas <geofft@ldpreload.com>
> Co-developed-by: Finn Behrens <me@kloenk.de>
> Signed-off-by: Finn Behrens <me@kloenk.de>
> Co-developed-by: Adam Bratschi-Kaye <ark.email@gmail.com>
> Signed-off-by: Adam Bratschi-Kaye <ark.email@gmail.com>
> Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Patch LGTM; please keep an eye on the series:
https://lore.kernel.org/lkml/20210707224310.1403944-2-ndesaulniers@google.com/

If that lands in kbuild before this, this patch will need to be
rebased to avoid a conflict in linux-next.

So (tentatively :-P):
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

If the patch needs to be rebased on the series linked above, please
drop my reviewed by tag and I will re-review. Perhaps putting me
explicitly on Cc: in the commit message will help notify me if there
are successive versions?

> ---
>  Makefile | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 0565caea036..6e823d8bd64 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -573,18 +573,23 @@ endif
>  # and from include/config/auto.conf.cmd to detect the compiler upgrade.
>  CC_VERSION_TEXT = $(subst $(pound),,$(shell $(CC) --version 2>/dev/null | head -n 1))
>
> -ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
> +TENTATIVE_CLANG_FLAGS := -Werror=unknown-warning-option
> +
>  ifneq ($(CROSS_COMPILE),)
> -CLANG_FLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
> +TENTATIVE_CLANG_FLAGS  += --target=$(notdir $(CROSS_COMPILE:%-=%))
>  endif
>  ifeq ($(LLVM_IAS),1)
> -CLANG_FLAGS    += -integrated-as
> +TENTATIVE_CLANG_FLAGS  += -integrated-as
>  else
> -CLANG_FLAGS    += -no-integrated-as
> +TENTATIVE_CLANG_FLAGS  += -no-integrated-as
>  GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
> -CLANG_FLAGS    += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
> +TENTATIVE_CLANG_FLAGS  += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
>  endif
> -CLANG_FLAGS    += -Werror=unknown-warning-option
> +
> +export TENTATIVE_CLANG_FLAGS
> +
> +ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
> +CLANG_FLAGS    += $(TENTATIVE_CLANG_FLAGS)
>  KBUILD_CFLAGS  += $(CLANG_FLAGS)
>  KBUILD_AFLAGS  += $(CLANG_FLAGS)
>  export CLANG_FLAGS
> --
> 2.32.0
>


-- 
Thanks,
~Nick Desaulniers