Re: [PATCH 03/17] Makefile: generate `CLANG_FLAGS` even in GCC builds

Nick Desaulniers
2021-07-14T18:14:14Z

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