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

[Nick Desaulniers <ndesaulniers@google.com>]

On Wed, Jul 14, 2021 at 11:13 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> 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

Also, consider whether `BINDGEN_FLAGS` would be more descriptive (and
less verbose) than `TENTATIVE_CLANG_FLAGS`.

> > +
> >  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



-- 
Thanks,
~Nick Desaulniers