On Thu, Sep 1, 2022 at 4:53 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote: > > On Wed, Aug 31, 2022 at 11:44:05AM -0700, Nick Desaulniers wrote: > > as-instr uses KBUILD_AFLAGS, but as-option uses KBUILD_CFLAGS. This can > > cause as-option to fail unexpectedly because clang will emit > > -Werror,-Wunused-command-line-argument for various -m and -f flags for > > assembler sources. > > Now that I am looking closer at it, where does that '-Werror' come from? The related commit is c3f0d0bc5b01ad90c45276952802455750444b4f The previous discussion with Arnd is https://lore.kernel.org/linux-kbuild/20170314213724.3836900-1-arnd@xxxxxxxx/ > For cc-option, we add it to elevate clang's warnings about unused '-f', > '-m', and '-W' flags to errors so that we do not add those flags. > However, I do not see '-Werror' in as-option. I am going to assume it > came from CONFIG_WERROR, as I believe Android has that turned on by > default. CONFIG_WERROR is added to CFLAGS. But, I guess it is more correct to do likewise for others. (https://patchwork.kernel.org/project/linux-kbuild/patch/20220905083619.672091-1-masahiroy@xxxxxxxxxx/) > I think that is the real problem: without '-Werror', the only > error that should come from as-option is when an option isn't supported > by the assembler, as clang will still warn but those will not be fatal > but with '-Werror', those warnings turn fatal, causing all subsequent > as-option calls to fail. Presumably, it is correct to add -Werror to as-option as well. We have no reason to add it to cc-option, but not to as-option. I also believe '-x assembler' should be changed to '-x assembler-with-cpp'. As I mentioned somewhere before, our assembly code (*.S) is always preprocessed. There is no *.s file in the kernel source tree. So, '-x assembler-with-cpp' matches the real situation. One interesting thing is, clang does not warn [-Wunused-command-line-argument] for *.S files. $ clang -fomit-frame-pointer -c -x assembler /dev/null -o /dev/null clang: warning: argument unused during compilation: '-fomit-frame-pointer' [-Wunused-command-line-argument] $ clang -fomit-frame-pointer -c -x assembler-with-cpp /dev/null -o /dev/null The root cause is we are using '-x assembler', which never happens in the kernel tree. To sum up, the code I think correct is: as-option = $(call try-run,\ $(CC) -Werror $(KBUILD_AFLAGS) $(1) -c -x assembler-with-cpp /dev/null -o "$$TMP",$(1),$(2)) > Do not get me wrong, I still believe this is the correct fix but I think > it would be good to describe exactly under which conditions this is a > real issue in case we ever have to revisit this. > > > Callers of as-option (and as-instr) likely want to be adding flags to > > KBUILD_AFLAGS/aflags-y, not KBUILD_CFLAGS/cflags-y. > > > > Link: https://github.com/ClangBuiltLinux/linux/issues/1699 > > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > > Regardless of changes to the commit message: > > Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx> > > > --- > > Changes v1 -> v2: > > * Split off changes to arch/x86/boot/compressed/Makefile into parent > > patch, as per Masahiro. > > > > scripts/Makefile.compiler | 6 +++--- > > 1 file changed, 3 insertions(+), 3 deletions(-) > > > > diff --git a/scripts/Makefile.compiler b/scripts/Makefile.compiler > > index 94d0d40cddb3..d1739f0d3ce3 100644 > > --- a/scripts/Makefile.compiler > > +++ b/scripts/Makefile.compiler > > @@ -29,13 +29,13 @@ try-run = $(shell set -e; \ > > fi) > > > > # as-option > > -# Usage: cflags-y += $(call as-option,-Wa$(comma)-isa=foo,) > > +# Usage: aflags-y += $(call as-option,-Wa$(comma)-isa=foo,) > > > > as-option = $(call try-run,\ > > - $(CC) $(KBUILD_CFLAGS) $(1) -c -x assembler /dev/null -o "$$TMP",$(1),$(2)) > > + $(CC) $(KBUILD_AFLAGS) $(1) -c -x assembler /dev/null -o "$$TMP",$(1),$(2)) > > > > # as-instr > > -# Usage: cflags-y += $(call as-instr,instr,option1,option2) > > +# Usage: aflags-y += $(call as-instr,instr,option1,option2) > > > > as-instr = $(call try-run,\ > > printf "%b\n" "$(1)" | $(CC) $(KBUILD_AFLAGS) -c -x assembler -o "$$TMP" -,$(2),$(3)) > > -- > > 2.37.2.672.g94769d06f0-goog > > -- Best Regards Masahiro Yamada
