linux-kernel - Re: [PATCH 1/3] Makefile.compiler: s/KBUILD_CFLAGS/KBUILD

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Message-ID: <CAKwvOdmR8D_GTM=ZhnhLzR3d9Ra46WpcO1W_CujGjMb+QR-_3w@mail.gmail.com>
Date:   Fri, 26 Aug 2022 11:34:50 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Tom Rix <trix@...hat.com>, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, x86@...nel.org,
        Dmitrii Bundin <dmitrii.bundin.a@...il.com>,
        Fangrui Song <maskray@...gle.com>,
        Alexey Alexandrov <aalexand@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH 1/3] Makefile.compiler: s/KBUILD_CFLAGS/KBUILD_AFLAGS/ for as-option

On Fri, Aug 26, 2022 at 11:21 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> Hi Nick,
>
> I think the title would be a little more readable if it was:
>
> Makefile.compiler: Use KBUILD_AFLAGS for as-option

Thanks, yeah that looks better; I expect a v2 of this series will be necessary.

>
> On Fri, Aug 26, 2022 at 11:10:33AM -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.
> >
> > 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@...gle.com>
> > ---
> >  arch/x86/boot/compressed/Makefile | 5 +++--
> >  scripts/Makefile.compiler         | 6 +++---
> >  2 files changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> > index 35ce1a64068b..fb3db714a028 100644
> > --- a/arch/x86/boot/compressed/Makefile
> > +++ b/arch/x86/boot/compressed/Makefile
> > @@ -48,8 +48,6 @@ KBUILD_CFLAGS += -Wno-pointer-sign
> >  KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
> >  KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> >  KBUILD_CFLAGS += -D__DISABLE_EXPORTS
> > -# Disable relocation relaxation in case the link is not PIE.
> > -KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no)
> >  KBUILD_CFLAGS += -include $(srctree)/include/linux/hidden.h
> >
> >  # sev.c indirectly inludes inat-table.h which is generated during
> > @@ -58,6 +56,9 @@ KBUILD_CFLAGS += -include $(srctree)/include/linux/hidden.h
> >  CFLAGS_sev.o += -I$(objtree)/arch/x86/lib/
> >
> >  KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
> > +# Disable relocation relaxation in case the link is not PIE.
> > +KBUILD_AFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no)
> > +
>
> Commit 09e43968db40 ("x86/boot/compressed: Disable relocation
> relaxation") added this to address
> https://github.com/ClangBuiltLinux/linux/issues/1121, is it correct to
> move it to only being used for the .S files in arch/x86/boot/compressed?

+ Arvind

Hmm...that makes me think we might need two different as-option flags;
one that does use KBUILD_CFLAGS, and whose result is meant to be added
to cflags-y, then another that uses KBUILD_AFLAGS and is added to
aflagsy-y.

(My patch 2/3 in the series would use the latter)

Let's see what thoughts Masahiro has.

>
> >  GCOV_PROFILE := n
> >  UBSAN_SANITIZE :=n
> >
> > 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
> >



-- 
Thanks,
~Nick Desaulniers