linux-kernel - Re: [tip: objtool/core] objtool: Fix HOSTCC flag usage

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Message-ID: <CAP-5=fXmwOb5ae-tejhhhsC8FF+ajivVjBhEjSyFb0z=uWPwow@mail.gmail.com>
Date:   Sat, 4 Feb 2023 13:21:49 -0800
From:   Ian Rogers <irogers@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>, Jiri Olsa <jolsa@...nel.org>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        x86@...nel.org, netdev@...r.kernel.org
Subject: Re: [tip: objtool/core] objtool: Fix HOSTCC flag usage

On Sat, Feb 4, 2023 at 9:05 AM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> On Fri, Feb 03, 2023 at 08:25:40PM +0200, Vladimir Oltean wrote:
> > On Wed, Feb 01, 2023 at 09:36:37AM -0800, Josh Poimboeuf wrote:
> > > On Wed, Feb 01, 2023 at 05:02:16PM +0000, Mark Rutland wrote:
> > > > Hi,
> > > >
> > > > I just spotted this breaks cross-compiling; details below.
> > >
> > > Thanks, we'll fix it up with
> > >
> > > diff --git a/tools/objtool/Makefile b/tools/objtool/Makefile
> > > index 29a8cd7449bf..83b100c1e7f6 100644
> > > --- a/tools/objtool/Makefile
> > > +++ b/tools/objtool/Makefile
> > > @@ -36,7 +36,7 @@ OBJTOOL_CFLAGS := -Werror $(WARNINGS) $(KBUILD_HOSTCFLAGS) -g $(INCLUDES) $(LIBE
> > >  OBJTOOL_LDFLAGS := $(LIBELF_LIBS) $(LIBSUBCMD) $(KBUILD_HOSTLDFLAGS)
> > >
> > >  # Allow old libelf to be used:
> > > -elfshdr := $(shell echo '$(pound)include <libelf.h>' | $(CC) $(CFLAGS) -x c -E - | grep elf_getshdr)
> > > +elfshdr := $(shell echo '$(pound)include <libelf.h>' | $(HOSTCC) $(OBJTOOL_CFLAGS) -x c -E - | grep elf_getshdr)
> > >  OBJTOOL_CFLAGS += $(if $(elfshdr),,-DLIBELF_USE_DEPRECATED)
> > >
> > >  # Always want host compilation.
> >
> > Profiting off of the occasion to point out that cross-compiling with
> > CONFIG_DEBUG_INFO_BTF=y is also broken (it builds the resolve_btfids
> > tool):
>
> The above patch was for objtool, though I'm guessing you were bitten by
> a similar patch for bpf:
>
>   13e07691a16f ("tools/resolve_btfids: Alter how HOSTCC is forced")
>
> It looks like it might have a similar problem we had for objtool.  Does
> this fix it?

Jiri Olsa has been exploring switching to using hostprogs (we need a
hostlibs notion), his patch is:
https://lore.kernel.org/bpf/20230202112839.1131892-1-jolsa@kernel.org/
With this thread giving context:
https://lore.kernel.org/lkml/20230201015015.359535-1-irogers@google.com/
If we have hostprogs and hostlibs then objtool should move to this
approach as changing CC leads to broken CFLAGS and the like.

Thanks,
Ian

> diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
> index daed388aa5d7..fff84cd914cd 100644
> --- a/tools/bpf/resolve_btfids/Makefile
> +++ b/tools/bpf/resolve_btfids/Makefile
> @@ -18,8 +18,8 @@ else
>  endif
>
>  # always use the host compiler
> -HOST_OVERRIDES := AR="$(HOSTAR)" CC="$(HOSTCC)" LD="$(HOSTLD)" ARCH="$(HOSTARCH)" \
> -                 EXTRA_CFLAGS="$(HOSTCFLAGS) $(KBUILD_HOSTCFLAGS)"
> +HOST_OVERRIDES := AR="$(HOSTAR)" CC="$(HOSTCC)" LD="$(HOSTLD)" ARCH="$(HOSTARCH)"
> +BTF_CFLAGS     := $(HOSTCFLAGS) $(KBUILD_HOSTCFLAGS)
>
>  RM      ?= rm
>  CROSS_COMPILE =
> @@ -53,23 +53,25 @@ $(OUTPUT) $(OUTPUT)/libsubcmd $(LIBBPF_OUT):
>
>  $(SUBCMDOBJ): fixdep FORCE | $(OUTPUT)/libsubcmd
>         $(Q)$(MAKE) -C $(SUBCMD_SRC) OUTPUT=$(SUBCMD_OUT) \
> -                   DESTDIR=$(SUBCMD_DESTDIR) $(HOST_OVERRIDES) prefix= subdir= \
> +                   $(HOST_OVERRIDES) EXTRA_CFLAGS="$(BTF_CFLAGS)" \
> +                   DESTDIR=$(LIBBPF_DESTDIR) prefix= subdir= \
>                     $(abspath $@) install_headers
>
>  $(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OUT)
>         $(Q)$(MAKE) $(submake_extras) -C $(LIBBPF_SRC) OUTPUT=$(LIBBPF_OUT)    \
> -                   DESTDIR=$(LIBBPF_DESTDIR) $(HOST_OVERRIDES) prefix= subdir= \
> +                   $(HOST_OVERRIDES) EXTRA_CFLAGS="$(BTF_CFLAGS)" \
> +                   DESTDIR=$(LIBBPF_DESTDIR) prefix= subdir= \
>                     $(abspath $@) install_headers
>
>  LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
>  LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
>
> -CFLAGS += -g \
> -          -I$(srctree)/tools/include \
> -          -I$(srctree)/tools/include/uapi \
> -          -I$(LIBBPF_INCLUDE) \
> -          -I$(SUBCMD_INCLUDE) \
> -          $(LIBELF_FLAGS)
> +BTF_CFLAGS += -g \
> +              -I$(srctree)/tools/include \
> +              -I$(srctree)/tools/include/uapi \
> +              -I$(LIBBPF_INCLUDE) \
> +              -I$(SUBCMD_INCLUDE) \
> +              $(LIBELF_FLAGS)
>
>  LIBS = $(LIBELF_LIBS) -lz
>
> @@ -77,7 +79,7 @@ export srctree OUTPUT CFLAGS Q
>  include $(srctree)/tools/build/Makefile.include
>
>  $(BINARY_IN): fixdep FORCE prepare | $(OUTPUT)
> -       $(Q)$(MAKE) $(build)=resolve_btfids $(HOST_OVERRIDES)
> +       $(Q)$(MAKE) $(build)=resolve_btfids $(HOST_OVERRIDES) CFLAGS="$(BTF_CFLAGS)"
>
>  $(BINARY): $(BPFOBJ) $(SUBCMDOBJ) $(BINARY_IN)
>         $(call msg,LINK,$@)