linux-kernel - Re: [PATCH v4 2/2] tools/resolve_btfids: Alter how HOSTCC is forced

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Date:   Tue, 31 Jan 2023 10:08:33 -0800
From:   Ian Rogers <irogers@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Connor OBrien <connoro@...gle.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v4 2/2] tools/resolve_btfids: Alter how HOSTCC is forced

On Tue, Jan 31, 2023 at 9:21 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> Hi Ian,
>
> On Mon, Jan 23, 2023 at 10:43:24PM -0800, Ian Rogers wrote:
> > HOSTCC is always wanted when building. Setting CC to HOSTCC happens
> > after tools/scripts/Makefile.include is included, meaning flags are
> > set assuming say CC is gcc, but then it can be later set to HOSTCC
> > which may be clang. tools/scripts/Makefile.include is needed for host
> > set up and common macros in objtool's Makefile. Rather than override
> > CC to HOSTCC, just pass CC as HOSTCC to Makefile.build, the libsubcmd
> > builds and the linkage step. This means the Makefiles don't see things
> > like CC changing and tool flag determination, and similar, work
> > properly.
> >
> > Also, clear the passed subdir as otherwise an outer build may break by
> > inadvertently passing an inappropriate value.
> >
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> >  tools/bpf/resolve_btfids/Makefile | 17 +++++++----------
> >  1 file changed, 7 insertions(+), 10 deletions(-)
> >
> > diff --git a/tools/bpf/resolve_btfids/Makefile b/tools/bpf/resolve_btfids/Makefile
> > index 1fe0082b2ecc..daed388aa5d7 100644
> > --- a/tools/bpf/resolve_btfids/Makefile
> > +++ b/tools/bpf/resolve_btfids/Makefile
> > @@ -18,14 +18,11 @@ else
> >  endif
> >
> >  # always use the host compiler
> > -AR       = $(HOSTAR)
> > -CC       = $(HOSTCC)
> > -LD       = $(HOSTLD)
> > -ARCH     = $(HOSTARCH)
> > +HOST_OVERRIDES := AR="$(HOSTAR)" CC="$(HOSTCC)" LD="$(HOSTLD)" ARCH="$(HOSTARCH)" \
> > +               EXTRA_CFLAGS="$(HOSTCFLAGS) $(KBUILD_HOSTCFLAGS)"
> > +
> >  RM      ?= rm
> >  CROSS_COMPILE =
> > -CFLAGS  := $(KBUILD_HOSTCFLAGS)
> > -LDFLAGS := $(KBUILD_HOSTLDFLAGS)
> >
> >  OUTPUT ?= $(srctree)/tools/bpf/resolve_btfids/
> >
> > @@ -56,12 +53,12 @@ $(OUTPUT) $(OUTPUT)/libsubcmd $(LIBBPF_OUT):
> >
> >  $(SUBCMDOBJ): fixdep FORCE | $(OUTPUT)/libsubcmd
> >       $(Q)$(MAKE) -C $(SUBCMD_SRC) OUTPUT=$(SUBCMD_OUT) \
> > -                 DESTDIR=$(SUBCMD_DESTDIR) prefix= \
> > +                 DESTDIR=$(SUBCMD_DESTDIR) $(HOST_OVERRIDES) 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=$(LIBBPF_DESTDIR) prefix= EXTRA_CFLAGS="$(CFLAGS)" \
> > +                 DESTDIR=$(LIBBPF_DESTDIR) $(HOST_OVERRIDES) prefix= subdir= \
> >                   $(abspath $@) install_headers
> >
> >  LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
> > @@ -80,11 +77,11 @@ export srctree OUTPUT CFLAGS Q
> >  include $(srctree)/tools/build/Makefile.include
> >
> >  $(BINARY_IN): fixdep FORCE prepare | $(OUTPUT)
> > -     $(Q)$(MAKE) $(build)=resolve_btfids
> > +     $(Q)$(MAKE) $(build)=resolve_btfids $(HOST_OVERRIDES)
> >
> >  $(BINARY): $(BPFOBJ) $(SUBCMDOBJ) $(BINARY_IN)
> >       $(call msg,LINK,$@)
> > -     $(Q)$(CC) $(BINARY_IN) $(LDFLAGS) -o $@ $(BPFOBJ) $(SUBCMDOBJ) $(LIBS)
> > +     $(Q)$(HOSTCC) $(BINARY_IN) $(KBUILD_HOSTLDFLAGS) -o $@ $(BPFOBJ) $(SUBCMDOBJ) $(LIBS)
> >
> >  clean_objects := $(wildcard $(OUTPUT)/*.o                \
> >                              $(OUTPUT)/.*.o.cmd           \
> > --
> > 2.39.0.246.g2a6d74b583-goog
> >
>
> I just bisected a linking failure when building resolve_btfids with
> clang to this change as commit 13e07691a16f ("tools/resolve_btfids:
> Alter how HOSTCC is forced") in the bpf-next tree.
>
> It appears to be related to whether or not CROSS_COMPILE is specified,
> which we have to do for certain architectures and configurations still.
> arm64 is not one of those but it helps demonstrate the issue.
>
>   # Turn off CONFIG_DEBUG_INFO_REDUCED and turn on CONFIG_DEBUG_INFO_BTF
>   $ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- HOSTLDFLAGS=-fuse-ld=lld LLVM=1 defconfig menuconfig
>
>   $ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- HOSTLDFLAGS=-fuse-ld=lld LLVM=1 prepare
>   ld.lld: error: $LINUX_SRC/tools/bpf/resolve_btfids//resolve_btfids-in.o is incompatible with elf64-x86-64
>   clang-17: error: linker command failed with exit code 1 (use -v to see invocation)
>   ...
>
> Before your change, with V=1, I see:
>
> clang -Wp,-MD,$LINUX_SRC/tools/bpf/resolve_btfids/.main.o.d -Wp,-MT,$LINUX_SRC/tools/bpf/resolve_btfids/main.o -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer -std=gnu11 -Wdeclaration-after-statement -g -I$LINUX_SRC/tools/include -I$LINUX_SRC/tools/include/uapi -I$LINUX_SRC/tools/bpf/resolve_btfids/libbpf/include -I$LINUX_SRC/tools/bpf/resolve_btfids/libsubcmd/include -D"BUILD_STR(s)=#s" -c -o $LINUX_SRC/tools/bpf/resolve_btfids/main.o main.c
>
> After, I see:
>
> clang -Wp,-MD,$LINUX_SRC/tools/bpf/resolve_btfids/.main.o.d -Wp,-MT,$LINUX_SRC/tools/bpf/resolve_btfids/main.o --target=aarch64-linux-gnu -g -I$LINUX_SRC/tools/include -I$LINUX_SRC/tools/include/uapi -I$LINUX_SRC/tools/bpf/resolve_btfids/libbpf/include -I$LINUX_SRC/tools/bpf/resolve_btfids/libsubcmd/include -D"BUILD_STR(s)=#s" -c -o $LINUX_SRC/tools/bpf/resolve_btfids/main.o main.c
>
> We seem to have taken on a '--target=aarch64-linux-gnu' (changing the
> target of resolve_btfids-in.o) and we dropped the warning flags.
>
> I think this comes from the clang block in
> tools/scripts/Makefile.include, which is included into the
> resolve_btfids Makefile via tools/lib/bpf/Makefile.
>
> I am not super familiar with the tools build system, otherwise I would
> try to provide a patch. I tried moving CROSS_COMPILE from a recursive to
> simple variable ('=' -> ':=') and moving it to HOST_OVERRIDES but those
> did not appear to resolve it for me.
>
> If there is any other information I can provide or patches I can test,
> please let me know.
>
> Cheers,
> Nathan

Thanks Nathan, and thanks for all the details in the bug report. I'm
looking into this.

Ian