Re: [PATCH v3 bpf-next 11/14] libbpf: makefile: add C/CXX/LDFLAGS to libbpf.so and test_libpf targets

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Yonghong Song <yhs@fb.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <jakub.kicinski@netronome.com>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	john fastabend <john.fastabend@gmail.com>,
	open list <linux-kernel@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	clang-built-linux@googlegroups.com,
	sergei.shtylyov@cogentembedded.com
Subject: Re: [PATCH v3 bpf-next 11/14] libbpf: makefile: add C/CXX/LDFLAGS to libbpf.so and test_libpf targets
Date: Wed, 18 Sep 2019 14:42:32 -0700	[thread overview]
Message-ID: <CAEf4BzZndo1z0A83MmcwtYdstkYKSa9Kzfmf1PRuUtS-D49oeg@mail.gmail.com> (raw)
In-Reply-To: <20190918110517.GD2908@khorivan>

On Wed, Sep 18, 2019 at 4:05 AM Ivan Khoronzhuk
<ivan.khoronzhuk@linaro.org> wrote:
>
> On Tue, Sep 17, 2019 at 10:19:22PM -0700, Andrii Nakryiko wrote:
> >On Mon, Sep 16, 2019 at 4:00 AM Ivan Khoronzhuk
> ><ivan.khoronzhuk@linaro.org> wrote:
> >>
> >> In case of LDFLAGS and EXTRA_CC/CXX flags there is no way to pass them
> >> correctly to build command, for instance when --sysroot is used or
> >> external libraries are used, like -lelf, wich can be absent in
> >> toolchain. This can be used for samples/bpf cross-compiling allowing
> >> to get elf lib from sysroot.
> >>
> >> Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
> >> ---
> >>  tools/lib/bpf/Makefile | 11 ++++++++---
> >>  1 file changed, 8 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
> >> index c6f94cffe06e..bccfa556ef4e 100644
> >> --- a/tools/lib/bpf/Makefile
> >> +++ b/tools/lib/bpf/Makefile
> >> @@ -94,6 +94,10 @@ else
> >>    CFLAGS := -g -Wall
> >>  endif
> >>
> >> +ifdef EXTRA_CXXFLAGS
> >> +  CXXFLAGS := $(EXTRA_CXXFLAGS)
> >> +endif
> >> +
> >>  ifeq ($(feature-libelf-mmap), 1)
> >>    override CFLAGS += -DHAVE_LIBELF_MMAP_SUPPORT
> >>  endif
> >> @@ -176,8 +180,9 @@ $(BPF_IN): force elfdep bpfdep
> >>  $(OUTPUT)libbpf.so: $(OUTPUT)libbpf.so.$(LIBBPF_VERSION)
> >>
> >>  $(OUTPUT)libbpf.so.$(LIBBPF_VERSION): $(BPF_IN)
> >> -       $(QUIET_LINK)$(CC) --shared -Wl,-soname,libbpf.so.$(LIBBPF_MAJOR_VERSION) \
> >> -                                   -Wl,--version-script=$(VERSION_SCRIPT) $^ -lelf -o $@
> >> +       $(QUIET_LINK)$(CC) $(LDFLAGS) \
> >> +               --shared -Wl,-soname,libbpf.so.$(LIBBPF_MAJOR_VERSION) \
> >> +               -Wl,--version-script=$(VERSION_SCRIPT) $^ -lelf -o $@
> >>         @ln -sf $(@F) $(OUTPUT)libbpf.so
> >>         @ln -sf $(@F) $(OUTPUT)libbpf.so.$(LIBBPF_MAJOR_VERSION)
> >>
> >> @@ -185,7 +190,7 @@ $(OUTPUT)libbpf.a: $(BPF_IN)
> >>         $(QUIET_LINK)$(RM) $@; $(AR) rcs $@ $^
> >>
> >>  $(OUTPUT)test_libbpf: test_libbpf.cpp $(OUTPUT)libbpf.a
> >> -       $(QUIET_LINK)$(CXX) $(INCLUDES) $^ -lelf -o $@
> >> +       $(QUIET_LINK)$(CXX) $(CXXFLAGS) $(LDFLAGS) $(INCLUDES) $^ -lelf -o $@
> >
> >Instead of doing ifdef EXTRA_CXXFLAGS bit above, you can just include
> >both $(CXXFLAGS) and $(EXTRA_CXXFLAGS), which will do the right thing
> >(and is actually recommended my make documentation way to do this).
> It's good practice to follow existent style, I've done similar way as for
> CFLAGS + EXTRACFLAGS here, didn't want to verify it can impact on
> smth else. And my goal is not to correct everything but embed my
> functionality, series tool large w/o it.

Alright, we'll have to eventually clean up this Makefile. What we do
with EXTRA_CFLAGS is not exactly correct, as in this Makefile
EXTRA_CFLAGS are overriding CFLAGS, instead of extending them, which
doesn't seem correct to me. BTW, bpftool does += instead of :=. All
this is avoided by just keeping CFLAGS and EXTRA_CFLAGS separate and
specifying both of them in $(CC)/$(CLANG) invocations. But feel free
to ignore this for now.

>
> >
> >But actually, there is no need to use C++ compiler here,
> >test_libbpf.cpp can just be plain C. Do you mind renaming it to .c and
> >using C compiler instead?
> Seems like, will try in next v.

Thanks!

>
> >
> >>
> >>  $(OUTPUT)libbpf.pc:
> >>         $(QUIET_GEN)sed -e "s|@PREFIX@|$(prefix)|" \
> >> --
> >> 2.17.1
> >>
>
> --
> Regards,
> Ivan Khoronzhuk