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

From: Ivan Khoronzhuk <[email-masked]>
Date: Wed, 18 Sep 2019 14:05:18 +0300

On Tue, Sep 17, 2019 at 10:19:22PM -0700, Andrii Nakryiko wrote:
>On Mon, Sep 16, 2019 at 4:00 AM Ivan Khoronzhuk
><[email-masked]> 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 <[email-masked]>
>> ---
>>  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.

>
>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.

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

-- 
Regards,
Ivan Khoronzhuk