Re: [PATCH bpf v3] bpftool: Allow to link libbpf dynamically

Toke Høiland-Jørgensen
2019-11-29T08:25:01Z

Jiri Olsa <jolsa@redhat.com> writes:

> On Thu, Nov 28, 2019 at 05:07:12PM +0100, Toke Høiland-Jørgensen wrote:
>
> SNIP
>
>> ifeq ($(srctree),)
>> srctree := $(patsubst %/,%,$(dir $(CURDIR)))
>> @@ -63,6 +72,19 @@ RM ?= rm -f
>> FEATURE_USER = .bpftool
>> FEATURE_TESTS = libbfd disassembler-four-args reallocarray zlib
>> FEATURE_DISPLAY = libbfd disassembler-four-args zlib
>> +ifdef LIBBPF_DYNAMIC
>> + FEATURE_TESTS += libbpf
>> + FEATURE_DISPLAY += libbpf
>> +
>> + # for linking with debug library run:
>> + # make LIBBPF_DYNAMIC=1 LIBBPF_DIR=/opt/libbpf
>> + ifdef LIBBPF_DIR
>> + LIBBPF_CFLAGS := -I$(LIBBPF_DIR)/include
>> + LIBBPF_LDFLAGS := -L$(LIBBPF_DIR)/$(libdir_relative)
>> + FEATURE_CHECK_CFLAGS-libbpf := $(LIBBPF_CFLAGS)
>> + FEATURE_CHECK_LDFLAGS-libbpf := $(LIBBPF_LDFLAGS)
>> + endif
>> +endif
>>
>> check_feat := 1
>> NON_CHECK_FEAT_TARGETS := clean uninstall doc doc-clean doc-install doc-uninstall
>> @@ -88,6 +110,18 @@ ifeq ($(feature-reallocarray), 0)
>> CFLAGS += -DCOMPAT_NEED_REALLOCARRAY
>> endif
>>
>> +ifdef LIBBPF_DYNAMIC
>> + ifeq ($(feature-libbpf), 1)
>> + # bpftool uses non-exported functions from libbpf, so just add the dynamic
>> + # version of libbpf and let the linker figure it out
>> + LIBS := -lbpf $(LIBS)
>
> nice, so linker will pick up the missing symbols and we
> don't need to check on particular libbpf version then

Yup, exactly. I verified with objdump that the end result is a
dynamically linked bpftool with LIBBPF_DYNAMIC is set, and a statically
linked one if it isn't; so the linker seems to be smart enough to just
figure out how to do the right thing :)

-Toke