Re: [PATCH bpf v2] bpftool: Allow to link libbpf dynamically

[Quentin Monnet <quentin.monnet@netronome.com>]

2019-11-28 15:53 UTC+0100 ~ Toke Høiland-Jørgensen <toke@redhat.com>
> From: Jiri Olsa <jolsa@kernel.org>
> 
> Currently we support only static linking with kernel's libbpf
> (tools/lib/bpf). This patch adds LIBBPF_DYNAMIC compile variable
> that triggers libbpf detection and bpf dynamic linking:
> 
>   $ make -C tools/bpf/bpftool make LIBBPF_DYNAMIC=1
> 
> If libbpf is not installed, build (with LIBBPF_DYNAMIC=1) stops with:
> 
>   $ make -C tools/bpf/bpftool LIBBPF_DYNAMIC=1
>     Auto-detecting system features:
>     ...                        libbfd: [ on  ]
>     ...        disassembler-four-args: [ on  ]
>     ...                          zlib: [ on  ]
>     ...                        libbpf: [ OFF ]
> 
>   Makefile:102: *** Error: No libbpf devel library found, please install-devel or libbpf-dev.
> 
> Adding LIBBPF_DIR compile variable to allow linking with
> libbpf installed into specific directory:
> 
>   $ make -C tools/lib/bpf/ prefix=/tmp/libbpf/ install_lib install_headers
>   $ make -C tools/bpf/bpftool/ LIBBPF_DYNAMIC=1 LIBBPF_DIR=/tmp/libbpf/
> 
> It might be needed to clean build tree first because features
> framework does not detect the change properly:
> 
>   $ make -C tools/build/feature clean
>   $ make -C tools/bpf/bpftool/ clean
> 
> Since bpftool uses bits of libbpf that are not exported as public API in
> the .so version, we also pass in libbpf.a to the linker, which allows it to
> pick up the private functions from the static library without having to
> expose them as ABI.
> 
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
> v2:
>   - Pass .a file to linker when dynamically linking, so bpftool can use
>     private functions from libbpf without exposing them as API.
>     
>  tools/bpf/bpftool/Makefile | 38 +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
> index 39bc6f0f4f0b..397051ed9e41 100644
> --- a/tools/bpf/bpftool/Makefile
> +++ b/tools/bpf/bpftool/Makefile
> @@ -1,6 +1,15 @@
>  # SPDX-License-Identifier: GPL-2.0-only
> +# LIBBPF_DYNAMIC to enable libbpf dynamic linking.
> +
>  include ../../scripts/Makefile.include
>  include ../../scripts/utilities.mak
> +include ../../scripts/Makefile.arch
> +
> +ifeq ($(LP64), 1)
> +  libdir_relative = lib64
> +else
> +  libdir_relative = lib
> +endif
>  
>  ifeq ($(srctree),)
>  srctree := $(patsubst %/,%,$(dir $(CURDIR)))
> @@ -55,7 +64,7 @@ ifneq ($(EXTRA_LDFLAGS),)
>  LDFLAGS += $(EXTRA_LDFLAGS)
>  endif
>  
> -LIBS = $(LIBBPF) -lelf -lz
> +LIBS = -lelf -lz

Hi Toke,

You don't need to remove $(LIBBPF) here, because you add it in both
cases below (whether $(LIBBPF_DYNAMIC) is defined or not).

>  
>  INSTALL ?= install
>  RM ?= rm -f
> @@ -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 := $(LIBBPF_LDFLAGS)
> +  endif
> +endif
>  
>  check_feat := 1
>  NON_CHECK_FEAT_TARGETS := clean uninstall doc doc-clean doc-install doc-uninstall
> @@ -88,6 +110,20 @@ ifeq ($(feature-reallocarray), 0)
>  CFLAGS += -DCOMPAT_NEED_REALLOCARRAY
>  endif
>  
> +ifdef LIBBPF_DYNAMIC
> +  ifeq ($(feature-libbpf), 1)
> +    # bpftool uses non-exported functions from libbpf, so pass both dynamic and
> +    # static versions and let the linker figure it out
> +    LIBS    := -lbpf $(LIBBPF) $(LIBS)

[$(LIBBPF) added to $(LIBS) here...]

> +    CFLAGS  += $(LIBBPF_CFLAGS)
> +    LDFLAGS += $(LIBBPF_LDFLAGS)
> +  else
> +    dummy := $(error Error: No libbpf devel library found, please install-devel or libbpf-dev.)

“install-devel” :)

> +  endif
> +else
> +  LIBS := $(LIBBPF) $(LIBS)

[... and here]

> +endif
> +
>  include $(wildcard $(OUTPUT)*.d)
>  
>  all: $(OUTPUT)bpftool
> 

Thanks,
Quentin