Re: [PATCH 3/3] bpftool: Allow to link libbpf dynamically

[Quentin Monnet]

2019-11-27 10:48 UTC+0100 ~ Jiri Olsa <jolsa@xxxxxxxxxx>
> 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: libbpf-devel is missing, please install it.  Stop.
> 
> Adding specific bpftool's libbpf check for libbpf_netlink_open (LIBBPF_0.0.6)
> which is the latest we need for bpftool at the moment.
> 
> 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
> 
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  tools/bpf/bpftool/Makefile        | 40 ++++++++++++++++++++++++++++++-
>  tools/build/feature/test-libbpf.c |  9 +++++++
>  2 files changed, 48 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
> index 39bc6f0f4f0b..2b6ed08cb31e 100644
> --- a/tools/bpf/bpftool/Makefile
> +++ b/tools/bpf/bpftool/Makefile

> @@ -55,7 +64,7 @@ ifneq ($(EXTRA_LDFLAGS),)
>  LDFLAGS += $(EXTRA_LDFLAGS)
>  endif
>  
> -LIBS = $(LIBBPF) -lelf -lz
> +LIBS = -lelf -lz

Hi Jiri,

This change seems to be breaking the build with the static library for
me. I know you add back $(LIBBPF) later in the Makefile, see at the end
of this email...

>  
>  INSTALL ?= install
>  RM ?= rm -f
> @@ -64,6 +73,23 @@ FEATURE_USER = .bpftool
>  FEATURE_TESTS = libbfd disassembler-four-args reallocarray zlib
>  FEATURE_DISPLAY = libbfd disassembler-four-args zlib
>  
> +ifdef LIBBPF_DYNAMIC
> +  # Add libbpf check with the flags to ensure bpftool
> +  # specific version is detected.

Nit: We do not check for a specific bpftool version, we check for a
recent enough libbpf version?

> +  FEATURE_CHECK_CFLAGS-libbpf := -DBPFTOOL
> +  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 := $(LIBBPF_LDFLAGS)
> +  endif
> +endif
> +
>  check_feat := 1
>  NON_CHECK_FEAT_TARGETS := clean uninstall doc doc-clean doc-install doc-uninstall
>  ifdef MAKECMDGOALS
> @@ -88,6 +114,18 @@ ifeq ($(feature-reallocarray), 0)
>  CFLAGS += -DCOMPAT_NEED_REALLOCARRAY
>  endif
>  
> +ifdef LIBBPF_DYNAMIC
> +  ifeq ($(feature-libbpf), 1)
> +    LIBS    += -lbpf
> +    CFLAGS  += $(LIBBPF_CFLAGS)
> +    LDFLAGS += $(LIBBPF_LDFLAGS)
> +  else
> +    dummy := $(error Error: No libbpf devel library found, please install libbpf-devel)

libbpf-devel sounds like a RH/Fedora package name, but other
distributions might have different names (Debian/Ubuntu would go by
libbpf-dev I suppose, although I don't believe such package exists at
the moment). Maybe use a more generic message?

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

... I believe the order of the libraries is relevant, and it seems the
static libbpf should be passed before the dynamic libs. Here I could fix
the build with the static library on my setup by prepending the library
path instead, like this:

	LIBS := $(LIBBPF) $(LIBS)

On the plus side, all build attempts from
tools/testing/selftests/bpf/test_bpftool_build.sh pass successfully on
my setup with dynamic linking from your branch.

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

Thanks,
Quentin