Re: [PATCH bpf-next] selftests/bpf: Use pkg-config to determine ld flags

[Daniel Borkmann <daniel@iogearbox.net>]

On 10/15/23 3:39 PM, Akihiko Odaki wrote:
> When linking statically, libraries may require other dependencies to be
> included to ld flags. In particular, libelf may require libzstd. Use
> pkg-config to determine such dependencies.

Is this not covered via -lz or is it that the name differs?

Anyway, this change breaks selftest build for BPF CI (see below), could this
either be made optional or detected differently?

https://github.com/kernel-patches/bpf/actions/runs/6524480596/job/17716170021

[...]
   make: pkg-config: Command not found
     CC       bench.o
   make: pkg-config: Command not found
   make: pkg-config: Command not found
     CC       bench_count.o
   make: pkg-config: Command not found
     BINARY   xdp_redirect_multi
     HOSTLD  /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/fixdep-in.o
     LINK    /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/fixdep
     CC       veristat.o
     INSTALL /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd//include/subcmd/exec-cmd.h
     CC      /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd/exec-cmd.o
   make: pkg-config: Command not found
     INSTALL /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd//include/subcmd/help.h
     CC      /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd/help.o
     INSTALL /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd//include/subcmd/pager.h
   make: pkg-config: Command not found
     BINARY   test_verifier
     CC      /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd/pager.o
     BINARY   test_tag
     CC      /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd/parse-options.o
   make: pkg-config: Command not found
     CC      /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd/run-command.o
     INSTALL /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd//include/subcmd/parse-options.h
   make: pkg-config: Command not found
     INSTALL /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd//include/subcmd/run-command.h
     CC      /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd/sigchain.o
     BINARY   flow_dissector_load
     CC      /tmp/work/bpf/bpf/tools/testing/selftests/bpf/tools/build/resolve_btfids/libsubcmd/subcmd-config.o
   make: pkg-config: Command not found
     BINARY   test_lirc_mode2_user
     INSTALL libsubcmd_headers
     BINARY   xdping
   make: pkg-config: Command not found

>   tools/testing/selftests/bpf/Makefile   | 3 ++-
>   tools/testing/selftests/bpf/README.rst | 2 +-
>   2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index caede9b574cb..833134aa2eda 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -4,6 +4,7 @@ include ../../../scripts/Makefile.arch
>   include ../../../scripts/Makefile.include
>   
>   CXX ?= $(CROSS_COMPILE)g++
> +PKG_CONFIG ?= $(CROSS_COMPILE)pkg-config
>   
>   CURDIR := $(abspath .)
>   TOOLSDIR := $(abspath ../../..)
> @@ -31,7 +32,7 @@ CFLAGS += -g -O0 -rdynamic -Wall -Werror $(GENFLAGS) $(SAN_CFLAGS)	\
>   	  -I$(CURDIR) -I$(INCLUDE_DIR) -I$(GENDIR) -I$(LIBDIR)		\
>   	  -I$(TOOLSINCDIR) -I$(APIDIR) -I$(OUTPUT)
>   LDFLAGS += $(SAN_LDFLAGS)
> -LDLIBS += -lelf -lz -lrt -lpthread
> +LDLIBS += $(shell $(PKG_CONFIG) --libs libelf zlib) -lrt -lpthread
>   
>   ifneq ($(LLVM),)
>   # Silence some warnings when compiled with clang
> diff --git a/tools/testing/selftests/bpf/README.rst b/tools/testing/selftests/bpf/README.rst
> index cb9b95702ac6..9af79c7a9b58 100644
> --- a/tools/testing/selftests/bpf/README.rst
> +++ b/tools/testing/selftests/bpf/README.rst
> @@ -77,7 +77,7 @@ In case of linker errors when running selftests, try using static linking:
>   
>   .. code-block:: console
>   
> -  $ LDLIBS=-static vmtest.sh
> +  $ LDLIBS=-static PKG_CONFIG='pkg-config --static' vmtest.sh
>   
>   .. note:: Some distros may not support static linking.
>   
>