Re: [PATCH bpf-next v4] selftests/bpf: Use pkg-config to determine ld flags

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On 2023/10/17 23:15, Daniel Borkmann wrote:
On 10/16/23 3:03 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.

Signed-off-by: Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>
---
V3 -> V4: Added "2> /dev/null".
V2 -> V3: Added missing "echo".
V1 -> V2: Implemented fallback, referring to HOSTPKG_CONFIG.

  tools/testing/selftests/bpf/Makefile   | 4 +++-
  tools/testing/selftests/bpf/README.rst | 2 +-
  2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index caede9b574cb..009e907a8abe 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 +32,8 @@ 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 $(PKG_CONFIG) --libs libelf zlib 2> /dev/null || echo -lelf -lz)    \
+      -lrt -lpthread
  ifneq ($(LLVM),)
  # Silence some warnings when compiled with clang

Staring at tools/bpf/resolve_btfids/Makefile, I'm trying to understand why we
cannot replicate something similar for BPF selftests?

For example, with your patch, why is it necessary to now have PKG_CONFIG and
another HOSTPKG_CONFIG var?

It's because at least Debian does have wrappers of pkg-config for cross compile targets. You can find them below:
https://packages.debian.org/search?searchon=contents&keywords=pkg-config&mode=path&suite=stable&arch=any


What about the below?

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 4225f975fce3..62166d2f937d 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -29,13 +29,17 @@ SAN_CFLAGS  ?=
  SAN_LDFLAGS    ?= $(SAN_CFLAGS)
  RELEASE                ?=
  OPT_FLAGS      ?= $(if $(RELEASE),-O2,-O0)
+
+LIBELF_FLAGS   := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
+LIBELF_LIBS    := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)

Having dedicated variables and checking --cflags are a good idea.



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