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

[Akihiko Odaki]

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.