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Message-ID: <512a8ed7-4321-4ffe-a569-da1bee288986@daynix.com>
Date:   Wed, 18 Oct 2023 04:26:41 +0900
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Subject: Re: [PATCH bpf-next v4] selftests/bpf: Use pkg-config to determine ld
 flags
Content-Language: en-US
To:     Daniel Borkmann <daniel@iogearbox.net>
Cc:     Andrii Nakryiko <andrii@kernel.org>,
        Mykola Lysenko <mykolal@fb.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>,
        Yonghong Song <yonghong.song@linux.dev>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Shuah Khan <shuah@kernel.org>, Nick Terrell <terrelln@fb.com>,
        bpf@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-kernel@vger.kernel.org, bjorn@kernel.org
References: <20231016130307.35104-1-akihiko.odaki@daynix.com>
 <4037a83a-c6b6-6eab-1cb1-93339686c4e5@iogearbox.net>
From:   Akihiko Odaki <akihiko.odaki@daynix.com>
In-Reply-To: <4037a83a-c6b6-6eab-1cb1-93339686c4e5@iogearbox.net>
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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@daynix.com>
>> ---
>> 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.