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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Wed, 28 Sep 2022 15:59:50 -0700
Message-ID: <CAEf4BzZp1oGtOy56noPtx+MimQq5ua7X25mB4Sf05CHLx=1m0g@mail.gmail.com>
Subject: Re: [PATCH 1/2] libbpf: add fPIC option for static library
To:     Xin Liu <liuxin350@huawei.com>
Cc:     ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
        martin.lau@linux.dev, song@kernel.org, yhs@fb.com,
        john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com,
        haoluo@google.com, jolsa@kernel.org, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, yanan@huawei.com,
        wuchangye@huawei.com, xiesongyang@huawei.com, zhudi2@huawei.com,
        kongweibin2@huawei.com
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On Sat, Sep 24, 2022 at 3:13 AM Xin Liu <liuxin350@huawei.com> wrote:
>
> Some programs depned on libbpf.a(eg:bpftool). If libbpf.a miss -fPIC,
> this will cause a similar error at compile time:
>
> /usr/bin/ld: .../libbpf.a(libbpf-in.o): relocation
> R_AARCH64_ADR_PREL_PG_HI21 against symbol `stderr@@GLIBC_2.17' which
> may bind externally can not be used when making a sharedobject;
> recompile with -fPIC
>
> Use -fPIC for static library compilation to solve this problem.
>
> Signed-off-by: Xin Liu <liuxin350@huawei.com>
> ---
>  tools/lib/bpf/Makefile | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
> index 4c904ef0b47e..427e971f4fcd 100644
> --- a/tools/lib/bpf/Makefile
> +++ b/tools/lib/bpf/Makefile
> @@ -91,9 +91,10 @@ override CFLAGS += $(INCLUDES)
>  override CFLAGS += -fvisibility=hidden
>  override CFLAGS += -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
>  override CFLAGS += $(CLANG_CROSS_FLAGS)
> +override CFLAGS += -fPIC
>

It seems wrong to force -fPIC for static library just because in some
situations users might want to statically link their *shared* library
with *static* libbpf. It's a bit unconventional, even though I see
situations in which this might be useful.

But I don't think this can be a default. I see three possible solutions:

1. Do nothing. Let users specify EXTRA_CFLAGS=-fPIC if they need
position-independent static lib
2. Let packagers decide this (again, through EXTRA_CFLAGS or by
patching Makefile, whichever is best). Or maybe build both PIC and
non-PIC static libraries and package both?
3. Produce PIC and non-PIC libbpf.a libraries from libbpf's Makefile.

I'm not sure which one is the best answer, would be nice to hear
opinions of people who do the packaging and distribution of libbpf in
distros.

>  # flags specific for shared library
> -SHLIB_FLAGS := -DSHARED -fPIC
> +SHLIB_FLAGS := -DSHARED
>
>  ifeq ($(VERBOSE),1)
>    Q =
> --
> 2.33.0
>