linux-kernel - Re: [PATCH 1/2] libbpf: add fPIC option for static library

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Message-ID: <6336516b63153_233df208c4@john.notmuch>
Date:   Thu, 29 Sep 2022 19:16:11 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Xin Liu <liuxin350@...wei.com>
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, yanan@...wei.com,
        wuchangye@...wei.com, xiesongyang@...wei.com, zhudi2@...wei.com,
        kongweibin2@...wei.com
Subject: Re: [PATCH 1/2] libbpf: add fPIC option for static library

Andrii Nakryiko wrote:
> On Sat, Sep 24, 2022 at 3:13 AM Xin Liu <liuxin350@...wei.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@...wei.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.

Not a distro or pkg maintainer but my $.02 is I would just leave it
for 1 and 2.

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