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

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Message-ID: <20220928083256.57679-1-liuxin350@huawei.com>
Date:   Wed, 28 Sep 2022 16:32:56 +0800
From:   Xin Liu <liuxin350@...wei.com>
To:     <quentin@...valent.com>
CC:     <andrii@...nel.org>, <ast@...nel.org>, <bpf@...r.kernel.org>,
        <daniel@...earbox.net>, <haoluo@...gle.com>,
        <john.fastabend@...il.com>, <jolsa@...nel.org>,
        <kongweibin2@...wei.com>, <kpsingh@...nel.org>,
        <linux-kernel@...r.kernel.org>, <liuxin350@...wei.com>,
        <martin.lau@...ux.dev>, <sdf@...gle.com>, <song@...nel.org>,
        <wuchangye@...wei.com>, <xiesongyang@...wei.com>,
        <yanan@...wei.com>, <yhs@...com>, <zhudi2@...wei.com>
Subject: Re: [PATCH 1/2] libbpf: add fPIC option for static library

On Tue, 27 Sep 2022 at 6:21:20 PM Quentin <quentin@...valent.com> wrote:
> Sat Sep 24 2022 11:12:08 GMT+0100 ~ Xin Liu <liuxin350@...wei.com>
> > Some programs depned on libbpf.a(eg:bpftool). If libbpf.a miss -fPIC,
> 
> Typo "depned"
> 
> > 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 += $(CLANG_CROSS_FLAGS)
> > +override CFLAGS += -fPIC
> >  
> >  # flags specific for shared library
> > -SHLIB_FLAGS := -DSHARED -fPIC
> > +SHLIB_FLAGS := -DSHARED
> >  
> >  ifeq ($(VERBOSE),1)
> >    Q =
> 
> Hi, the two patches look OK to me, but it would be nice to have a bit
> more context on what the flags do other than “fixing this particular
> issue” and how they improve bpftool security. It would also be
> interesting to have a note on what it does on various architectures, my
> understanding is that only some archs are supported (I read AArch64,
> m68k, PowerPC and SPARC), I guess the flags are silently ignored on x86
> for example?
> 
> Thanks,
> Quentin
> 
This advice is very useful to me. Thank you very much for your reply.
I'll fix it in the V2 version.

Thansk,
Xin Liu