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

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Date:   Tue, 27 Sep 2022 11:21:20 +0100
From:   Quentin Monnet <quentin@...valent.com>
To:     Xin Liu <liuxin350@...wei.com>, 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
Cc:     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

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