netdev - Re: [PATCH bpf] libbpf: add macro __BUILD_STATIC_LIBBPF_

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Date:   Fri, 27 Sep 2019 14:48:20 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, Alexei Starovoitov <ast@...com>,
        kernel-team@...com
Subject: Re: [PATCH bpf] libbpf: add macro __BUILD_STATIC_LIBBPF__ to guard
 .symver

On Thu, Sep 26, 2019 at 04:02:04PM -0700, Yonghong Song wrote:
> bcc uses libbpf repo as a submodule. It brings in libbpf source
> code and builds everything together to produce shared libraries.
> With latest libbpf, I got the following errors:
>   /bin/ld: libbcc_bpf.so.0.10.0: version node not found for symbol xsk_umem__create@...BPF_0.0.2
>   /bin/ld: failed to set dynamic section sizes: Bad value
>   collect2: error: ld returned 1 exit status
>   make[2]: *** [src/cc/libbcc_bpf.so.0.10.0] Error 1
> 
> In xsk.c, we have
>   asm(".symver xsk_umem__create_v0_0_2, xsk_umem__create@...BPF_0.0.2");
>   asm(".symver xsk_umem__create_v0_0_4, xsk_umem__create@@LIBBPF_0.0.4");
> The linker thinks the built is for LIBBPF but cannot find proper version
> LIBBPF_0.0.2/4, so emit errors.
> 
> I also confirmed that using libbpf.a to produce a shared library also
> has issues:
>   -bash-4.4$ cat t.c
>   extern void *xsk_umem__create;
>   void * test() { return xsk_umem__create; }
>   -bash-4.4$ gcc -c t.c
>   -bash-4.4$ gcc -shared t.o libbpf.a -o t.so
>   /bin/ld: t.so: version node not found for symbol xsk_umem__create@...BPF_0.0.2
>   /bin/ld: failed to set dynamic section sizes: Bad value
>   collect2: error: ld returned 1 exit status
>   -bash-4.4$
> 
> To fix the problem, I simply added a macro __BUILD_STATIC_LIBBPF__
> which will prevent issuing .symver assembly codes when enabled.
> The .symver assembly codes are still issued by default.
> This will at least give other libbpf users to build libbpf
> without these versioned symbols.
> 
> I did not touch Makefile to actually use this macro to build
> static library as I want to check whether this is desirable or not.

Isn't there any better way on how we can detect this? Asking users to
pass this macro to the build seems a suboptimal user experience. How
are other libraries solving this given this seems really not specific
to libbpf?

> Signed-off-by: Yonghong Song <yhs@...com>
> ---
>  tools/lib/bpf/xsk.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index 24fa313524fb..76c12c4c5c70 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -261,8 +261,11 @@ int xsk_umem__create_v0_0_2(struct xsk_umem **umem_ptr, void *umem_area,
>  	return xsk_umem__create_v0_0_4(umem_ptr, umem_area, size, fill, comp,
>  					&config);
>  }
> +
> +#ifndef __BUILD_STATIC_LIBBPF__
>  asm(".symver xsk_umem__create_v0_0_2, xsk_umem__create@...BPF_0.0.2");
>  asm(".symver xsk_umem__create_v0_0_4, xsk_umem__create@@LIBBPF_0.0.4");
> +#endif
>  
>  static int xsk_load_xdp_prog(struct xsk_socket *xsk)
>  {
> -- 
> 2.17.1
>