netdev - Re: [PATCH bpf v1] tools/libbpf: avoid counting local symbols in ABI check

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Date:   Tue, 8 Sep 2020 12:53:10 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Tony Ambardar <tony.ambardar@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Andrey Ignatov <rdna@...com>
Subject: Re: [PATCH bpf v1] tools/libbpf: avoid counting local symbols in ABI check

On Sat, Sep 5, 2020 at 2:49 PM Tony Ambardar <tony.ambardar@...il.com> wrote:
>
> Encountered the following failure building libbpf from kernel 5.8.5 sources
> with GCC 8.4.0 and binutils 2.34: (long paths shortened)
>
>   Warning: Num of global symbols in sharedobjs/libbpf-in.o (234) does NOT
>   match with num of versioned symbols in libbpf.so (236). Please make sure
>   all LIBBPF_API symbols are versioned in libbpf.map.
>   --- libbpf_global_syms.tmp    2020-09-02 07:30:58.920084380 +0000
>   +++ libbpf_versioned_syms.tmp 2020-09-02 07:30:58.924084388 +0000
>   @@ -1,3 +1,5 @@
>   +_fini
>   +_init
>    bpf_btf_get_fd_by_id
>    bpf_btf_get_next_id
>    bpf_create_map
>   make[4]: *** [Makefile:210: check_abi] Error 1
>
> Investigation shows _fini and _init are actually local symbols counted
> amongst global ones:
>
>   $ readelf --dyn-syms --wide libbpf.so|head -10
>
>   Symbol table '.dynsym' contains 343 entries:
>      Num:    Value  Size Type    Bind   Vis      Ndx Name
>        0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
>        1: 00004098     0 SECTION LOCAL  DEFAULT   11
>        2: 00004098     8 FUNC    LOCAL  DEFAULT   11 _init@@LIBBPF_0.0.1
>        3: 00023040     8 FUNC    LOCAL  DEFAULT   14 _fini@@LIBBPF_0.0.1
>        4: 00000000     0 OBJECT  GLOBAL DEFAULT  ABS LIBBPF_0.0.4
>        5: 00000000     0 OBJECT  GLOBAL DEFAULT  ABS LIBBPF_0.0.1
>        6: 0000ffa4     8 FUNC    GLOBAL DEFAULT   12 bpf_object__find_map_by_offset@@LIBBPF_0.0.1
>
> A previous commit filtered global symbols in sharedobjs/libbpf-in.o. Do the
> same with the libbpf.so DSO for consistent comparison.
>
> Fixes: 306b267cb3c4 ("libbpf: Verify versioned symbols")
>
> Signed-off-by: Tony Ambardar <Tony.Ambardar@...il.com>
> ---

LGTM, thanks!

Acked-by: Andrii Nakryiko <andriin@...com>

>  tools/lib/bpf/Makefile | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
> index b78484e7a608..9ae8f4ef0aac 100644
> --- a/tools/lib/bpf/Makefile
> +++ b/tools/lib/bpf/Makefile
> @@ -152,6 +152,7 @@ GLOBAL_SYM_COUNT = $(shell readelf -s --wide $(BPF_IN_SHARED) | \
>                            awk '/GLOBAL/ && /DEFAULT/ && !/UND/ {print $$NF}' | \
>                            sort -u | wc -l)
>  VERSIONED_SYM_COUNT = $(shell readelf --dyn-syms --wide $(OUTPUT)libbpf.so | \
> +                             awk '/GLOBAL/ && /DEFAULT/ && !/UND/ {print $$NF}' | \
>                               grep -Eo '[^ ]+@...BPF_' | cut -d@ -f1 | sort -u | wc -l)
>
>  CMD_TARGETS = $(LIB_TARGET) $(PC_FILE)
> @@ -219,6 +220,7 @@ check_abi: $(OUTPUT)libbpf.so
>                     awk '/GLOBAL/ && /DEFAULT/ && !/UND/ {print $$NF}'|  \
>                     sort -u > $(OUTPUT)libbpf_global_syms.tmp;           \
>                 readelf --dyn-syms --wide $(OUTPUT)libbpf.so |           \
> +                   awk '/GLOBAL/ && /DEFAULT/ && !/UND/ {print $$NF}'|  \
>                     grep -Eo '[^ ]+@...BPF_' | cut -d@ -f1 |             \
>                     sort -u > $(OUTPUT)libbpf_versioned_syms.tmp;        \
>                 diff -u $(OUTPUT)libbpf_global_syms.tmp                  \
> --
> 2.25.1
>