netdev - Re: [PATCH iproute2] bpf: check map symbol type properly with newer llvm compiler

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Date:   Tue, 30 Oct 2018 01:08:18 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>, ast@...com, netdev@...r.kernel.org
Cc:     kernel-team@...com
Subject: Re: [PATCH iproute2] bpf: check map symbol type properly with newer
 llvm compiler

On 10/29/2018 11:32 PM, Yonghong Song wrote:
> With llvm 7.0 or earlier, the map symbol type is STT_NOTYPE.
>   -bash-4.4$ cat t.c
>   __attribute__((section("maps"))) int g;
>   -bash-4.4$ clang -target bpf -O2 -c t.c
>   -bash-4.4$ readelf -s t.o
> 
>   Symbol table '.symtab' contains 2 entries:
>      Num:    Value          Size Type    Bind   Vis      Ndx Name
>        0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
>        1: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT    3 g
>   -bash-4.4$
> 
> The following llvm commit enables BPF target to generate
> proper symbol type and size.
>   commit bf6ec206615b9718869d48b4e5400d0c6e3638dd
>   Author: Yonghong Song <yhs@...com>
>   Date:   Wed Sep 19 16:04:13 2018 +0000
> 
>       [bpf] Symbol sizes and types in object file
> 
>       Clang-compiled object files currently don't include the symbol sizes and
>       types.  Some tools however need that information.  For example, ctfconvert
>       uses that information to generate FreeBSD's CTF representation from ELF
>       files.
>       With this patch, symbol sizes and types are included in object files.
> 
>       Signed-off-by: Paul Chaignon <paul.chaignon@...nge.com>
>       Reported-by: Yutaro Hayakawa <yhayakawa3720@...il.com>
> 
> Hence, for llvm 8.0.0 (currently trunk), symbol type will be not NOTYPE, but OBJECT.
>   -bash-4.4$ clang -target bpf -O2 -c t.c
>   -bash-4.4$ readelf -s t.o
> 
>   Symbol table '.symtab' contains 3 entries:
>      Num:    Value          Size Type    Bind   Vis      Ndx Name
>        0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
>        1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS t.c
>        2: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    3 g
> -bash-4.4$
> 
> This patch makes sure bpf library accepts both NOTYPE and OBJECT types
> of global map symbols.
> 
> Signed-off-by: Yonghong Song <yhs@...com>

Thanks!

Acked-by: Daniel Borkmann <daniel@...earbox.net>