netdev - Re: [PATCH bpf-next] libbpf: ignore .eh_frame sections when parsing elf files

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Date:   Fri, 2 Jul 2021 18:10:13 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next] libbpf: ignore .eh_frame sections when parsing
 elf files

On 6/29/21 1:09 PM, Toke Høiland-Jørgensen wrote:
> The .eh_frame and .rel.eh_frame sections will be present in BPF object
> files when compiled using a multi-stage compile pipe like in samples/bpf.
> This produces errors when loading such a file with libbpf. While the errors
> are technically harmless, they look odd and confuse users. So add .eh_frame
> sections to is_sec_name_dwarf() so they will also be ignored by libbpf
> processing. This gets rid of output like this from samples/bpf:
> 
> libbpf: elf: skipping unrecognized data section(32) .eh_frame
> libbpf: elf: skipping relo section(33) .rel.eh_frame for section(32) .eh_frame
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>

For the samples/bpf case, could we instead just add a -fno-asynchronous-unwind-tables
to clang as cflags to avoid .eh_frame generation in the first place?

> ---
>   tools/lib/bpf/libbpf.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 1e04ce724240..676af6be5961 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -2906,7 +2906,8 @@ static Elf_Data *elf_sec_data(const struct bpf_object *obj, Elf_Scn *scn)
>   static bool is_sec_name_dwarf(const char *name)
>   {
>   	/* approximation, but the actual list is too long */
> -	return strncmp(name, ".debug_", sizeof(".debug_") - 1) == 0;
> +	return (strncmp(name, ".debug_", sizeof(".debug_") - 1) == 0 ||
> +		strncmp(name, ".eh_frame", sizeof(".eh_frame") - 1) == 0);
>   }
>   
>   static bool ignore_elf_section(GElf_Shdr *hdr, const char *name)
> 

Thanks,
Daniel