netdev - Re: [PATCH bpf-next 3/9] libbpf: simplify endianness check

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Date:   Wed, 29 May 2019 10:09:31 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 3/9] libbpf: simplify endianness check

On Tue, May 28, 2019 at 6:14 PM Andrii Nakryiko <andriin@...com> wrote:
>
> Rewrite endianness check to use "more canonical" way, using
> compiler-defined macros, similar to few other places in libbpf. It also
> is more obvious and shorter.
>
> Signed-off-by: Andrii Nakryiko <andriin@...com>
Acked-by: Song Liu <songliubraving@...com>

> ---
>  tools/lib/bpf/libbpf.c | 37 ++++++++++++-------------------------
>  1 file changed, 12 insertions(+), 25 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 7b80b9ae8a1f..c98f9942fba4 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -607,31 +607,18 @@ static int bpf_object__elf_init(struct bpf_object *obj)
>         return err;
>  }
>
> -static int
> -bpf_object__check_endianness(struct bpf_object *obj)
> -{
> -       static unsigned int const endian = 1;
> -
> -       switch (obj->efile.ehdr.e_ident[EI_DATA]) {
> -       case ELFDATA2LSB:
> -               /* We are big endian, BPF obj is little endian. */
> -               if (*(unsigned char const *)&endian != 1)
> -                       goto mismatch;
> -               break;
> -
> -       case ELFDATA2MSB:
> -               /* We are little endian, BPF obj is big endian. */
> -               if (*(unsigned char const *)&endian != 0)
> -                       goto mismatch;
> -               break;
> -       default:
> -               return -LIBBPF_ERRNO__ENDIAN;
> -       }
> -
> -       return 0;
> -
> -mismatch:
> -       pr_warning("Error: endianness mismatch.\n");
> +static int bpf_object__check_endianness(struct bpf_object *obj)
> +{
> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +       if (obj->efile.ehdr.e_ident[EI_DATA] == ELFDATA2LSB)
> +               return 0;
> +#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> +       if (obj->efile.ehdr.e_ident[EI_DATA] == ELFDATA2MSB)
> +               return 0;
> +#else
> +# error "Unrecognized __BYTE_ORDER__"
> +#endif
> +       pr_warning("endianness mismatch.\n");
>         return -LIBBPF_ERRNO__ENDIAN;
>  }
>
> --
> 2.17.1
>