Re: [PATCH bpf-next 2/3] libbpf: Fix relocating big-endian bitfields

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Fri, 2021-10-22 at 01:46 +0200, Ilya Leoshkevich wrote:
> This is the same as commit c9e982b87946 ("libbpf: Fix dumping
> big-endian bitfields"), but for CO-RE. Make the code structure as
> similar as possible to that of btf_dump_get_bitfield_value().
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>  tools/lib/bpf/relo_core.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/bpf/relo_core.c b/tools/lib/bpf/relo_core.c
> index b5b8956a1be8..fd814b985e1e 100644
> --- a/tools/lib/bpf/relo_core.c
> +++ b/tools/lib/bpf/relo_core.c
> @@ -661,13 +661,18 @@ static int bpf_core_calc_field_relo(const char
> *prog_name,
>                 if (validate)
>                         *validate = true; /* signedness is never
> ambiguous */
>                 break;
> -       case BPF_FIELD_LSHIFT_U64:
> +       case BPF_FIELD_LSHIFT_U64: {
> +               __u32 bits_offset = bit_off - byte_off * 8;
> +               __u8 nr_copy_bits;
> +
>  #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> -               *val = 64 - (bit_off + bit_sz - byte_off  * 8);
> +               nr_copy_bits = bit_sz + bits_offset;
>  #else
> -               *val = (8 - byte_sz) * 8 + (bit_off - byte_off * 8);
> +               nr_copy_bits = byte_sz * 8 - bits_offset;
>  #endif
> +               *val = 64 - nr_copy_bits;
>                 break;
> +       }
>         case BPF_FIELD_RSHIFT_U64:
>                 *val = 64 - bit_sz;
>                 if (validate)

At a closer look this patch is not necessary: the new and the old
expressions yield the same result. Please disregard it.

Best regards,
Ilya