Re: [PATCH bpf-next 3/3] libbpf: Fix dumping __int128

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Oct 12, 2021 at 4:32 AM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> On s390 __int128 can be 8-byte aligned, therefore in libbpf will
> occasionally consider variables of this type non-aligned and try to
> dump them as a bitfield, which is supported for at most 64-bit
> integers.
>
> Fix by using the same trick as btf_dump_float_data(): copy non-aligned
> values to the local buffer.
>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>  tools/lib/bpf/btf_dump.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> index ab45771d0cb4..d8264c1762e8 100644
> --- a/tools/lib/bpf/btf_dump.c
> +++ b/tools/lib/bpf/btf_dump.c
> @@ -1672,9 +1672,10 @@ static int btf_dump_int_data(struct btf_dump *d,
>  {
>         __u8 encoding = btf_int_encoding(t);
>         bool sign = encoding & BTF_INT_SIGNED;
> +       char buf[16] __aligned(16);
>         int sz = t->size;
>
> -       if (sz == 0) {
> +       if (sz == 0 || sz > sizeof(buf)) {
>                 pr_warn("unexpected size %d for id [%u]\n", sz, type_id);
>                 return -EINVAL;
>         }
> @@ -1682,8 +1683,10 @@ static int btf_dump_int_data(struct btf_dump *d,
>         /* handle packed int data - accesses of integers not aligned on
>          * int boundaries can cause problems on some platforms.
>          */
> -       if (!ptr_is_aligned(data, sz))
> -               return btf_dump_bitfield_data(d, t, data, 0, 0);
> +       if (!ptr_is_aligned(data, sz)) {

I think ptr_is_aligned() logic is wrong. We should probably not assume
that __int128 has 16-byte alignment. Can you try fixing it by using
btf__align_of() API to get the natural alignment?

> +               memcpy(buf, data, sz);
> +               data = buf;
> +       }
>
>         switch (sz) {
>         case 16: {
> --
> 2.31.1
>