netdev - Re: [PATCH bpf] tools: bpftool: fix a bitfield pretty print issue

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Date:   Wed, 28 Nov 2018 15:28:11 -0800
From:   Song Liu <liu.song.a23@...il.com>
To:     yhs@...com
Cc:     ast@...com, Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>, kernel-team@...com
Subject: Re: [PATCH bpf] tools: bpftool: fix a bitfield pretty print issue

On Wed, Nov 28, 2018 at 10:09 AM Yonghong Song <yhs@...com> wrote:
>
> Commit b12d6ec09730 ("bpf: btf: add btf print functionality")
> added btf pretty print functionality to bpftool.
> There is a problem though in printing a bitfield whose type
> has modifiers.
>
> For example, for a type like
>   typedef int ___int;
>   struct tmp_t {
>           int a:3;
>           ___int b:3;
>   };
> Suppose we have a map
>   struct bpf_map_def SEC("maps") tmpmap = {
>           .type = BPF_MAP_TYPE_HASH,
>           .key_size = sizeof(__u32),
>           .value_size = sizeof(struct tmp_t),
>           .max_entries = 1,
>   };
> and the hash table is populated with one element with
> key 0 and value (.a = 1 and .b = 2).
>
> In BTF, the struct member "b" will have a type "typedef" which
> points to an int type. The current implementation does not
> pass the bit offset during transition from typedef to int type,
> hence incorrectly print the value as
>   $ bpftool m d id 79
>   [{
>           "key": 0,
>           "value": {
>               "a": 0x1,
>               "b": 0x1
>           }
>       }
>   ]
>
> This patch fixed the issue by carrying bit_offset along the type
> chain during bit_field print. The correct result can be printed as
>   $ bpftool m d id 76
>   [{
>           "key": 0,
>           "value": {
>               "a": 0x1,
>               "b": 0x2
>           }
>       }
>   ]
>
> The kernel pretty print is implemented correctly and does not
> have this issue.
>
> Fixes: b12d6ec09730 ("bpf: btf: add btf print functionality")
> Signed-off-by: Yonghong Song <yhs@...com>

Acked-by: Song Liu <songliubraving@...com>

> ---
>  tools/bpf/bpftool/btf_dumper.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/bpf/bpftool/btf_dumper.c b/tools/bpf/bpftool/btf_dumper.c
> index 55bc512a1831..e4e6e2b3fd84 100644
> --- a/tools/bpf/bpftool/btf_dumper.c
> +++ b/tools/bpf/bpftool/btf_dumper.c
> @@ -32,7 +32,7 @@ static void btf_dumper_ptr(const void *data, json_writer_t *jw,
>  }
>
>  static int btf_dumper_modifier(const struct btf_dumper *d, __u32 type_id,
> -                              const void *data)
> +                              __u8 bit_offset, const void *data)
>  {
>         int actual_type_id;
>
> @@ -40,7 +40,7 @@ static int btf_dumper_modifier(const struct btf_dumper *d, __u32 type_id,
>         if (actual_type_id < 0)
>                 return actual_type_id;
>
> -       return btf_dumper_do_type(d, actual_type_id, 0, data);
> +       return btf_dumper_do_type(d, actual_type_id, bit_offset, data);
>  }
>
>  static void btf_dumper_enum(const void *data, json_writer_t *jw)
> @@ -237,7 +237,7 @@ static int btf_dumper_do_type(const struct btf_dumper *d, __u32 type_id,
>         case BTF_KIND_VOLATILE:
>         case BTF_KIND_CONST:
>         case BTF_KIND_RESTRICT:
> -               return btf_dumper_modifier(d, type_id, data);
> +               return btf_dumper_modifier(d, type_id, bit_offset, data);
>         default:
>                 jsonw_printf(d->jw, "(unsupported-kind");
>                 return -EINVAL;
> --
> 2.17.1
>