Re: [PATCH v10 bpf-next 08/10] bpftool: add BTF dump "format meta" to dump header/metadata

On Fri, Feb 27, 2026 at 2:05 AM Alan Maguire <[email protected]> wrote:
>
> Provide a way to dump BTF metadata info via bpftool; this
> consists of BTF size, header fields and layout info
> (if available); for example
>
> $ bpftool btf dump file vmlinux format meta
> size 6516862
> magic 0xeb9f
> version 1
> flags 0x0
> hdr_len 32
> type_len 3929096
> type_off 0
> str_len 2587654
> str_off 3929176
> layout_len 80
> layout_off 3929096
> kind 0    UNKNOWN    info_sz 0    elem_sz 0    flags 0
> kind 1    INT        info_sz 4    elem_sz 0    flags 0
> kind 2    PTR        info_sz 0    elem_sz 0    flags 0
> kind 3    ARRAY      info_sz 12   elem_sz 0    flags 0
> kind 4    STRUCT     info_sz 0    elem_sz 12   flags 0
> kind 5    UNION      info_sz 0    elem_sz 12   flags 0
> kind 6    ENUM       info_sz 0    elem_sz 8    flags 0
> kind 7    FWD        info_sz 0    elem_sz 0    flags 0
> kind 8    TYPEDEF    info_sz 0    elem_sz 0    flags 0
> kind 9    VOLATILE   info_sz 0    elem_sz 0    flags 0
> kind 10   CONST      info_sz 0    elem_sz 0    flags 0
> kind 11   RESTRICT   info_sz 0    elem_sz 0    flags 0
> kind 12   FUNC       info_sz 0    elem_sz 0    flags 0
> kind 13   FUNC_PROTO info_sz 0    elem_sz 8    flags 0
> kind 14   VAR        info_sz 4    elem_sz 0    flags 0
> kind 15   DATASEC    info_sz 0    elem_sz 12   flags 0
> kind 16   FLOAT      info_sz 0    elem_sz 0    flags 0
> kind 17   DECL_TAG   info_sz 4    elem_sz 0    flags 0
> kind 18   TYPE_TAG   info_sz 0    elem_sz 0    flags 0
> kind 19   ENUM64     info_sz 0    elem_sz 12   flags 0
>
> JSON output is also supported:
>
> $ bpftool -p btf dump file /var/tmp/vmlinux.kind_layout format meta
> {
>     "size": 6516862,
>     "header": {
>         "magic": 60319,
>         "version": 1,
>         "flags": 0,
>         "hdr_len": 32,
>         "type_len": 3929096,
>         "type_off": 0,
>         "str_len": 2587654,
>         "str_off": 3929176,
>         "layout_len": 80,
>         "layout_off": 3929096
>     },
>     "layouts": [{
>             "kind": 0,
>             "name": "UNKNOWN",
>             "info_sz": 0,
>             "elem_sz": 0,
>             "flags": 0
>         },{
>             "kind": 1,
>             "name": "INT",
>             "info_sz": 4,
>             "elem_sz": 0,
>             "flags": 0
>         },{
>             "kind": 2,
>             "name": "PTR",
>             "info_sz": 0,
>             "elem_sz": 0,
>             "flags": 0
>         },{
> ...
>
> Signed-off-by: Alan Maguire <[email protected]>
> ---
>  tools/bpf/bpftool/bash-completion/bpftool |  2 +-
>  tools/bpf/bpftool/btf.c                   | 91 ++++++++++++++++++++++-
>  2 files changed, 89 insertions(+), 4 deletions(-)
>
> diff --git a/tools/bpf/bpftool/bash-completion/bpftool b/tools/bpf/bpftool/bash-completion/bpftool
> index a28f0cc522e4..d49d6a70cb74 100644
> --- a/tools/bpf/bpftool/bash-completion/bpftool
> +++ b/tools/bpf/bpftool/bash-completion/bpftool
> @@ -950,7 +950,7 @@ _bpftool()
>                              return 0
>                              ;;
>                          format)
> -                            COMPREPLY=( $( compgen -W "c raw" -- "$cur" ) )
> +                            COMPREPLY=( $( compgen -W "c raw meta" -- "$cur" ) )
>                              ;;
>                          root_id)
>                              return 0;
> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index 946612029dee..08dc6a86bc85 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c
> @@ -835,6 +835,87 @@ static int dump_btf_c(const struct btf *btf,
>         return err;
>  }
>
> +static int dump_btf_meta(const struct btf *btf)
> +{
> +       __u32 layout_off = 0, layout_len = 0;
> +       __u32 data_sz, i, nr_kinds = 0;
> +       const struct btf_header *hdr;
> +       const struct btf_layout *l;
> +       const void *data;
> +
> +       data = btf__raw_data(btf, &data_sz);
> +       if (!data)
> +               return -ENOMEM;
> +       hdr = data;
> +       if (json_output) {
> +               jsonw_start_object(json_wtr);           /* metadata object */
> +               jsonw_uint_field(json_wtr, "size", data_sz);
> +               jsonw_name(json_wtr, "header");
> +               jsonw_start_object(json_wtr);           /* header object */
> +               jsonw_uint_field(json_wtr, "magic", hdr->magic);
> +               jsonw_uint_field(json_wtr, "version", hdr->version);
> +               jsonw_uint_field(json_wtr, "flags", hdr->flags);
> +               jsonw_uint_field(json_wtr, "hdr_len", hdr->hdr_len);
> +               jsonw_uint_field(json_wtr, "type_len", hdr->type_len);
> +               jsonw_uint_field(json_wtr, "type_off", hdr->type_off);
> +               jsonw_uint_field(json_wtr, "str_len", hdr->str_len);
> +               jsonw_uint_field(json_wtr, "str_off", hdr->str_off);

I don't think this part is useful.
People can hex dump it if they really want to.

> +       } else {
> +               printf("size %-10u\n", data_sz);
> +               printf("magic 0x%-10x\nversion %-10d\nflags 0x%-10x\nhdr_len %-10u\n",
> +                      hdr->magic, hdr->version, hdr->flags, hdr->hdr_len);
> +               printf("type_len %-10u\ntype_off %-10u\n", hdr->type_len, hdr->type_off);
> +               printf("str_len %-10u\nstr_off %-10u\n", hdr->str_len, hdr->str_off);
> +       }
> +
> +       if (data_sz >= hdr->hdr_len && hdr->hdr_len >= sizeof(struct btf_header)) {
> +               layout_off = hdr->layout_off;
> +               layout_len = hdr->layout_len;
> +               if (layout_len > 0 &&
> +                   (long long)layout_off + layout_len + hdr->hdr_len <= data_sz) {
> +                       l = (void *)hdr + hdr->hdr_len + layout_off;
> +                       nr_kinds = layout_len / sizeof(*l);
> +               }
> +       } else {
> +               if (!json_output)
> +                       return 0;
> +       }
> +
> +       /* unconditionally display (possibly empty) layout info for json only */
> +       if (json_output) {
> +               jsonw_uint_field(json_wtr, "layout_len", layout_len);
> +               jsonw_uint_field(json_wtr, "layout_off", layout_off);
> +               jsonw_end_object(json_wtr);             /* end header object */
> +
> +               jsonw_name(json_wtr, "layouts");
> +               jsonw_start_array(json_wtr);
> +               for (i = 0; i < nr_kinds; i++) {
> +                       jsonw_start_object(json_wtr);
> +                       jsonw_uint_field(json_wtr, "kind", i);
> +                       if (i < NR_BTF_KINDS)
> +                               jsonw_string_field(json_wtr, "name", btf_kind_str[i]);
> +                       else
> +                               jsonw_null_field(json_wtr, "name");
> +                       jsonw_uint_field(json_wtr, "info_sz", l[i].info_sz);
> +                       jsonw_uint_field(json_wtr, "elem_sz", l[i].elem_sz);
> +                       jsonw_uint_field(json_wtr, "flags", l[i].flags);
> +                       jsonw_end_object(json_wtr);
> +               }

This part, I guess, is ok-ish.
I wouldn't do it either though. Feels like a debug feature.