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.

Sure, we can omit the bpftool changes for v11 and add them later if there's a need.