linux-kernel - Re: [RFC bpf-next 1/2] bpftool: Print map ID upon creation and support JSON output

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Message-ID: <7a6be149-9e04-444e-a433-49450385d6a0@linux.dev>
Date: Wed, 29 Oct 2025 10:56:23 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>,
 bpf@...r.kernel.org
Cc: alan.maguire@...cle.com, Quentin Monnet <qmo@...nel.org>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, John Fastabend <john.fastabend@...il.com>,
 KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
 Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
 Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [RFC bpf-next 1/2] bpftool: Print map ID upon creation and
 support JSON output



On 10/29/25 9:05 AM, Harshit Mogalapalli wrote:
> Hi Yonghong,
>
>
> On 29/10/25 07:44, Yonghong Song wrote:
>>
>>
>> On 10/28/25 5:57 AM, Harshit Mogalapalli wrote:
>>> It is useful to print map ID on successful creation.
>>>
>>> JSON case:
>>> $ ./bpftool -j map create /sys/fs/bpf/test_map4 type hash key 4 
>>> value 8 entries 128 name map4
>>> {"id":12}
>>>
>>> Generic case:
>>> $ ./bpftool  map create /sys/fs/bpf/test_map5 type hash key 4 value 
>>> 8 entries 128 name map5
>>> Map successfully created with ID: 15
>>>
>>> Bpftool Issue: https://github.com/libbpf/bpftool/issues/121
>>> Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
>>> ---
>>>   tools/bpf/bpftool/map.c | 24 ++++++++++++++++++++----
>>>   1 file changed, 20 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
>>> index c9de44a45778..b6580f25361d 100644
>>> --- a/tools/bpf/bpftool/map.c
>>> +++ b/tools/bpf/bpftool/map.c
>>> @@ -1251,6 +1251,8 @@ static int do_create(int argc, char **argv)
>>>       LIBBPF_OPTS(bpf_map_create_opts, attr);
>>>       enum bpf_map_type map_type = BPF_MAP_TYPE_UNSPEC;
>>>       __u32 key_size = 0, value_size = 0, max_entries = 0;
>>> +    struct bpf_map_info map_info = {};
>>> +    __u32 map_info_len = sizeof(map_info);
>>>       const char *map_name = NULL;
>>>       const char *pinfile;
>>>       int err = -1, fd;
>>> @@ -1353,13 +1355,27 @@ static int do_create(int argc, char **argv)
>>>       }
>>>       err = do_pin_fd(fd, pinfile);
>>> -    close(fd);
>>> -    if (err)
>>> +    if (err) {
>>> +        close(fd);
>>
>> I think you can remove close(fd) here,
>>
>>>           goto exit;
>>> +    }
>>> -    if (json_output)
>>> -        jsonw_null(json_wtr);
>>> +    err = bpf_obj_get_info_by_fd(fd, &map_info, &map_info_len);
>>> +    if (err) {
>>> +        p_err("Failed to fetch map info: %s\n", strerror(errno));
>>> +        close(fd);
>>
>> and here
>>
>>> +        goto exit;
>>> +    }
>>> +    close(fd);
>>
>> and here,
>>
>>> +
>>> +    if (json_output) {
>>> +        jsonw_start_object(json_wtr);
>>> +        jsonw_int_field(json_wtr, "id", map_info.id);
>>> +        jsonw_end_object(json_wtr);
>>> +    } else {
>>> +        printf("Map successfully created with ID: %u\n", map_info.id);
>>> +    }
>>>   exit:
>>
>> and put close(fd) here.
>
> I think we need one more close_fd: label and then put a close(fd); 
> here. As there are other gotos to exit earlier in this function when 
> fd is uninitialized, which can the error like:
>
> map.c: In function ‘do_create’:
> map.c:1375:9: warning: ‘fd’ may be used uninitialized 
> [-Wmaybe-uninitialized]
>  1375 |         close(fd);
>       |         ^~~~~~~~~
> map.c:1258:23: note: ‘fd’ was declared here
>  1258 |         int err = -1, fd;
>       |                       ^~
>
>
>
> So, maybe we could do something like this:
>
>         err = do_pin_fd(fd, pinfile);
> -       close(fd);
>         if (err)
> -               goto exit;
> +               goto close_fd;
>
> -       if (json_output)
> -               jsonw_null(json_wtr);
> +       err = bpf_obj_get_info_by_fd(fd, &map_info, &map_info_len);
> +       if (err) {
> +               p_err("Failed to fetch map info: %s\n", strerror(errno));
> +               goto close_fd;
> +       }
>
> +       if (json_output) {
> +               jsonw_start_object(json_wtr);
> +               jsonw_int_field(json_wtr, "id", map_info.id);
> +               jsonw_end_object(json_wtr);
> +       } else {
> +               printf("Map successfully created with ID: %u\n", 
> map_info.id);
> +       }
> +close_fd:
> +       close(fd);
>  exit:
>         if (attr.inner_map_fd > 0)
>                 close(attr.inner_map_fd);
>
> I can prepare a v2 with this change, but wouldn't it be simpler to add a
> direct close(fd); on the few error paths instead of introducing an
> additional label for close(fd);?

The above change LGTM. Thanks!

>
> Thoughts/Suggestions ?
>
> Thanks,
> Harshit
>
>>
>>>       if (attr.inner_map_fd > 0)
>>>           close(attr.inner_map_fd);
>>
>>
>
>