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

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Message-ID: <89b12696-26ff-411f-9cd3-74361f0f1ecd@linux.dev>
Date: Tue, 28 Oct 2025 19:14:52 -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 10/28/25 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.

>   	if (attr.inner_map_fd > 0)
>   		close(attr.inner_map_fd);