Message-ID: <7874cfab-3f96-4cfb-9e52-b9d8108bc536@kernel.org>
Date: Thu, 6 Nov 2025 02:05:33 +0000
From: Quentin Monnet <qmo@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>,
 bpf <bpf@...r.kernel.org>, Alan Maguire <alan.maguire@...cle.com>,
 Yonghong Song <yonghong.song@...ux.dev>, 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>, LKML <linux-kernel@...r.kernel.org>,
 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] bpftool: Print map ID upon creation and support
 JSON output

2025-11-05 17:29 UTC-0800 ~ Alexei Starovoitov
<alexei.starovoitov@...il.com>
> On Wed, Nov 5, 2025 at 1:38 AM Quentin Monnet <qmo@...nel.org> wrote:
>>
>> 2025-11-04 09:54 UTC-0800 ~ Alexei Starovoitov
>> <alexei.starovoitov@...il.com>
>>> On Sat, Nov 1, 2025 at 12:34 PM Harshit Mogalapalli
>>> <harshit.m.mogalapalli@...cle.com> 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
>>>> Acked-by: Yonghong Song <yonghong.song@...ux.dev>
>>>> Reviewed-by: Quentin Monnet <qmo@...nel.org>
>>>> Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
>>>> ---
>>>> v2->v3: remove a line break("\n" ) in p_err statement. [Thanks Quentin]
>>>> ---
>>>>  tools/bpf/bpftool/map.c | 21 +++++++++++++++++----
>>>>  1 file changed, 17 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
>>>> index c9de44a45778..f32ae5476d76 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 +1355,24 @@ static int do_create(int argc, char **argv)
>>>>         }
>>>>
>>>>         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", 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);
>>>> +       }
>>>
>>> bpftool doesn't print it today and some scripts may depend on that.
>>
>>
>> Hi Alexei, are you sure we can't add any input at all? I'm concerned
>> that users won't ever find the IDs for created maps they might want to
>> use, if they never see it in the plain output.
>>
>>
>>> Let's drop this 'printf'. Json can do it unconditionally, since
>>> json parsing scripts should filter things they care about.
>>
>> I'd say the risk is the same. Scripts should filter things, but in
>> practise they might just as well be comparing to "null" today, given
>> that we didn't have any other output for the command so far. Conversely,
>> what scripts should not do is rely on plain output, we've always
>> recommended using bpftool's JSON for automation (or the exit code, in
>> the case of map creation). So I'm not convinced it's justified to
>> introduce a difference between plain and JSON in the current case.
> 
> tbh the "map create" feature suppose to create and pin and if both
> are successful then the map will be there and bpftool will
> exit with success.
> Now you're arguing that there could be a race with another
> bpftool/something that pins a different map in the same location
> and success of bpftool doesn't mean that exact that map is there.
> Other tool could have unpinned/deleted map, pinned another one, etc.
> Sure, such races are possible, but returning map id still
> looks pointless. It doesn't solve any race.
> So the whole 'lets print id' doesn't quite make sense to me.

OK "solving races" is not accurate, but returning the ID gives a unique
handle to work with the map, if a user runs a follow-up invocation to
update entries using the ID they can be sure they're working with the
same map - whatever happened with the bpffs. Or they can have the update
fail if you really want that particular map but, for example, it's been
recreated in the meantime. At the moment there's no way to uniquely
identify the map we've created with bpftool, and that seems weird to me.

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