Message-ID: <CAADnVQL7cLYPKEQOLWi1DjTZjhE_Fy4zWLrWG+=NSeN821SyMw@mail.gmail.com>
Date: Wed, 5 Nov 2025 18:14:02 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Quentin Monnet <qmo@...nel.org>
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

On Wed, Nov 5, 2025 at 6:05 PM Quentin Monnet <qmo@...nel.org> wrote:
>
> 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.

ID is not unique. If somebody rm -rf bpffs. That ID will not point anywhere.
Also it's 31-bit space and folks in the past demonstrated an attack
to recycle the same ID.
So the users cannot be sure what ID is this.

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