Re: [PATCH v3 1/2] bpftool: Print map ID upon creation and support JSON output

2025-11-04 09:54 UTC-0800 ~ Alexei Starovoitov alexei.starovoitov@gmail.com

On Sat, Nov 1, 2025 at 12:34 PM Harshit Mogalapalli harshit.m.mogalapalli@oracle.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@linux.dev Reviewed-by: Quentin Monnet qmo@kernel.org Signed-off-by: Harshit Mogalapalli harshit.m.mogalapalli@oracle.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.

Quentin