2025-11-05 18:14 UTC-0800 ~ Alexei Starovoitov <[email protected]> > On Wed, Nov 5, 2025 at 6:05 PM Quentin Monnet <[email protected]> wrote: >> >> 2025-11-05 17:29 UTC-0800 ~ Alexei Starovoitov >> <[email protected]> >>> On Wed, Nov 5, 2025 at 1:38 AM Quentin Monnet <[email protected]> wrote: >>>> >>>> 2025-11-04 09:54 UTC-0800 ~ Alexei Starovoitov >>>> <[email protected]> >>>>> On Sat, Nov 1, 2025 at 12:34 PM Harshit Mogalapalli >>>>> <[email protected]> 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 <[email protected]> >>>>>> Reviewed-by: Quentin Monnet <[email protected]> >>>>>> Signed-off-by: Harshit Mogalapalli <[email protected]> >>>>>> --- >>>>>> 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. >
Ah. I did assume it was unique :/. My bad, then in that case it doesn't make too much sense, indeed.
