Hi Alexei and Quentin,
On 06/11/25 07:55, Quentin Monnet wrote:
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.
Thanks a lot for your inputs on this. I have learnt a lot by following
this discussion.
Thanks,
Harshit
