Re: How about adding a name for bpftool self created maps?

[Hangbin Liu <liuhangbin@gmail.com>]

On Tue, Jun 21, 2022 at 10:28:27PM +0100, Quentin Monnet wrote:
> Hi Hangbin,
> 
> No plan currently. Adding names has been suggested before, but it's
> not compatible with some older kernels that don't support map names
> [0]. Maybe one solution would be to probe the kernel for map name
> support, and to add a name if supported.

Hi Quentin,

I looked into this issue this week. And I have some questions.
Can we just use the probe_kern_prog_name() function directly? e.g.

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index e89cc9c885b3..f7d1580cd54e 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4476,7 +4476,10 @@ static int probe_kern_global_data(void)
        };
        int ret, map, insn_cnt = ARRAY_SIZE(insns);

-       map = bpf_map_create(BPF_MAP_TYPE_ARRAY, NULL, sizeof(int), 32, 1, NULL);
+       if (probe_kern_prog_name() > 0)
+               map = bpf_map_create(BPF_MAP_TYPE_ARRAY, "global_data", sizeof(int), 32, 1, NULL);
+       else
+               map = bpf_map_create(BPF_MAP_TYPE_ARRAY, NULL, sizeof(int), 32, 1, NULL);
        if (map < 0) {
                ret = -errno;
                cp = libbpf_strerror_r(ret, errmsg, sizeof(errmsg));

I know the map name and prog name supports are not in the same patch. But they are
added to kernel in one patch series. I doubt any one will backport them separately.

And I also have a question about function probe_kern_prog_name(). I only
saw it created a prog with name "test". But I didn't find the function check
if the prog are really has name "test". If a old kernel doesn't support prog
name, I think it will just ignore the name field. No?

Another way I think we can use to probe if kernel supports map name is try
to attach a kprobe/bpf_obj_name_cpy. If attach success, the kernel should support
the prog/map name. WDYT?

Thanks
Hangbin