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

[Hangbin Liu <liuhangbin@gmail.com>]

On Thu, Aug 04, 2022 at 11:01:48AM +0100, Quentin Monnet wrote:
> Hi! It would look much cleaner to have something specific to map names.
> It does not have to be a dedicated probe in my opinion, maybe we can
> just try loading with a name and retry if this fails with -EINVAL (a bit
> like we retry with another prog type in bpf_object__probe_loading(), if
> the first one fails). Something like this (not tested):
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 50d41815f431..abcafdf8ae7e 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4430,7 +4430,10 @@ 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);
> +	map = bpf_map_create(BPF_MAP_TYPE_ARRAY, "global_data", sizeof(int), 32, 1, NULL);
> +	if (map < 0 && errno == EINVAL)
> +		/* Retry without name */
> +		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));
> 
> (Maybe with a small wrapper, given that we'd also need this in
> probe_prog_bind_map() and probe_kern_array_mmap() as well.)

Ah, this looks more clean and easier.

> 
> > 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?
> 
> No, "if (CHECK_ATTR(BPF_PROG_LOAD))" should fail in bpf_prog_load() in
> kernel/bpf/syscall.c, and the syscall should fail with -EINVAL.
> 
> If older kernels simply ignored the "name" field for programs and maps,
> we wouldn't have to probe or retry for the current case in the first
> place :).

Thanks for the explanation. I will try add a wrapper first.

Regards
Hangbin