Re: [RFC PATCH bpf-next v2 2/3] libbpf: Add FEAT_KPROBE_MULTI_LINK feature probe.

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Mar 30, 2026 at 04:30:18PM +0530, Varun R Mallya wrote:
> Add FEAT_KPROBE_MULTI_LINK, similar to UPROBE_MULTI_LINK
> by loading and creating a link for a small BPF program with
> BPF_TRACE_KPROBE_MULTI as the expected attach type, and
> then checking the success of the operation.
> 
> Signed-off-by: Varun R Mallya <varunrmallya@gmail.com>
> ---
>  tools/lib/bpf/features.c        | 37 +++++++++++++++++++++++++++++++++
>  tools/lib/bpf/libbpf_internal.h |  2 ++
>  2 files changed, 39 insertions(+)
> 
> diff --git a/tools/lib/bpf/features.c b/tools/lib/bpf/features.c
> index adcad221c601..13227c9ea69d 100644
> --- a/tools/lib/bpf/features.c
> +++ b/tools/lib/bpf/features.c
> @@ -424,6 +424,40 @@ static int probe_uprobe_multi_link(int token_fd)
>  	return link_fd < 0 && err == -EINVAL;
>  }
>  
> +static int probe_kprobe_multi_link(int token_fd)
> +{
> +	LIBBPF_OPTS(bpf_prog_load_opts, load_opts,
> +		    .expected_attach_type = BPF_TRACE_KPROBE_MULTI,
> +		    .token_fd = token_fd,
> +		    .prog_flags = token_fd ? BPF_F_TOKEN_FD : 0,
> +	);
> +	LIBBPF_OPTS(bpf_link_create_opts, link_opts);
> +	struct bpf_insn insns[] = {
> +		BPF_MOV64_IMM(BPF_REG_0, 0),
> +		BPF_EXIT_INSN(),
> +	};
> +	int prog_fd, link_fd, err;
> +	const char *sym = "bpf_map_lookup_elem"; /* stable, always present */
> +
> +	prog_fd = bpf_prog_load(BPF_PROG_TYPE_KPROBE, NULL, "GPL",
> +				insns, ARRAY_SIZE(insns), &load_opts);
> +	if (prog_fd < 0)
> +		return -errno;
> +
> +	/* attaching to a valid symbol should succeed */
> +	link_opts.kprobe_multi.syms = &sym;
> +	link_opts.kprobe_multi.cnt = 1;

maybe we could set cnt > MAX_KPROBE_MULTI_CNT and check if the fail
is E2BIG ? this could save some cycles, but haven't tried that

jirka

> +	link_fd = bpf_link_create(prog_fd, -1, BPF_TRACE_KPROBE_MULTI, &link_opts);
> +	err = -errno;
> +	if (link_fd >= 0)
> +		close(link_fd);
> +	close(prog_fd);
> +	/* if kprobe_multi is supported, link creation either succeeds or
> +	 * fails with something other than -EINVAL due to permissions,
> +	 */
> +	return link_fd >= 0 || (err != -EINVAL);
> +}
> +

SNIP