Re: [PATCH bpf-next v2 1/2] libbpf: Add libbpf_probe_bpf_kfunc API

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2025-01-24 at 01:05 +0800, Tao Chen wrote:

[...]

> +int libbpf_probe_bpf_kfunc(enum bpf_prog_type prog_type, int kfunc_id,
> +			   __s16 off, const void *opts)
                                 ^^^
Nit:                      maybe name this btf_fd?

> +{

In v2 function looks identical to libbpf_probe_bpf_helper,
do we want to copy-paste it or introduce a utility:

  static int probe_insn(enum bpf_prog_type prog_type, struct bpf_insn insn,
                        const char **accepted_msgs)

And call it from both libbpf_probe_bpf_{helper,kfunc}?

> +	struct bpf_insn insns[] = {
> +		BPF_EXIT_INSN(),
> +		BPF_EXIT_INSN(),
> +	};
> +	const size_t insn_cnt = ARRAY_SIZE(insns);
> +	int err;
> +	char buf[4096];
> +
> +	if (opts)
> +		return libbpf_err(-EINVAL);
> +
> +	/* Same logic as probe_bpf_helper check */
> +	switch (prog_type) {
> +	case BPF_PROG_TYPE_TRACING:
> +	case BPF_PROG_TYPE_EXT:
> +	case BPF_PROG_TYPE_LSM:
> +	case BPF_PROG_TYPE_STRUCT_OPS:
> +		return -EOPNOTSUPP;
> +	default:
> +		break;
> +	}
> +
> +	insns[0].code = BPF_JMP | BPF_CALL;
> +	insns[0].src_reg = BPF_PSEUDO_KFUNC_CALL;
> +	insns[0].imm = kfunc_id;
> +	insns[0].off = off;
> +
> +	buf[0] = '\0';
> +	err = probe_prog_load(prog_type, insns, insn_cnt, buf, sizeof(buf));
> +	if (err < 0)
> +		return libbpf_err(err);
> +
> +	/* If BPF verifier recognizes BPF kfunc but it's not supported for
> +	 * given BPF program type, it will emit "calling kernel function
> +	 * bpf_cpumask_create is not allowed", if the kfunc id is invalid,
> +	 * it will emit "kernel btf_id 4294967295 is not a function"
> +	 */
> +	if (err == 0 && (strstr(buf, "not allowed") || strstr(buf, "not a function")))
> +		return 0;
> +
> +	return 1; /* assume supported */
> +}
> +
>  int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type, enum bpf_func_id helper_id,
>  			    const void *opts)
>  {