Re: [PATCH bpf-next v5 3/4] libbpf: Add libbpf_probe_bpf_kfunc API

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Feb 10, 2025 at 01:59:44PM +0800, Tao Chen wrote:

SNIP

> diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
> index e142130cb83c..53f1196394bf 100644
> --- a/tools/lib/bpf/libbpf_probes.c
> +++ b/tools/lib/bpf/libbpf_probes.c
> @@ -433,6 +433,54 @@ static bool can_probe_prog_type(enum bpf_prog_type prog_type)
>  	return true;
>  }
>  
> +int libbpf_probe_bpf_kfunc(enum bpf_prog_type prog_type, int kfunc_id, int btf_fd,
> +			   const void *opts)
> +{
> +	struct bpf_insn insns[] = {
> +		BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, BPF_PSEUDO_KFUNC_CALL, 1, kfunc_id),
> +		BPF_EXIT_INSN(),
> +	};
> +	const size_t insn_cnt = ARRAY_SIZE(insns);
> +	char buf[4096];
> +	int fd_array[2] = {-1};
> +	int ret;
> +
> +	if (opts)
> +		return libbpf_err(-EINVAL);
> +
> +	if (!can_probe_prog_type(prog_type))
> +		return -EOPNOTSUPP;
> +
> +	if (btf_fd >= 0) {
> +		fd_array[1] = btf_fd;
> +	} else if (btf_fd == -1) {
> +		/* insn.off = 0, means vmlinux btf */
> +		insns[0].off = 0;
> +	} else {
> +		return libbpf_err(-EINVAL);
> +	}
> +
> +	buf[0] = '\0';
> +	ret = probe_prog_load(prog_type, insns, insn_cnt, btf_fd >= 0 ? fd_array : NULL,
> +			      0, buf, sizeof(buf));

hum, you pass fd_array_cnt as 0, which IIUC will work properly

but I guess then we don't need to have fd_array_cnt argument in
probe_prog_load if all callers pass 0 ?

jirka

> +	if (ret < 0)
> +		return libbpf_err(ret);
> +
> +	/* 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 btf fd
> +	 * invalid in module btf, it will emit "invalid module BTF fd specified" or
> +	 * "negative offset disallowed for kernel module function call"
> +	 */
> +	if (ret == 0 && (strstr(buf, "not allowed") || strstr(buf, "not a function") ||
> +			(strstr(buf, "invalid module BTF fd")) ||
> +			(strstr(buf, "negative offset disallowed"))))
> +		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)
>  {
> -- 
> 2.43.0
>