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

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Jan 23, 2025 at 01:06:19AM +0800, Tao Chen wrote:
> Similarly to libbpf_probe_bpf_helper, the libbpf_probe_bpf_kfunc
> used to test the availability of the different eBPF kfuncs on the
> current system.

hi,
there's "bpf_kfunc" DECL_TAG for each kfunc in BTF data,
I think that should do the same job? please check [1] and
related commits for details

jirka


[1] 770abbb5a25a bpftool: Support dumping kfunc prototypes from BTF

> 
> Signed-off-by: Tao Chen <chen.dylane@gmail.com>
> ---
>  tools/lib/bpf/libbpf.h        | 16 +++++++++++++++-
>  tools/lib/bpf/libbpf.map      |  1 +
>  tools/lib/bpf/libbpf_probes.c | 36 +++++++++++++++++++++++++++++++++++
>  3 files changed, 52 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 3020ee45303a..3b6d33578a16 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -1680,7 +1680,21 @@ LIBBPF_API int libbpf_probe_bpf_map_type(enum bpf_map_type map_type, const void
>   */
>  LIBBPF_API int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type,
>  				       enum bpf_func_id helper_id, const void *opts);
> -
> +/**
> + * @brief **libbpf_probe_bpf_kfunc()** detects if host kernel supports the
> + * use of a given BPF kfunc from specified BPF program type.
> + * @param prog_type BPF program type used to check the support of BPF kfunc
> + * @param kfunc_id The btf ID of BPF kfunc to check support for
> + * @param opts reserved for future extensibility, should be NULL
> + * @return 1, if given combination of program type and kfunc is supported; 0,
> + * if the combination is not supported; negative error code if feature
> + * detection for provided input arguments failed or can't be performed
> + *
> + * Make sure the process has required set of CAP_* permissions (or runs as
> + * root) when performing feature checking.
> + */
> +LIBBPF_API int libbpf_probe_bpf_kfunc(enum bpf_prog_type prog_type,
> +				      int kfunc_id, const void *opts);
>  /**
>   * @brief **libbpf_num_possible_cpus()** is a helper function to get the
>   * number of possible CPUs that the host kernel supports and expects.
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index a8b2936a1646..e93fae101efd 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -436,4 +436,5 @@ LIBBPF_1.6.0 {
>  		bpf_linker__add_buf;
>  		bpf_linker__add_fd;
>  		bpf_linker__new_fd;
> +		libbpf_probe_bpf_kfunc;
>  } LIBBPF_1.5.0;
> diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
> index 9dfbe7750f56..bc1cf2afbe87 100644
> --- a/tools/lib/bpf/libbpf_probes.c
> +++ b/tools/lib/bpf/libbpf_probes.c
> @@ -413,6 +413,42 @@ int libbpf_probe_bpf_map_type(enum bpf_map_type map_type, const void *opts)
>  	return libbpf_err(ret);
>  }
>  
> +int libbpf_probe_bpf_kfunc(enum bpf_prog_type prog_type, int kfunc_id,
> +			   const void *opts)
> +{
> +	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);
> +
> +	insns[0].code = BPF_JMP | BPF_CALL;
> +	insns[0].src_reg = BPF_PSEUDO_KFUNC_CALL;
> +	insns[0].imm = kfunc_id;
> +
> +	/* Now only support kfunc from vmlinux */
> +	insns[0].off = 0;
> +
> +	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 (err == 0 && strstr(buf, "not allowed"))
> +		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
>