Re: [PATCH bpf-next v3 2/3] libbpf: Add libbpf_probe_bpf_kfunc API

Andrii Nakryiko
2025-01-24T18:46:45Z

On Fri, Jan 24, 2025 at 6:44 AM Tao Chen <chen.dylane@gmail.com> 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.
>
> Signed-off-by: Tao Chen <chen.dylane@gmail.com>
> ---
> tools/lib/bpf/libbpf.h | 17 ++++++++++++++++-
> tools/lib/bpf/libbpf.map | 1 +
> tools/lib/bpf/libbpf_probes.c | 30 ++++++++++++++++++++++++++++++
> 3 files changed, 47 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 3020ee45303a..035829e22099 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -1680,7 +1680,22 @@ 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 btf_fd The module BTF FD, 0 for vmlinux
> + * @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, __s16 btf_fd, 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 b73345977b4e..cd7d16c1cc49 100644
> --- a/tools/lib/bpf/libbpf_probes.c
> +++ b/tools/lib/bpf/libbpf_probes.c
> @@ -446,6 +446,36 @@ static int probe_func_comm(enum bpf_prog_type prog_type, struct bpf_insn insn,
> return 0;
> }
>
> +int libbpf_probe_bpf_kfunc(enum bpf_prog_type prog_type, int kfunc_id,
> + __s16 btf_fd, const void *opts)

btf_fd should be int (and mention in documentation that if kfunc is
defined in kernel module btf_fd is used to point to module's BTF)

> +{
> + struct bpf_insn insn;
> + int err;
> + char buf[4096];
> +
> + if (opts)
> + return libbpf_err(-EINVAL);
> +
> + insn.code = BPF_JMP | BPF_CALL;
> + insn.src_reg = BPF_PSEUDO_KFUNC_CALL;
> + insn.imm = kfunc_id;
> + insn.off = btf_fd;
> +
> + err = probe_func_comm(prog_type, insn, buf, sizeof(buf));
> + if (err)
> + return 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)
> {
> --
> 2.43.0
>