On Tue, Feb 11, 2025 at 3:19 AM Tao Chen <chen.dylane@xxxxxxxxx> 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.
>
> Cc: Tao Chen <dylane.chen@xxxxxxxxxxxxxx>
> Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> ---
> tools/lib/bpf/libbpf.h | 19 +++++++++++++-
> tools/lib/bpf/libbpf.map | 1 +
> tools/lib/bpf/libbpf_probes.c | 48 +++++++++++++++++++++++++++++++++++
> 3 files changed, 67 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 3020ee45303a..e796e38cf255 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -1680,7 +1680,24 @@ 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, if kfunc is defined in kernel module,
> + * btf_fd is used to point to module's BTF, which is >= 0, and -1 means kfunc
> + * defined in 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, int 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 b5a838de6f47..3bbfe13aeb6a 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -438,4 +438,5 @@ LIBBPF_1.6.0 {
> bpf_linker__new_fd;
> btf__add_decl_attr;
> btf__add_type_attr;
> + 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 8ed92ea922b3..ab5591c385de 100644
> --- a/tools/lib/bpf/libbpf_probes.c
> +++ b/tools/lib/bpf/libbpf_probes.c
> @@ -431,6 +431,54 @@ 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;
libbpf_err() here
pw-bot: cr
> +
> + if (btf_fd >= 0) {
> + fd_array[1] = btf_fd;
> + } else if (btf_fd == -1) {
let's not hard-code the equality, use < 0 (though I'd follow
verifier's offset == 0 convention for vmlinux BTF here as well to stay
conceptually consistent)
> + /* 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,
> + buf, sizeof(buf));
> + 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,
bpf_cpumask_create -> <name> to keep comments generic?
> + * it will emit "kernel btf_id 4294967295 is not a function". If btf fd
same as above, use <id> placeholder instead of specific number?
and keep BTF (all caps) use consistent, please
> + * invalid in module btf, it will emit "invalid module BTF fd specified" or
ditto, btf -> BTF
> + * "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"))))
stylistically, given amount of checks, I'd probably go with the
following structure
if (ret > 0)
return 1;
if (strstr(buf, "not allowed") ||
strstr(buf, "not a function") ||
...)
return 0;
> + 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
>
