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Sat, 08 Feb 2025 22:57:01 -0800 (PST) Message-ID: Date: Sun, 9 Feb 2025 14:56:56 +0800 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH bpf-next v4 3/4] libbpf: Add libbpf_probe_bpf_kfunc API To: Eduard Zingerman , ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, haoluo@google.com, jolsa@kernel.org, qmo@kernel.org Cc: bpf@vger.kernel.org, linux-kernel@vger.kernel.org References: <20250206051557.27913-1-chen.dylane@gmail.com> <20250206051557.27913-4-chen.dylane@gmail.com> <7d667c037e7396fb88cf243162c5aa8a537858bb.camel@gmail.com> From: Tao Chen In-Reply-To: <7d667c037e7396fb88cf243162c5aa8a537858bb.camel@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 在 2025/2/8 06:35, Eduard Zingerman 写道: > On Thu, 2025-02-06 at 13:15 +0800, Tao Chen wrote: > > [...] > >> 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, 0 means kfunc defined in vmlinux. > > Regarding '0' as special value: > in general FD is considered invalid only if it's negative, 0 is a valid FD. > Andrii, I remember there was a lengthy discussion about FD==0 and BPF, > but I don't remember the conclusion. > Hi Eduard, As you said, so what about "-1 means kfunc defined in vmlinux", -1 just used to distinguish whether it is vmlinux，then processing in libbpf_probe_bpf_kfunc like: offset = 0; // vmlinux btf offset default is 0 insn.off = offset; if (btf_fd >= 0) { offset = 1; insn.off = offset; fd_array[offset] = btf_fd; } What do you think? >> + * @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 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; > > This is now in conflict with bpf-next. > >> } LIBBPF_1.5.0; >> diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c >> index e142130cb83c..c7f2b2dfbcf1 100644 >> --- a/tools/lib/bpf/libbpf_probes.c >> +++ b/tools/lib/bpf/libbpf_probes.c >> @@ -433,6 +433,61 @@ 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, btf_fd, kfunc_id), >> + BPF_EXIT_INSN(), >> + }; >> + const size_t insn_cnt = ARRAY_SIZE(insns); >> + char buf[4096]; >> + int *fd_array = NULL; >> + size_t fd_array_cnt = 0, fd_array_cap = fd_array_cnt; >> + int ret; >> + >> + if (opts) >> + return libbpf_err(-EINVAL); >> + >> + if (!can_probe_prog_type(prog_type)) >> + return -EOPNOTSUPP; >> + >> + if (btf_fd) { >> + ret = libbpf_ensure_mem((void **)&fd_array, &fd_array_cap, >> + sizeof(int), fd_array_cnt + btf_fd); > > Please take a look at the tools/testing/selftests/bpf/prog_tests/fd_array.c, > e.g. test case check_fd_array_cnt__fd_array_ok(). The offset field of the > call instruction does not have to be an fd (as it only has 16 bits), > instead it's an offset inside the fd_array. > Here it would be sufficient to allocate a small array on stack. > >> + if (ret) >> + return ret; >> + >> + /* In kernel, obtain the btf fd by means of the offset of >> + * the fd_array, and the offset is the btf fd. >> + */ >> + fd_array[btf_fd] = btf_fd; >> + } > > [...] > -- Best Regards Dylane Chen