[PATCH bpf-next v8 3/4] libbpf: Add libbpf_probe_bpf_kfunc API

Tao Chen
2025-02-21T16:36:31Z

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@didiglobal.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Tao Chen <chen.dylane@linux.dev>
---
tools/lib/bpf/libbpf.h | 19 ++++++++++++-
tools/lib/bpf/libbpf.map | 1 +
tools/lib/bpf/libbpf_probes.c | 51 +++++++++++++++++++++++++++++++++++
3 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 3020ee45303a..c79b4475b956 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 < 0 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 de2b1205b436..8efebc18a215 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -431,6 +431,57 @@ 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 libbpf_err(-EOPNOTSUPP);
+
+ if (btf_fd >= 0)
+ fd_array[1] = btf_fd;
+ else
+ /* insn.off = 0, means vmlinux btf */
+ insns[0].off = 0;
+
+ 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 (ret > 0)
+ return 1; /* assume supported */
+
+ /* If BPF verifier recognizes BPF kfunc but it's not supported for
+ * given BPF program type, it will emit "calling kernel function
+ * <name> is not allowed". If the kfunc id is invalid,
+ * it will emit "kernel btf_id <id> 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
+ * kfunc prog not dev buound, it will emit "metadata kfuncs require
+ * device-bound program".
+ */
+ if (strstr(buf, "not allowed") || strstr(buf, "not a function") ||
+ strstr(buf, "invalid module BTF fd") ||
+ strstr(buf, "negative offset disallowed") ||
+ strstr(buf, "device-bound program"))
+ return 0;
+
+ return 1;
+}
+
int libbpf_probe_bpf_helper(enum bpf_prog_type prog_type, enum bpf_func_id helper_id,
const void *opts)
{
--
2.43.0