[PATCH bpf-next v2 1/2] libbpf: Add libbpf_probe_bpf_kfunc API

[Tao Chen <chen.dylane@xxxxxxxxx>]

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@xxxxxxxxx>
---
 tools/lib/bpf/libbpf.h        | 17 ++++++++++++-
 tools/lib/bpf/libbpf.map      |  1 +
 tools/lib/bpf/libbpf_probes.c | 47 +++++++++++++++++++++++++++++++++++
 3 files changed, 64 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 3020ee45303a..ac0b46d8d8f9 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 off 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 off, 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..641b1b008eeb 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -413,6 +413,53 @@ 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,
+			   __s16 off, 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);
+
+	/* Same logic as probe_bpf_helper check */
+	switch (prog_type) {
+	case BPF_PROG_TYPE_TRACING:
+	case BPF_PROG_TYPE_EXT:
+	case BPF_PROG_TYPE_LSM:
+	case BPF_PROG_TYPE_STRUCT_OPS:
+		return -EOPNOTSUPP;
+	default:
+		break;
+	}
+
+	insns[0].code = BPF_JMP | BPF_CALL;
+	insns[0].src_reg = BPF_PSEUDO_KFUNC_CALL;
+	insns[0].imm = kfunc_id;
+	insns[0].off = off;
+
+	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 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