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

Tao Chen
2025-01-24T14:44:50Z

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)
+{
+ 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