libbpf: Clarify raw-address single kprobe attach behavior

bpf_program__attach_kprobe_opts() documents single-kprobe attach
through func_name, with an optional offset. For the PMU-based path,
func_name = NULL with an absolute address in offset already works as
well, but that is not described in the API.

This commit clarifies this existing non-legacy behavior. For PMU-based
attach, callers can use func_name = NULL with an absolute address in
offset as the raw-address form. For legacy tracefs/debugfs kprobes,
reject this form explicitly.

Signed-off-by: Hoyeon Lee <hoyeon.lee@suse.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20260401143116.185049-3-hoyeon.lee@suse.com

src/libbpf.c

@@ -11843,6 +11843,8 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
 	default:
 		return libbpf_err_ptr(-EINVAL);
 	}
+	if (!func_name && legacy)
+		return libbpf_err_ptr(-EOPNOTSUPP);
 
 	if (!legacy) {
 		pfd = perf_event_open_probe(false /* uprobe */, retprobe,
@@ -11863,20 +11865,20 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
 	}
 	if (pfd < 0) {
 		err = pfd;
-		pr_warn("prog '%s': failed to create %s '%s+0x%zx' perf event: %s\n",
+		pr_warn("prog '%s': failed to create %s '%s%s0x%zx' perf event: %s\n",
 			prog->name, retprobe ? "kretprobe" : "kprobe",
-			func_name, offset,
-			errstr(err));
+			func_name ?: "", func_name ? "+" : "",
+			offset, errstr(err));
 		goto err_out;
 	}
 	link = bpf_program__attach_perf_event_opts(prog, pfd, &pe_opts);
 	err = libbpf_get_error(link);
 	if (err) {
 		close(pfd);
-		pr_warn("prog '%s': failed to attach to %s '%s+0x%zx': %s\n",
+		pr_warn("prog '%s': failed to attach to %s '%s%s0x%zx': %s\n",
 			prog->name, retprobe ? "kretprobe" : "kprobe",
-			func_name, offset,
-			errstr(err));
+			func_name ?: "", func_name ? "+" : "",
+			offset, errstr(err));
 		goto err_clean_legacy;
 	}
 	if (legacy) {

src/libbpf.h

@@ -557,7 +557,7 @@ struct bpf_kprobe_opts {
 	size_t sz;
 	/* custom user-provided value fetchable through bpf_get_attach_cookie() */
 	__u64 bpf_cookie;
-	/* function's offset to install kprobe to */
+	/* function offset, or raw address if func_name == NULL */
 	size_t offset;
 	/* kprobe is return probe */
 	bool retprobe;
@@ -565,11 +565,36 @@ struct bpf_kprobe_opts {
 	enum probe_attach_mode attach_mode;
 	size_t :0;
 };
+
 #define bpf_kprobe_opts__last_field attach_mode
 
+/**
+ * @brief **bpf_program__attach_kprobe()** attaches a BPF program to a
+ * kernel function entry or return.
+ *
+ * @param prog BPF program to attach
+ * @param retprobe Attach to function return
+ * @param func_name Name of the kernel function to attach to
+ * @return Reference to the newly created BPF link; or NULL is returned on
+ * error, error code is stored in errno
+ */
 LIBBPF_API struct bpf_link *
 bpf_program__attach_kprobe(const struct bpf_program *prog, bool retprobe,
 			   const char *func_name);
+
+/**
+ * @brief **bpf_program__attach_kprobe_opts()** is just like
+ * bpf_program__attach_kprobe() except with an options struct
+ * for various configurations.
+ *
+ * @param prog BPF program to attach
+ * @param func_name Name of the kernel function to attach to. If NULL,
+ * opts->offset is treated as a raw kernel address. Raw-address attach
+ * is supported with PROBE_ATTACH_MODE_PERF and PROBE_ATTACH_MODE_LINK.
+ * @param opts Options for altering program attachment
+ * @return Reference to the newly created BPF link; or NULL is returned on
+ * error, error code is stored in errno
+ */
 LIBBPF_API struct bpf_link *
 bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
                                 const char *func_name,