libbpf: Allow "incomplete" basic tracing SEC() definitions

In a lot of cases the target of kprobe/kretprobe, tracepoint, raw
tracepoint, etc BPF program might not be known at the compilation time
and will be discovered at runtime. This was always a supported case by
libbpf, with APIs like bpf_program__attach_{kprobe,tracepoint,etc}()
accepting full target definition, regardless of what was defined in
SEC() definition in BPF source code.

Unfortunately, up till now libbpf still enforced users to specify at
least something for the fake target, e.g., SEC("kprobe/whatever"), which
is cumbersome and somewhat misleading.

This patch allows target-less SEC() definitions for basic tracing BPF
program types:

  - kprobe/kretprobe;
  - multi-kprobe/multi-kretprobe;
  - tracepoints;
  - raw tracepoints.

Such target-less SEC() definitions are meant to specify declaratively
proper BPF program type only. Attachment of them will have to be handled
programmatically using correct APIs. As such, skeleton's auto-attachment
of such BPF programs is skipped and generic bpf_program__attach() will
fail, if attempted, due to the lack of enough target information.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20220428185349.3799599-2-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -8853,22 +8853,22 @@ static const struct bpf_sec_def section_defs[] = {
 	SEC_DEF("socket",		SOCKET_FILTER, 0, SEC_NONE | SEC_SLOPPY_PFX),
 	SEC_DEF("sk_reuseport/migrate",	SK_REUSEPORT, BPF_SK_REUSEPORT_SELECT_OR_MIGRATE, SEC_ATTACHABLE | SEC_SLOPPY_PFX),
 	SEC_DEF("sk_reuseport",		SK_REUSEPORT, BPF_SK_REUSEPORT_SELECT, SEC_ATTACHABLE | SEC_SLOPPY_PFX),
-	SEC_DEF("kprobe/",		KPROBE,	0, SEC_NONE, attach_kprobe),
+	SEC_DEF("kprobe+",		KPROBE,	0, SEC_NONE, attach_kprobe),
 	SEC_DEF("uprobe+",		KPROBE,	0, SEC_NONE, attach_uprobe),
-	SEC_DEF("kretprobe/",		KPROBE, 0, SEC_NONE, attach_kprobe),
+	SEC_DEF("kretprobe+",		KPROBE, 0, SEC_NONE, attach_kprobe),
 	SEC_DEF("uretprobe+",		KPROBE, 0, SEC_NONE, attach_uprobe),
-	SEC_DEF("kprobe.multi/",	KPROBE,	BPF_TRACE_KPROBE_MULTI, SEC_NONE, attach_kprobe_multi),
-	SEC_DEF("kretprobe.multi/",	KPROBE,	BPF_TRACE_KPROBE_MULTI, SEC_NONE, attach_kprobe_multi),
+	SEC_DEF("kprobe.multi+",	KPROBE,	BPF_TRACE_KPROBE_MULTI, SEC_NONE, attach_kprobe_multi),
+	SEC_DEF("kretprobe.multi+",	KPROBE,	BPF_TRACE_KPROBE_MULTI, SEC_NONE, attach_kprobe_multi),
 	SEC_DEF("usdt+",		KPROBE,	0, SEC_NONE, attach_usdt),
 	SEC_DEF("tc",			SCHED_CLS, 0, SEC_NONE),
 	SEC_DEF("classifier",		SCHED_CLS, 0, SEC_NONE | SEC_SLOPPY_PFX | SEC_DEPRECATED),
 	SEC_DEF("action",		SCHED_ACT, 0, SEC_NONE | SEC_SLOPPY_PFX),
-	SEC_DEF("tracepoint/",		TRACEPOINT, 0, SEC_NONE, attach_tp),
-	SEC_DEF("tp/",			TRACEPOINT, 0, SEC_NONE, attach_tp),
-	SEC_DEF("raw_tracepoint/",	RAW_TRACEPOINT, 0, SEC_NONE, attach_raw_tp),
-	SEC_DEF("raw_tp/",		RAW_TRACEPOINT, 0, SEC_NONE, attach_raw_tp),
-	SEC_DEF("raw_tracepoint.w/",	RAW_TRACEPOINT_WRITABLE, 0, SEC_NONE, attach_raw_tp),
-	SEC_DEF("raw_tp.w/",		RAW_TRACEPOINT_WRITABLE, 0, SEC_NONE, attach_raw_tp),
+	SEC_DEF("tracepoint+",		TRACEPOINT, 0, SEC_NONE, attach_tp),
+	SEC_DEF("tp+",			TRACEPOINT, 0, SEC_NONE, attach_tp),
+	SEC_DEF("raw_tracepoint+",	RAW_TRACEPOINT, 0, SEC_NONE, attach_raw_tp),
+	SEC_DEF("raw_tp+",		RAW_TRACEPOINT, 0, SEC_NONE, attach_raw_tp),
+	SEC_DEF("raw_tracepoint.w+",	RAW_TRACEPOINT_WRITABLE, 0, SEC_NONE, attach_raw_tp),
+	SEC_DEF("raw_tp.w+",		RAW_TRACEPOINT_WRITABLE, 0, SEC_NONE, attach_raw_tp),
 	SEC_DEF("tp_btf/",		TRACING, BPF_TRACE_RAW_TP, SEC_ATTACH_BTF, attach_trace),
 	SEC_DEF("fentry/",		TRACING, BPF_TRACE_FENTRY, SEC_ATTACH_BTF, attach_trace),
 	SEC_DEF("fmod_ret/",		TRACING, BPF_MODIFY_RETURN, SEC_ATTACH_BTF, attach_trace),
@@ -10595,6 +10595,12 @@ static int attach_kprobe(const struct bpf_program *prog, long cookie, struct bpf
 	char *func;
 	int n;
 
+	*link = NULL;
+
+	/* no auto-attach for SEC("kprobe") and SEC("kretprobe") */
+	if (strcmp(prog->sec_name, "kprobe") == 0 || strcmp(prog->sec_name, "kretprobe") == 0)
+		return 0;
+
 	opts.retprobe = str_has_pfx(prog->sec_name, "kretprobe/");
 	if (opts.retprobe)
 		func_name = prog->sec_name + sizeof("kretprobe/") - 1;
@@ -10625,6 +10631,13 @@ static int attach_kprobe_multi(const struct bpf_program *prog, long cookie, stru
 	char *pattern;
 	int n;
 
+	*link = NULL;
+
+	/* no auto-attach for SEC("kprobe.multi") and SEC("kretprobe.multi") */
+	if (strcmp(prog->sec_name, "kprobe.multi") == 0 ||
+	    strcmp(prog->sec_name, "kretprobe.multi") == 0)
+		return 0;
+
 	opts.retprobe = str_has_pfx(prog->sec_name, "kretprobe.multi/");
 	if (opts.retprobe)
 		spec = prog->sec_name + sizeof("kretprobe.multi/") - 1;
@@ -11329,6 +11342,12 @@ static int attach_tp(const struct bpf_program *prog, long cookie, struct bpf_lin
 	if (!sec_name)
 		return -ENOMEM;
 
+	*link = NULL;
+
+	/* no auto-attach for SEC("tp") or SEC("tracepoint") */
+	if (strcmp(prog->sec_name, "tp") == 0 || strcmp(prog->sec_name, "tracepoint") == 0)
+		return 0;
+
 	/* extract "tp/<category>/<name>" or "tracepoint/<category>/<name>" */
 	if (str_has_pfx(prog->sec_name, "tp/"))
 		tp_cat = sec_name + sizeof("tp/") - 1;
@@ -11380,20 +11399,34 @@ struct bpf_link *bpf_program__attach_raw_tracepoint(const struct bpf_program *pr
 static int attach_raw_tp(const struct bpf_program *prog, long cookie, struct bpf_link **link)
 {
 	static const char *const prefixes[] = {
-		"raw_tp/",
-		"raw_tracepoint/",
-		"raw_tp.w/",
-		"raw_tracepoint.w/",
+		"raw_tp",
+		"raw_tracepoint",
+		"raw_tp.w",
+		"raw_tracepoint.w",
 	};
 	size_t i;
 	const char *tp_name = NULL;
 
+	*link = NULL;
+
 	for (i = 0; i < ARRAY_SIZE(prefixes); i++) {
-		if (str_has_pfx(prog->sec_name, prefixes[i])) {
-			tp_name = prog->sec_name + strlen(prefixes[i]);
-			break;
-		}
+		size_t pfx_len;
+
+		if (!str_has_pfx(prog->sec_name, prefixes[i]))
+			continue;
+
+		pfx_len = strlen(prefixes[i]);
+		/* no auto-attach case of, e.g., SEC("raw_tp") */
+		if (prog->sec_name[pfx_len] == '\0')
+			return 0;
+
+		if (prog->sec_name[pfx_len] != '/')
+			continue;
+
+		tp_name = prog->sec_name + pfx_len + 1;
+		break;
 	}
+
 	if (!tp_name) {
 		pr_warn("prog '%s': invalid section name '%s'\n",
 			prog->name, prog->sec_name);