libbpf: Show supported ELF section names when failing to guess prog/attach type

We need to let users check their wrong ELF section name with proper
ELF section names when they fail to get a prog/attach type from it.
Because users can't realize libbpf guess prog/attach types from given
ELF section names. For example, when a 'cgroup' section name of a
BPF program is used, show available ELF section names(types).

Before:

    $ bpftool prog load bpf-prog.o /sys/fs/bpf/prog1
    Error: failed to guess program type based on ELF section name cgroup

After:

    libbpf: failed to guess program type based on ELF section name 'cgroup'
    libbpf: supported section(type) names are: socket kprobe/ kretprobe/ classifier action tracepoint/ raw_tracepoint/ xdp perf_event lwt_in lwt_out lwt_xmit lwt_seg6local cgroup_skb/ingress cgroup_skb/egress cgroup/skb cgroup/sock cgroup/post_bind4 cgroup/post_bind6 cgroup/dev sockops sk_skb/stream_parser sk_skb/stream_verdict sk_skb sk_msg lirc_mode2 flow_dissector cgroup/bind4 cgroup/bind6 cgroup/connect4 cgroup/connect6 cgroup/sendmsg4 cgroup/sendmsg6

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Quentin Monnet <quentin.monnet@netronome.com>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Andrey Ignatov <rdna@fb.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -2667,9 +2667,38 @@ static const struct {
 #undef BPF_EAPROG_SEC
 #undef BPF_APROG_COMPAT
 
+#define MAX_TYPE_NAME_SIZE 32
+
+static char *libbpf_get_type_names(bool attach_type)
+{
+	int i, len = ARRAY_SIZE(section_names) * MAX_TYPE_NAME_SIZE;
+	char *buf;
+
+	buf = malloc(len);
+	if (!buf)
+		return NULL;
+
+	buf[0] = '\0';
+	/* Forge string buf with all available names */
+	for (i = 0; i < ARRAY_SIZE(section_names); i++) {
+		if (attach_type && !section_names[i].is_attachable)
+			continue;
+
+		if (strlen(buf) + strlen(section_names[i].sec) + 2 > len) {
+			free(buf);
+			return NULL;
+		}
+		strcat(buf, " ");
+		strcat(buf, section_names[i].sec);
+	}
+
+	return buf;
+}
+
 int libbpf_prog_type_by_name(const char *name, enum bpf_prog_type *prog_type,
 			     enum bpf_attach_type *expected_attach_type)
 {
+	char *type_names;
 	int i;
 
 	if (!name)
@@ -2682,12 +2711,20 @@ int libbpf_prog_type_by_name(const char *name, enum bpf_prog_type *prog_type,
 		*expected_attach_type = section_names[i].expected_attach_type;
 		return 0;
 	}
+	pr_warning("failed to guess program type based on ELF section name '%s'\n", name);
+	type_names = libbpf_get_type_names(false);
+	if (type_names != NULL) {
+		pr_info("supported section(type) names are:%s\n", type_names);
+		free(type_names);
+	}
+
 	return -EINVAL;
 }
 
 int libbpf_attach_type_by_name(const char *name,
 			       enum bpf_attach_type *attach_type)
 {
+	char *type_names;
 	int i;
 
 	if (!name)
@@ -2701,6 +2738,13 @@ int libbpf_attach_type_by_name(const char *name,
 		*attach_type = section_names[i].attach_type;
 		return 0;
 	}
+	pr_warning("failed to guess attach type based on ELF section name '%s'\n", name);
+	type_names = libbpf_get_type_names(true);
+	if (type_names != NULL) {
+		pr_info("attachable section(type) names are:%s\n", type_names);
+		free(type_names);
+	}
+
 	return -EINVAL;
 }
 
@@ -2907,8 +2951,6 @@ int bpf_prog_load_xattr(const struct bpf_prog_load_attr *attr,
 			err = bpf_program__identify_section(prog, &prog_type,
 							    &expected_attach_type);
 			if (err < 0) {
-				pr_warning("failed to guess program type based on section name %s\n",
-					   prog->section_name);
 				bpf_object__close(obj);
 				return -EINVAL;
 			}