samples/bpf: Fix sockex3 error: Missing BPF prog type

since commit 450b167fb9be("libbpf: clean up SEC() handling"),
sec_def_matches() does not recognize "socket/xxx" as "socket", therefore,
the BPF program type is not recognized.

Instead of sockex3_user.c parsing section names to get the BPF program fd.
We use the program array map to assign a static index to each BPF program
(get inspired by selftests/bpf progs/test_prog_array_init.c).
Therefore, use SEC("socket") as section name instead of SEC("socket/xxx"),
so that the BPF program is parsed to SOCKET_FILTER type. The "missing BPF
prog type" problem is solved.

How to reproduce this error:
$ cd samples/bpf
$ sudo ./sockex3
libbpf: prog 'bpf_func_PARSE_IP': missing BPF prog type, check ELF section name 'socket/3'
libbpf: prog 'bpf_func_PARSE_IP': failed to load: -22
libbpf: failed to load object './sockex3_kern.o'
ERROR: loading BPF object file failed

Signed-off-by: Rong Tao <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

samples/bpf/sockex3_kern.c

···
 #define IP_MF		0x2000
 #define IP_OFFSET	0x1FFF
 
-#define PROG(F) SEC("socket/"__stringify(F)) int bpf_func_##F
-
-struct {
-	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
-	__uint(key_size, sizeof(u32));
-	__uint(value_size, sizeof(u32));
-	__uint(max_entries, 8);
-} jmp_table SEC(".maps");
-
 #define PARSE_VLAN 1
 #define PARSE_MPLS 2
 #define PARSE_IP 3
 #define PARSE_IPV6 4
-
-/* Protocol dispatch routine. It tail-calls next BPF program depending
- * on eth proto. Note, we could have used ...
- *
- *   bpf_tail_call(skb, &jmp_table, proto);
- *
- * ... but it would need large prog_array and cannot be optimised given
- * the map key is not static.
- */
-static inline void parse_eth_proto(struct __sk_buff *skb, u32 proto)
-{
-	switch (proto) {
-	case ETH_P_8021Q:
-	case ETH_P_8021AD:
-		bpf_tail_call(skb, &jmp_table, PARSE_VLAN);
-		break;
-	case ETH_P_MPLS_UC:
-	case ETH_P_MPLS_MC:
-		bpf_tail_call(skb, &jmp_table, PARSE_MPLS);
-		break;
-	case ETH_P_IP:
-		bpf_tail_call(skb, &jmp_table, PARSE_IP);
-		break;
-	case ETH_P_IPV6:
-		bpf_tail_call(skb, &jmp_table, PARSE_IPV6);
-		break;
-	}
-}
 
 struct vlan_hdr {
 	__be16 h_vlan_TCI;
···
 	};
 	__u32 ip_proto;
 };
+
+static inline void parse_eth_proto(struct __sk_buff *skb, u32 proto);
 
 static inline int ip_is_fragment(struct __sk_buff *ctx, __u64 nhoff)
 {
···
 	}
 }
 
-PROG(PARSE_IP)(struct __sk_buff *skb)
+SEC("socket")
+int bpf_func_ip(struct __sk_buff *skb)
 {
 	struct globals *g = this_cpu_globals();
 	__u32 nhoff, verlen, ip_proto;
···
 	return 0;
 }
 
-PROG(PARSE_IPV6)(struct __sk_buff *skb)
+SEC("socket")
+int bpf_func_ipv6(struct __sk_buff *skb)
 {
 	struct globals *g = this_cpu_globals();
 	__u32 nhoff, ip_proto;
···
 	return 0;
 }
 
-PROG(PARSE_VLAN)(struct __sk_buff *skb)
+SEC("socket")
+int bpf_func_vlan(struct __sk_buff *skb)
 {
 	__u32 nhoff, proto;
 
···
 	return 0;
 }
 
-PROG(PARSE_MPLS)(struct __sk_buff *skb)
+SEC("socket")
+int bpf_func_mpls(struct __sk_buff *skb)
 {
 	__u32 nhoff, label;
 
···
 	return 0;
 }
 
-SEC("socket/0")
+struct {
+	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
+	__uint(key_size, sizeof(u32));
+	__uint(max_entries, 8);
+	__array(values, u32 (void *));
+} prog_array_init SEC(".maps") = {
+	.values = {
+		[PARSE_VLAN] = (void *)&bpf_func_vlan,
+		[PARSE_IP]   = (void *)&bpf_func_ip,
+		[PARSE_IPV6] = (void *)&bpf_func_ipv6,
+		[PARSE_MPLS] = (void *)&bpf_func_mpls,
+	},
+};
+
+/* Protocol dispatch routine. It tail-calls next BPF program depending
+ * on eth proto. Note, we could have used ...
+ *
+ *   bpf_tail_call(skb, &prog_array_init, proto);
+ *
+ * ... but it would need large prog_array and cannot be optimised given
+ * the map key is not static.
+ */
+static inline void parse_eth_proto(struct __sk_buff *skb, u32 proto)
+{
+	switch (proto) {
+	case ETH_P_8021Q:
+	case ETH_P_8021AD:
+		bpf_tail_call(skb, &prog_array_init, PARSE_VLAN);
+		break;
+	case ETH_P_MPLS_UC:
+	case ETH_P_MPLS_MC:
+		bpf_tail_call(skb, &prog_array_init, PARSE_MPLS);
+		break;
+	case ETH_P_IP:
+		bpf_tail_call(skb, &prog_array_init, PARSE_IP);
+		break;
+	case ETH_P_IPV6:
+		bpf_tail_call(skb, &prog_array_init, PARSE_IPV6);
+		break;
+	}
+}
+
+SEC("socket")
 int main_prog(struct __sk_buff *skb)
 {
 	__u32 nhoff = ETH_HLEN;

samples/bpf/sockex3_user.c

···
 
 int main(int argc, char **argv)
 {
-	int i, sock, key, fd, main_prog_fd, jmp_table_fd, hash_map_fd;
+	int i, sock, fd, main_prog_fd, hash_map_fd;
 	struct bpf_program *prog;
 	struct bpf_object *obj;
-	const char *section;
 	char filename[256];
 	FILE *f;
 
···
 		goto cleanup;
 	}
 
-	jmp_table_fd = bpf_object__find_map_fd_by_name(obj, "jmp_table");
 	hash_map_fd = bpf_object__find_map_fd_by_name(obj, "hash_map");
-	if (jmp_table_fd < 0 || hash_map_fd < 0) {
+	if (hash_map_fd < 0) {
 		fprintf(stderr, "ERROR: finding a map in obj file failed\n");
 		goto cleanup;
 	}
 
+	/* find BPF main program */
+	main_prog_fd = 0;
 	bpf_object__for_each_program(prog, obj) {
 		fd = bpf_program__fd(prog);
 
-		section = bpf_program__section_name(prog);
-		if (sscanf(section, "socket/%d", &key) != 1) {
-			fprintf(stderr, "ERROR: finding prog failed\n");
-			goto cleanup;
-		}
-
-		if (key == 0)
+		if (!strcmp(bpf_program__name(prog), "main_prog"))
 			main_prog_fd = fd;
-		else
-			bpf_map_update_elem(jmp_table_fd, &key, &fd, BPF_ANY);
+	}
+
+	if (main_prog_fd == 0) {
+		fprintf(stderr, "ERROR: can't find main_prog\n");
+		goto cleanup;
 	}
 
 	sock = open_raw_sock("lo");