bpf: Introduce BPF_ANNOTATE_KV_PAIR

This patch introduces BPF_ANNOTATE_KV_PAIR to signal the
bpf loader about the btf key_type and value_type of a bpf map.
Please refer to the changes in test_btf_haskv.c for its usage.
Both iproute2 and libbpf loader will then have the same
convention to find out the map's btf_key_type_id and
btf_value_type_id from a map's name.

Fixes: 8a138aed ("bpf: btf: Add BTF support to libbpf")
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/btf.c

@@ -189,8 +189,7 @@ static int btf_parse_type_sec(struct btf *btf, btf_print_fn_t err_log)
 	return 0;
 }
 
-static const struct btf_type *btf_type_by_id(const struct btf *btf,
-					     __u32 type_id)
+const struct btf_type *btf__type_by_id(const struct btf *btf, __u32 type_id)
 {
 	if (type_id > btf->nr_types)
 		return NULL;
@@ -233,7 +232,7 @@ __s64 btf__resolve_size(const struct btf *btf, __u32 type_id)
 	__s64 size = -1;
 	int i;
 
-	t = btf_type_by_id(btf, type_id);
+	t = btf__type_by_id(btf, type_id);
 	for (i = 0; i < MAX_RESOLVE_DEPTH && !btf_type_is_void_or_null(t);
 	     i++) {
 		size = btf_type_size(t);
@@ -258,7 +257,7 @@ __s64 btf__resolve_size(const struct btf *btf, __u32 type_id)
 			return -EINVAL;
 		}
 
-		t = btf_type_by_id(btf, type_id);
+		t = btf__type_by_id(btf, type_id);
 	}
 
 	if (size < 0)

tools/lib/bpf/btf.h

@@ -9,6 +9,7 @@
 #define BTF_ELF_SEC ".BTF"
 
 struct btf;
+struct btf_type;
 
 typedef int (*btf_print_fn_t)(const char *, ...)
 	__attribute__((format(printf, 1, 2)));
@@ -16,6 +17,7 @@ typedef int (*btf_print_fn_t)(const char *, ...)
 void btf__free(struct btf *btf);
 struct btf *btf__new(__u8 *data, __u32 size, btf_print_fn_t err_log);
 __s32 btf__find_by_name(const struct btf *btf, const char *type_name);
+const struct btf_type *btf__type_by_id(const struct btf *btf, __u32 id);
 __s64 btf__resolve_size(const struct btf *btf, __u32 type_id);
 int btf__fd(const struct btf *btf);
 

tools/lib/bpf/libbpf.c

@@ -36,6 +36,7 @@
 #include <linux/err.h>
 #include <linux/kernel.h>
 #include <linux/bpf.h>
+#include <linux/btf.h>
 #include <linux/list.h>
 #include <linux/limits.h>
 #include <sys/stat.h>
@@ -1014,68 +1015,72 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
 
 static int bpf_map_find_btf_info(struct bpf_map *map, const struct btf *btf)
 {
+	const struct btf_type *container_type;
+	const struct btf_member *key, *value;
 	struct bpf_map_def *def = &map->def;
 	const size_t max_name = 256;
+	char container_name[max_name];
 	__s64 key_size, value_size;
-	__s32 key_id, value_id;
-	char name[max_name];
+	__s32 container_id;
 
-	/* Find key type by name from BTF */
-	if (snprintf(name, max_name, "%s_key", map->name) == max_name) {
-		pr_warning("map:%s length of BTF key_type:%s_key is too long\n",
+	if (snprintf(container_name, max_name, "____btf_map_%s", map->name) ==
+	    max_name) {
+		pr_warning("map:%s length of '____btf_map_%s' is too long\n",
 			   map->name, map->name);
 		return -EINVAL;
 	}
 
-	key_id = btf__find_by_name(btf, name);
-	if (key_id < 0) {
-		pr_debug("map:%s key_type:%s cannot be found in BTF\n",
-			 map->name, name);
-		return key_id;
+	container_id = btf__find_by_name(btf, container_name);
+	if (container_id < 0) {
+		pr_debug("map:%s container_name:%s cannot be found in BTF. Missing BPF_ANNOTATE_KV_PAIR?\n",
+			 map->name, container_name);
+		return container_id;
 	}
 
-	key_size = btf__resolve_size(btf, key_id);
-	if (key_size < 0) {
-		pr_warning("map:%s key_type:%s cannot get the BTF type_size\n",
-			   map->name, name);
-		return key_size;
+	container_type = btf__type_by_id(btf, container_id);
+	if (!container_type) {
+		pr_warning("map:%s cannot find BTF type for container_id:%u\n",
+			   map->name, container_id);
+		return -EINVAL;
 	}
 
-	if (def->key_size != key_size) {
-		pr_warning("map:%s key_type:%s has BTF type_size:%u != key_size:%u\n",
-			   map->name, name, (unsigned int)key_size, def->key_size);
+	if (BTF_INFO_KIND(container_type->info) != BTF_KIND_STRUCT ||
+	    BTF_INFO_VLEN(container_type->info) < 2) {
+		pr_warning("map:%s container_name:%s is an invalid container struct\n",
+			   map->name, container_name);
 		return -EINVAL;
 	}
 
-	/* Find value type from BTF */
-	if (snprintf(name, max_name, "%s_value", map->name) == max_name) {
-		pr_warning("map:%s length of BTF value_type:%s_value is too long\n",
-			  map->name, map->name);
-		return -EINVAL;
+	key = (struct btf_member *)(container_type + 1);
+	value = key + 1;
+
+	key_size = btf__resolve_size(btf, key->type);
+	if (key_size < 0) {
+		pr_warning("map:%s invalid BTF key_type_size\n",
+			   map->name);
+		return key_size;
 	}
 
-	value_id = btf__find_by_name(btf, name);
-	if (value_id < 0) {
-		pr_debug("map:%s value_type:%s cannot be found in BTF\n",
-			 map->name, name);
-		return value_id;
+	if (def->key_size != key_size) {
+		pr_warning("map:%s btf_key_type_size:%u != map_def_key_size:%u\n",
+			   map->name, (__u32)key_size, def->key_size);
+		return -EINVAL;
 	}
 
-	value_size = btf__resolve_size(btf, value_id);
+	value_size = btf__resolve_size(btf, value->type);
 	if (value_size < 0) {
-		pr_warning("map:%s value_type:%s cannot get the BTF type_size\n",
-			   map->name, name);
+		pr_warning("map:%s invalid BTF value_type_size\n", map->name);
 		return value_size;
 	}
 
 	if (def->value_size != value_size) {
-		pr_warning("map:%s value_type:%s has BTF type_size:%u != value_size:%u\n",
-			   map->name, name, (unsigned int)value_size, def->value_size);
+		pr_warning("map:%s btf_value_type_size:%u != map_def_value_size:%u\n",
+			   map->name, (__u32)value_size, def->value_size);
 		return -EINVAL;
 	}
 
-	map->btf_key_type_id = key_id;
-	map->btf_value_type_id = value_id;
+	map->btf_key_type_id = key->type;
+	map->btf_value_type_id = value->type;
 
 	return 0;
 }

tools/testing/selftests/bpf/bpf_helpers.h

@@ -158,6 +158,15 @@ struct bpf_map_def {
 	unsigned int numa_node;
 };
 
+#define BPF_ANNOTATE_KV_PAIR(name, type_key, type_val)		\
+	struct ____btf_map_##name {				\
+		type_key key;					\
+		type_val value;					\
+	};							\
+	struct ____btf_map_##name				\
+	__attribute__ ((section(".maps." #name), used))		\
+		____btf_map_##name = { }
+
 static int (*bpf_skb_load_bytes)(void *ctx, int off, void *to, int len) =
 	(void *) BPF_FUNC_skb_load_bytes;
 static int (*bpf_skb_store_bytes)(void *ctx, int off, void *from, int len, int flags) =

tools/testing/selftests/bpf/test_btf_haskv.c

@@ -10,11 +10,6 @@ struct ipv_counts {
 	unsigned int v6;
 };
 
-typedef int btf_map_key;
-typedef struct ipv_counts btf_map_value;
-btf_map_key dumm_key;
-btf_map_value dummy_value;
-
 struct bpf_map_def SEC("maps") btf_map = {
 	.type = BPF_MAP_TYPE_ARRAY,
 	.key_size = sizeof(int),
@@ -22,6 +17,8 @@ struct bpf_map_def SEC("maps") btf_map = {
 	.max_entries = 4,
 };
 
+BPF_ANNOTATE_KV_PAIR(btf_map, int, struct ipv_counts);
+
 struct dummy_tracepoint_args {
 	unsigned long long pad;
 	struct sock *sock;