libbpf: deprecate legacy BPF map definitions

Enact deprecation of legacy BPF map definition in SEC("maps") ([0]). For
the definitions themselves introduce LIBBPF_STRICT_MAP_DEFINITIONS flag
for libbpf strict mode. If it is set, error out on any struct
bpf_map_def-based map definition. If not set, libbpf will print out
a warning for each legacy BPF map to raise awareness that it goes away.

For any use of BPF_ANNOTATE_KV_PAIR() macro providing a legacy way to
associate BTF key/value type information with legacy BPF map definition,
warn through libbpf's pr_warn() error message (but don't fail BPF object
open).

BPF-side struct bpf_map_def is marked as deprecated. User-space struct
bpf_map_def has to be used internally in libbpf, so it is left
untouched. It should be enough for bpf_map__def() to be marked
deprecated to raise awareness that it goes away.

bpftool is an interesting case that utilizes libbpf to open BPF ELF
object to generate skeleton. As such, even though bpftool itself uses
full on strict libbpf mode (LIBBPF_STRICT_ALL), it has to relax it a bit
for BPF map definition handling to minimize unnecessary disruptions. So
opt-out of LIBBPF_STRICT_MAP_DEFINITIONS for bpftool. User's code that
will later use generated skeleton will make its own decision whether to
enforce LIBBPF_STRICT_MAP_DEFINITIONS or not.

There are few tests in selftests/bpf that are consciously using legacy
BPF map definitions to test libbpf functionality. For those, temporary
opt out of LIBBPF_STRICT_MAP_DEFINITIONS mode for the duration of those
tests.

  [0] Closes: https://github.com/libbpf/libbpf/issues/272

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220120060529.1890907-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/main.c

@@ -478,7 +478,14 @@ int main(int argc, char **argv)
 	}
 
 	if (!legacy_libbpf) {
-		ret = libbpf_set_strict_mode(LIBBPF_STRICT_ALL);
+		enum libbpf_strict_mode mode;
+
+		/* Allow legacy map definitions for skeleton generation.
+		 * It will still be rejected if users use LIBBPF_STRICT_ALL
+		 * mode for loading generated skeleton.
+		 */
+		mode = (__LIBBPF_STRICT_LAST - 1) & ~LIBBPF_STRICT_MAP_DEFINITIONS;
+		ret = libbpf_set_strict_mode(mode);
 		if (ret)
 			p_err("failed to enable libbpf strict mode: %d", ret);
 	}

tools/lib/bpf/bpf_helpers.h

@@ -133,7 +133,7 @@ struct bpf_map_def {
 	unsigned int value_size;
 	unsigned int max_entries;
 	unsigned int map_flags;
-};
+} __attribute__((deprecated("use BTF-defined maps in .maps section")));
 
 enum libbpf_pin_type {
 	LIBBPF_PIN_NONE,

tools/lib/bpf/libbpf.c

@@ -1937,6 +1937,11 @@ static int bpf_object__init_user_maps(struct bpf_object *obj, bool strict)
 	if (obj->efile.maps_shndx < 0)
 		return 0;
 
+	if (libbpf_mode & LIBBPF_STRICT_MAP_DEFINITIONS) {
+		pr_warn("legacy map definitions in SEC(\"maps\") are not supported\n");
+		return -EOPNOTSUPP;
+	}
+
 	if (!symbols)
 		return -EINVAL;
 
@@ -1999,6 +2004,8 @@ static int bpf_object__init_user_maps(struct bpf_object *obj, bool strict)
 			return -LIBBPF_ERRNO__FORMAT;
 		}
 
+		pr_warn("map '%s' (legacy): legacy map definitions are deprecated, use BTF-defined maps instead\n", map_name);
+
 		if (ELF64_ST_BIND(sym->st_info) == STB_LOCAL) {
 			pr_warn("map '%s' (legacy): static maps are not supported\n", map_name);
 			return -ENOTSUP;
@@ -4190,6 +4197,7 @@ static int bpf_map_find_btf_info(struct bpf_object *obj, struct bpf_map *map)
 		return 0;
 
 	if (!bpf_map__is_internal(map)) {
+		pr_warn("Use of BPF_ANNOTATE_KV_PAIR is deprecated, use BTF-defined maps in .maps section instead\n");
 		ret = btf__get_map_kv_tids(obj->btf, map->name, def->key_size,
 					   def->value_size, &key_type_id,
 					   &value_type_id);

tools/lib/bpf/libbpf_legacy.h

@@ -73,6 +73,11 @@ enum libbpf_strict_mode {
 	 * operation.
 	 */
 	LIBBPF_STRICT_AUTO_RLIMIT_MEMLOCK = 0x10,
+	/*
+	 * Error out on any SEC("maps") map definition, which are deprecated
+	 * in favor of BTF-defined map definitions in SEC(".maps").
+	 */
+	LIBBPF_STRICT_MAP_DEFINITIONS = 0x20,
 
 	__LIBBPF_STRICT_LAST,
 };

tools/testing/selftests/bpf/prog_tests/btf.c

@@ -4560,6 +4560,8 @@ static void do_test_file(unsigned int test_num)
 	has_btf_ext = btf_ext != NULL;
 	btf_ext__free(btf_ext);
 
+	/* temporary disable LIBBPF_STRICT_MAP_DEFINITIONS to test legacy maps */
+	libbpf_set_strict_mode((__LIBBPF_STRICT_LAST - 1) & ~LIBBPF_STRICT_MAP_DEFINITIONS);
 	obj = bpf_object__open(test->file);
 	err = libbpf_get_error(obj);
 	if (CHECK(err, "obj: %d", err))
@@ -4684,6 +4686,8 @@ skip:
 	fprintf(stderr, "OK");
 
 done:
+	libbpf_set_strict_mode(LIBBPF_STRICT_ALL);
+
 	btf__free(btf);
 	free(func_info);
 	bpf_object__close(obj);