libbpf: Reject static maps

Static maps never really worked with libbpf, because all such maps were always
silently resolved to the very first map. Detect static maps (both legacy and
BTF-defined) and report user-friendly error.

Tested locally by switching few maps (legacy and BTF-defined) in selftests to
static ones and verifying that now libbpf rejects them loudly.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210513233643.194711-2-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -1795,7 +1795,6 @@ static int bpf_object__init_user_maps(struct bpf_object *obj, bool strict)
 	if (!symbols)
 		return -EINVAL;
 
-
 	scn = elf_sec_by_idx(obj, obj->efile.maps_shndx);
 	data = elf_sec_data(obj, scn);
 	if (!scn || !data) {
@@ -1855,6 +1854,12 @@ static int bpf_object__init_user_maps(struct bpf_object *obj, bool strict)
 			return -LIBBPF_ERRNO__FORMAT;
 		}
 
+		if (GELF_ST_TYPE(sym.st_info) == STT_SECTION
+		    || GELF_ST_BIND(sym.st_info) == STB_LOCAL) {
+			pr_warn("map '%s' (legacy): static maps are not supported\n", map_name);
+			return -ENOTSUP;
+		}
+
 		map->libbpf_type = LIBBPF_MAP_UNSPEC;
 		map->sec_idx = sym.st_shndx;
 		map->sec_offset = sym.st_value;
@@ -2262,6 +2267,16 @@ static void fill_map_from_def(struct bpf_map *map, const struct btf_map_def *def
 		pr_debug("map '%s': found inner map definition.\n", map->name);
 }
 
+static const char *btf_var_linkage_str(__u32 linkage)
+{
+	switch (linkage) {
+	case BTF_VAR_STATIC: return "static";
+	case BTF_VAR_GLOBAL_ALLOCATED: return "global";
+	case BTF_VAR_GLOBAL_EXTERN: return "extern";
+	default: return "unknown";
+	}
+}
+
 static int bpf_object__init_user_btf_map(struct bpf_object *obj,
 					 const struct btf_type *sec,
 					 int var_idx, int sec_idx,
@@ -2294,10 +2309,9 @@ static int bpf_object__init_user_btf_map(struct bpf_object *obj,
 			map_name, btf_kind_str(var));
 		return -EINVAL;
 	}
-	if (var_extra->linkage != BTF_VAR_GLOBAL_ALLOCATED &&
-	    var_extra->linkage != BTF_VAR_STATIC) {
-		pr_warn("map '%s': unsupported var linkage %u.\n",
-			map_name, var_extra->linkage);
+	if (var_extra->linkage != BTF_VAR_GLOBAL_ALLOCATED) {
+		pr_warn("map '%s': unsupported map linkage %s.\n",
+			map_name, btf_var_linkage_str(var_extra->linkage));
 		return -EOPNOTSUPP;
 	}