[PATCH bpf-next 3/4] libbpf: add subskeleton scaffolding

[Delyan Kratunov <delyank@fb.com>]

In symmetry with bpf_object__open_skeleton(),
bpf_object__open_subskeleton() performs the actual walking and linking
of symbols described by bpf_sym_skeleton objects.

Signed-off-by: Delyan Kratunov <delyank@fb.com>
---
 tools/lib/bpf/libbpf.c   | 76 ++++++++++++++++++++++++++++++++++++++++
 tools/lib/bpf/libbpf.h   | 21 +++++++++++
 tools/lib/bpf/libbpf.map |  2 ++
 3 files changed, 99 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index d20ae8f225ee..e6c27f4b9dea 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -11748,6 +11748,82 @@ int bpf_object__open_skeleton(struct bpf_object_skeleton *s,
 	return 0;
 }
 
+int bpf_object__open_subskeleton(struct bpf_object_subskeleton *s)
+{
+	int i, len, map_type_id, sym_idx;
+	const char *var_name;
+	struct bpf_map *map;
+	struct btf *btf;
+	const struct btf_type *map_type, *var_type;
+	const struct bpf_sym_skeleton *sym;
+	struct btf_var_secinfo *var;
+	struct bpf_map *last_map = NULL;
+	const struct btf_type *last_map_type = NULL;
+
+	if (!s->obj)
+		return libbpf_err(-EINVAL);
+
+	btf = bpf_object__btf(s->obj);
+	if (!btf)
+		return libbpf_err(errno);
+
+	for (sym_idx = 0; sym_idx < s->sym_cnt; sym_idx++) {
+		sym = &s->syms[sym_idx];
+		if (last_map && (strcmp(sym->section, bpf_map__section_name(last_map)) == 0)) {
+			map = last_map;
+			map_type = last_map_type;
+		} else {
+			map = bpf_object__find_map_by_name(s->obj, sym->section);
+			if (!map) {
+				pr_warn("Could not find map for section %1$s, symbol %2$s",
+					sym->section, s->syms[i].name);
+				return libbpf_err(-EINVAL);
+			}
+			map_type_id = btf__find_by_name_kind(btf, sym->section, BTF_KIND_DATASEC);
+			if (map_type_id < 0) {
+				pr_warn("Could not find map type in btf for section %1$s (due to symbol %2$s)",
+					sym->section, sym->name);
+				return libbpf_err(-EINVAL);
+			}
+			map_type = btf__type_by_id(btf, map_type_id);
+		}
+
+		/* We have a section and a corresponding type, now find the
+		 * symbol in the loaded map. This is clearly quadratic in the
+		 * number of symbols in the section, but that's easy to optimize
+		 * once the need arises.
+		 */
+
+		len = btf_vlen(map_type);
+		for (i = 0, var = btf_var_secinfos(map_type); i < len; i++, var++) {
+			var_type = btf__type_by_id(btf, var->type);
+			if (!var_type) {
+				pr_warn("Could not find var type for item %1$d in section %2$s",
+					i, sym->section);
+				return libbpf_err(-EINVAL);
+			}
+			var_name = btf__name_by_offset(btf, var_type->name_off);
+			if (strcmp(var_name, sym->name) == 0) {
+				*sym->addr = (char *) map->mmaped + var->offset;
+				break;
+			}
+		}
+
+		last_map = map;
+		last_map_type = map_type;
+	}
+	return 0;
+}
+
+void bpf_object__destroy_subskeleton(struct bpf_object_subskeleton *s)
+{
+	if (!s)
+		return;
+	if (s->syms)
+		free(s->syms);
+	free(s);
+}
+
 int bpf_object__load_skeleton(struct bpf_object_skeleton *s)
 {
 	int i, err;
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 7b66794f1c0a..915d59c31ad5 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -1291,6 +1291,27 @@ LIBBPF_API int bpf_object__attach_skeleton(struct bpf_object_skeleton *s);
 LIBBPF_API void bpf_object__detach_skeleton(struct bpf_object_skeleton *s);
 LIBBPF_API void bpf_object__destroy_skeleton(struct bpf_object_skeleton *s);
 
+struct bpf_sym_skeleton {
+	const char *name;
+	const char *section;
+	void **addr;
+};
+
+struct bpf_object_subskeleton {
+	size_t sz; /* size of this struct, for forward/backward compatibility */
+
+	const struct bpf_object *obj;
+
+	int sym_cnt;
+	int sym_skel_sz;
+	struct bpf_sym_skeleton *syms;
+};
+
+LIBBPF_API int
+bpf_object__open_subskeleton(struct bpf_object_subskeleton *s);
+LIBBPF_API void
+bpf_object__destroy_subskeleton(struct bpf_object_subskeleton *s);
+
 struct gen_loader_opts {
 	size_t sz; /* size of this struct, for forward/backward compatiblity */
 	const char *data;
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 5c85d297d955..81a1d0259866 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -443,4 +443,6 @@ LIBBPF_0.7.0 {
 LIBBPF_0.8.0 {
 	global:
     bpf_map__section_name;
+    bpf_object__open_subskeleton;
+    bpf_object__destroy_subskeleton;
 } LIBBPF_0.7.0;
-- 
2.34.1