libbpf: Split bpf object load into prepare/load

Introduce bpf_object__prepare API: additional intermediate preparation
step that performs ELF processing, relocations, prepares final state of
BPF program instructions (accessible with bpf_program__insns()), creates
and (potentially) pins maps, and stops short of loading BPF programs.

We anticipate few use cases for this API, such as:
* Use prepare to initialize bpf_token, without loading freplace
programs, unlocking possibility to lookup BTF of other programs.
* Execute prepare to obtain finalized BPF program instructions without
loading programs, enabling tools like veristat to process one program at
a time, without incurring cost of ELF parsing and processing.

Signed-off-by: Mykyta Yatsenko <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/lib/bpf/libbpf.c

···
 
 	for (i = 0; i < obj->nr_programs; i++) {
 		prog = &obj->programs[i];
-		err = bpf_object__sanitize_prog(obj, prog);
-		if (err)
-			return err;
-	}
-
-	for (i = 0; i < obj->nr_programs; i++) {
-		prog = &obj->programs[i];
 		if (prog_is_subprog(obj, prog))
 			continue;
 		if (!prog->autoload) {
···
 	}
 
 	bpf_object__free_relocs(obj);
+	return 0;
+}
+
+static int bpf_object_prepare_progs(struct bpf_object *obj)
+{
+	struct bpf_program *prog;
+	size_t i;
+	int err;
+
+	for (i = 0; i < obj->nr_programs; i++) {
+		prog = &obj->programs[i];
+		err = bpf_object__sanitize_prog(obj, prog);
+		if (err)
+			return err;
+	}
 	return 0;
 }
 
···
 	return 0;
 }
 
+static void bpf_object_unpin(struct bpf_object *obj)
+{
+	int i;
+
+	/* unpin any maps that were auto-pinned during load */
+	for (i = 0; i < obj->nr_maps; i++)
+		if (obj->maps[i].pinned && !obj->maps[i].reused)
+			bpf_map__unpin(&obj->maps[i], NULL);
+}
+
+static void bpf_object_post_load_cleanup(struct bpf_object *obj)
+{
+	int i;
+
+	/* clean up fd_array */
+	zfree(&obj->fd_array);
+
+	/* clean up module BTFs */
+	for (i = 0; i < obj->btf_module_cnt; i++) {
+		close(obj->btf_modules[i].fd);
+		btf__free(obj->btf_modules[i].btf);
+		free(obj->btf_modules[i].name);
+	}
+	obj->btf_module_cnt = 0;
+	zfree(&obj->btf_modules);
+
+	/* clean up vmlinux BTF */
+	btf__free(obj->btf_vmlinux);
+	obj->btf_vmlinux = NULL;
+}
+
+static int bpf_object_prepare(struct bpf_object *obj, const char *target_btf_path)
+{
+	int err;
+
+	if (obj->state >= OBJ_PREPARED) {
+		pr_warn("object '%s': prepare loading can't be attempted twice\n", obj->name);
+		return -EINVAL;
+	}
+
+	err = bpf_object_prepare_token(obj);
+	err = err ? : bpf_object__probe_loading(obj);
+	err = err ? : bpf_object__load_vmlinux_btf(obj, false);
+	err = err ? : bpf_object__resolve_externs(obj, obj->kconfig);
+	err = err ? : bpf_object__sanitize_maps(obj);
+	err = err ? : bpf_object__init_kern_struct_ops_maps(obj);
+	err = err ? : bpf_object_adjust_struct_ops_autoload(obj);
+	err = err ? : bpf_object__relocate(obj, obj->btf_custom_path ? : target_btf_path);
+	err = err ? : bpf_object__sanitize_and_load_btf(obj);
+	err = err ? : bpf_object__create_maps(obj);
+	err = err ? : bpf_object_prepare_progs(obj);
+
+	if (err) {
+		bpf_object_unpin(obj);
+		bpf_object_unload(obj);
+		obj->state = OBJ_LOADED;
+		return err;
+	}
+
+	obj->state = OBJ_PREPARED;
+	return 0;
+}
+
 static int bpf_object_load(struct bpf_object *obj, int extra_log_level, const char *target_btf_path)
 {
-	int err, i;
+	int err;
 
 	if (!obj)
 		return libbpf_err(-EINVAL);
···
 		return libbpf_err(-LIBBPF_ERRNO__ENDIAN);
 	}
 
-	err = bpf_object_prepare_token(obj);
-	err = err ? : bpf_object__probe_loading(obj);
-	err = err ? : bpf_object__load_vmlinux_btf(obj, false);
-	err = err ? : bpf_object__resolve_externs(obj, obj->kconfig);
-	err = err ? : bpf_object__sanitize_maps(obj);
-	err = err ? : bpf_object__init_kern_struct_ops_maps(obj);
-	err = err ? : bpf_object_adjust_struct_ops_autoload(obj);
-	err = err ? : bpf_object__relocate(obj, obj->btf_custom_path ? : target_btf_path);
-	err = err ? : bpf_object__sanitize_and_load_btf(obj);
-	err = err ? : bpf_object__create_maps(obj);
-	err = err ? : bpf_object__load_progs(obj, extra_log_level);
+	if (obj->state < OBJ_PREPARED) {
+		err = bpf_object_prepare(obj, target_btf_path);
+		if (err)
+			return libbpf_err(err);
+	}
+	err = bpf_object__load_progs(obj, extra_log_level);
 	err = err ? : bpf_object_init_prog_arrays(obj);
 	err = err ? : bpf_object_prepare_struct_ops(obj);
 
···
 			err = bpf_gen__finish(obj->gen_loader, obj->nr_programs, obj->nr_maps);
 	}
 
-	/* clean up fd_array */
-	zfree(&obj->fd_array);
-
-	/* clean up module BTFs */
-	for (i = 0; i < obj->btf_module_cnt; i++) {
-		close(obj->btf_modules[i].fd);
-		btf__free(obj->btf_modules[i].btf);
-		free(obj->btf_modules[i].name);
-	}
-	free(obj->btf_modules);
-
-	/* clean up vmlinux BTF */
-	btf__free(obj->btf_vmlinux);
-	obj->btf_vmlinux = NULL;
-
+	bpf_object_post_load_cleanup(obj);
 	obj->state = OBJ_LOADED; /* doesn't matter if successfully or not */
-	if (err)
-		goto out;
+
+	if (err) {
+		bpf_object_unpin(obj);
+		bpf_object_unload(obj);
+		pr_warn("failed to load object '%s'\n", obj->path);
+		return libbpf_err(err);
+	}
 
 	return 0;
-out:
-	/* unpin any maps that were auto-pinned during load */
-	for (i = 0; i < obj->nr_maps; i++)
-		if (obj->maps[i].pinned && !obj->maps[i].reused)
-			bpf_map__unpin(&obj->maps[i], NULL);
+}
 
-	bpf_object_unload(obj);
-	pr_warn("failed to load object '%s'\n", obj->path);
-	return libbpf_err(err);
+int bpf_object__prepare(struct bpf_object *obj)
+{
+	return libbpf_err(bpf_object_prepare(obj, NULL));
 }
 
 int bpf_object__load(struct bpf_object *obj)
···
 
 	if (IS_ERR_OR_NULL(obj))
 		return;
+
+	/*
+	 * if user called bpf_object__prepare() without ever getting to
+	 * bpf_object__load(), we need to clean up stuff that is normally
+	 * cleaned up at the end of loading step
+	 */
+	bpf_object_post_load_cleanup(obj);
 
 	usdt_manager_free(obj->usdt_man);
 	obj->usdt_man = NULL;

tools/lib/bpf/libbpf.h

···
 		     const struct bpf_object_open_opts *opts);
 
 /**
+ * @brief **bpf_object__prepare()** prepares BPF object for loading:
+ * performs ELF processing, relocations, prepares final state of BPF program
+ * instructions (accessible with bpf_program__insns()), creates and
+ * (potentially) pins maps. Leaves BPF object in the state ready for program
+ * loading.
+ * @param obj Pointer to a valid BPF object instance returned by
+ * **bpf_object__open*()** API
+ * @return 0, on success; negative error code, otherwise, error code is
+ * stored in errno
+ */
+int bpf_object__prepare(struct bpf_object *obj);
+
+/**
  * @brief **bpf_object__load()** loads BPF object into kernel.
  * @param obj Pointer to a valid BPF object instance returned by
  * **bpf_object__open*()** APIs

tools/lib/bpf/libbpf.map

···
 		bpf_linker__add_buf;
 		bpf_linker__add_fd;
 		bpf_linker__new_fd;
+		bpf_object__prepare;
 		btf__add_decl_attr;
 		btf__add_type_attr;
 } LIBBPF_1.5.0;