libbpf: split initialization and loading of BTF

Libbpf does sanitization of BTF before loading it into kernel, if kernel
doesn't support some of newer BTF features. This removes some of the
important information from BTF (e.g., DATASEC and VAR description),
which will be used for map construction. This patch splits BTF
processing into initialization step, in which BTF is initialized from
ELF and all the original data is still preserved; and
sanitization/loading step, which ensures that BTF is safe to load into
kernel. This allows to use full BTF information to construct maps, while
still loading valid BTF into older kernels.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -1113,7 +1113,7 @@ static void bpf_object__sanitize_btf_ext(struct bpf_object *obj)
 	}
 }
 
-static int bpf_object__load_btf(struct bpf_object *obj,
+static int bpf_object__init_btf(struct bpf_object *obj,
 				Elf_Data *btf_data,
 				Elf_Data *btf_ext_data)
 {
@@ -1132,13 +1132,6 @@ static int bpf_object__load_btf(struct bpf_object *obj,
 				   BTF_ELF_SEC, err);
 			goto out;
 		}
-		bpf_object__sanitize_btf(obj);
-		err = btf__load(obj->btf);
-		if (err) {
-			pr_warning("Error loading %s into kernel: %d.\n",
-				   BTF_ELF_SEC, err);
-			goto out;
-		}
 	}
 	if (btf_ext_data) {
 		if (!obj->btf) {
@@ -1154,7 +1147,6 @@ static int bpf_object__load_btf(struct bpf_object *obj,
 			obj->btf_ext = NULL;
 			goto out;
 		}
-		bpf_object__sanitize_btf_ext(obj);
 	}
 out:
 	if (err || IS_ERR(obj->btf)) {
@@ -1165,6 +1157,26 @@ static int bpf_object__load_btf(struct bpf_object *obj,
 	return 0;
 }
 
+static int bpf_object__sanitize_and_load_btf(struct bpf_object *obj)
+{
+	int err = 0;
+
+	if (!obj->btf)
+		return 0;
+
+	bpf_object__sanitize_btf(obj);
+	bpf_object__sanitize_btf_ext(obj);
+
+	err = btf__load(obj->btf);
+	if (err) {
+		pr_warning("Error loading %s into kernel: %d.\n",
+			   BTF_ELF_SEC, err);
+		btf__free(obj->btf);
+		obj->btf = NULL;
+	}
+	return 0;
+}
+
 static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
 {
 	Elf *elf = obj->efile.elf;
@@ -1296,9 +1308,11 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
 		pr_warning("Corrupted ELF file: index of strtab invalid\n");
 		return -LIBBPF_ERRNO__FORMAT;
 	}
-	err = bpf_object__load_btf(obj, btf_data, btf_ext_data);
+	err = bpf_object__init_btf(obj, btf_data, btf_ext_data);
 	if (!err)
 		err = bpf_object__init_maps(obj, flags);
+	if (!err)
+		err = bpf_object__sanitize_and_load_btf(obj);
 	if (!err)
 		err = bpf_object__init_prog_names(obj);
 	return err;