libbpf: Prevent loading vmlinux BTF twice

Prevent loading/parsing vmlinux BTF twice in some cases: for CO-RE relocations
and for BTF-aware hooks (tp_btf, fentry/fexit, etc).

Fixes: a6ed02ca ("libbpf: Load btf_vmlinux only once per object.")
Signed-off-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200624043805.1794620-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -2504,11 +2504,24 @@ static inline bool libbpf_prog_needs_vmlinux_btf(struct bpf_program *prog)
 
 static int bpf_object__load_vmlinux_btf(struct bpf_object *obj)
 {
+	bool need_vmlinux_btf = false;
 	struct bpf_program *prog;
 	int err;
 
+	/* CO-RE relocations need kernel BTF */
+	if (obj->btf_ext && obj->btf_ext->field_reloc_info.len)
+		need_vmlinux_btf = true;
+
 	bpf_object__for_each_program(prog, obj) {
 		if (libbpf_prog_needs_vmlinux_btf(prog)) {
+			need_vmlinux_btf = true;
+			break;
+		}
+	}
+
+	if (!need_vmlinux_btf)
+		return 0;
+
 	obj->btf_vmlinux = libbpf_find_kernel_btf();
 	if (IS_ERR(obj->btf_vmlinux)) {
 		err = PTR_ERR(obj->btf_vmlinux);
@@ -2516,10 +2529,6 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj)
 		obj->btf_vmlinux = NULL;
 		return err;
 	}
-			return 0;
-		}
-	}
-
 	return 0;
 }
 
@@ -4945,8 +4954,8 @@ bpf_core_reloc_fields(struct bpf_object *obj, const char *targ_btf_path)
 	if (targ_btf_path)
 		targ_btf = btf__parse_elf(targ_btf_path, NULL);
 	else
-		targ_btf = libbpf_find_kernel_btf();
-	if (IS_ERR(targ_btf)) {
+		targ_btf = obj->btf_vmlinux;
+	if (IS_ERR_OR_NULL(targ_btf)) {
 		pr_warn("failed to get target BTF: %ld\n", PTR_ERR(targ_btf));
 		return PTR_ERR(targ_btf);
 	}
@@ -4987,6 +4996,8 @@ bpf_core_reloc_fields(struct bpf_object *obj, const char *targ_btf_path)
 	}
 
 out:
+	/* obj->btf_vmlinux is freed at the end of object load phase */
+	if (targ_btf != obj->btf_vmlinux)
 		btf__free(targ_btf);
 	if (!IS_ERR_OR_NULL(cand_cache)) {
 		hashmap__for_each_entry(cand_cache, entry, i) {