bpf: Return -ENOTSUPP when attaching to non-kernel BTF

Return -ENOTSUPP if tracing BPF program is attempted to be attached with
specified attach_btf_obj_fd pointing to non-kernel (neither vmlinux nor
module) BTF object. This scenario might be supported in the future and isn't
outright invalid, so -EINVAL isn't the most appropriate error code.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201208064326.667389-1-andrii@kernel.org

kernel/bpf/syscall.c

@@ -2121,8 +2121,11 @@ static int bpf_prog_load(union bpf_attr *attr, union bpf_attr __user *uattr)
 			if (IS_ERR(attach_btf))
 				return -EINVAL;
 			if (!btf_is_kernel(attach_btf)) {
+				/* attaching through specifying bpf_prog's BTF
+				 * objects directly might be supported eventually
+				 */
 				btf_put(attach_btf);
-				return -EINVAL;
+				return -ENOTSUPP;
 			}
 		}
 	} else if (attr->attach_btf_id) {