bpf: Allow empty module BTFs

mainline inclusion
from mainline-5.11-rc5
commit bcc5e616
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EUVD
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bcc5e6162d66d44f7929f30fce032f95855fc8b4

-------------------------------------------------

Some modules don't declare any new types and end up with an empty BTF,
containing only valid BTF header and no types or strings sections. This
currently causes BTF validation error. There is nothing wrong with such BTF,
so fix the issue by allowing module BTFs with no types or strings.

Fixes: 36e68442 ("bpf: Load and verify kernel module BTFs")
Reported-by: NChristopher William Snowhill <chris@kode54.net>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210110070341.1380086-1-andrii@kernel.org
(cherry picked from commit bcc5e616)
Signed-off-by: NWang Yufen <wangyufen@huawei.com>

kernel/bpf/btf.c

@@ -4172,7 +4172,7 @@ static int btf_parse_hdr(struct btf_verifier_env *env)
 		return -ENOTSUPP;
 	}
 
-	if (btf_data_size == hdr->hdr_len) {
+	if (!btf->base_btf && btf_data_size == hdr->hdr_len) {
 		btf_verifier_log(env, "No data");
 		return -EINVAL;
 	}