libbpf: Allow loading empty BTFs

stable inclusion
from stable-5.10.80
commit 255eb8f8affd23a37c8823c167da9f9e9ad416d0
bugzilla: 185821 https://gitee.com/openeuler/kernel/issues/I4L7CG

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=255eb8f8affd23a37c8823c167da9f9e9ad416d0

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

[ Upstream commit b8d52264 ]

Empty BTFs do come up (e.g., simple kernel modules with no new types and
strings, compared to the vmlinux BTF) and there is nothing technically wrong
with them. So remove unnecessary check preventing loading empty BTFs.

Fixes: d8123624 ("libbpf: Fix BTF data layout checks and allow empty BTF")
Reported-by: NChristopher William Snowhill <chris@kode54.net>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210110070341.1380086-2-andrii@kernel.orgSigned-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Reviewed-by: NWeilong Chen <chenweilong@huawei.com>
Acked-by: NWeilong Chen <chenweilong@huawei.com>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

tools/lib/bpf/btf.c

@@ -210,11 +210,6 @@ static int btf_parse_hdr(struct btf *btf)
 	}
 
 	meta_left = btf->raw_size - sizeof(*hdr);
-	if (!meta_left) {
-		pr_debug("BTF has no data\n");
-		return -EINVAL;
-	}
-
 	if (meta_left < hdr->str_off + hdr->str_len) {
 		pr_debug("Invalid BTF total size:%u\n", btf->raw_size);
 		return -EINVAL;