libbpf: Introduce bpf_link_get_fd_by_id_opts()

Introduce bpf_link_get_fd_by_id_opts(), for symmetry with
bpf_map_get_fd_by_id_opts(), to let the caller pass the newly introduced
data structure bpf_get_fd_by_id_opts. Keep the existing
bpf_link_get_fd_by_id(), and call bpf_link_get_fd_by_id_opts() with NULL as
opts argument, to prevent setting open_flags.

Currently, the kernel does not support non-zero open_flags for
bpf_link_get_fd_by_id_opts(), and a call with them will result in an error
returned by the bpf() system call. The caller should always pass zero
open_flags.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221006110736.84253-6-roberto.sassu@huaweicloud.com

tools/lib/bpf/bpf.c

@@ -1004,19 +1004,29 @@ int bpf_btf_get_fd_by_id(__u32 id)
 	return bpf_btf_get_fd_by_id_opts(id, NULL);
 }
 
-int bpf_link_get_fd_by_id(__u32 id)
+int bpf_link_get_fd_by_id_opts(__u32 id,
+			       const struct bpf_get_fd_by_id_opts *opts)
 {
 	const size_t attr_sz = offsetofend(union bpf_attr, open_flags);
 	union bpf_attr attr;
 	int fd;
 
+	if (!OPTS_VALID(opts, bpf_get_fd_by_id_opts))
+		return libbpf_err(-EINVAL);
+
 	memset(&attr, 0, attr_sz);
 	attr.link_id = id;
+	attr.open_flags = OPTS_GET(opts, open_flags, 0);
 
 	fd = sys_bpf_fd(BPF_LINK_GET_FD_BY_ID, &attr, attr_sz);
 	return libbpf_err_errno(fd);
 }
 
+int bpf_link_get_fd_by_id(__u32 id)
+{
+	return bpf_link_get_fd_by_id_opts(id, NULL);
+}
+
 int bpf_obj_get_info_by_fd(int bpf_fd, void *info, __u32 *info_len)
 {
 	const size_t attr_sz = offsetofend(union bpf_attr, info);

tools/lib/bpf/bpf.h

@@ -383,6 +383,8 @@ LIBBPF_API int bpf_btf_get_fd_by_id(__u32 id);
 LIBBPF_API int bpf_btf_get_fd_by_id_opts(__u32 id,
 				const struct bpf_get_fd_by_id_opts *opts);
 LIBBPF_API int bpf_link_get_fd_by_id(__u32 id);
+LIBBPF_API int bpf_link_get_fd_by_id_opts(__u32 id,
+				const struct bpf_get_fd_by_id_opts *opts);
 LIBBPF_API int bpf_obj_get_info_by_fd(int bpf_fd, void *info, __u32 *info_len);
 
 struct bpf_prog_query_opts {

tools/lib/bpf/libbpf.map

@@ -372,6 +372,7 @@ LIBBPF_1.0.0 {
 LIBBPF_1.1.0 {
 	global:
 		bpf_btf_get_fd_by_id_opts;
+		bpf_link_get_fd_by_id_opts;
 		bpf_map_get_fd_by_id_opts;
 		bpf_prog_get_fd_by_id_opts;
 		user_ring_buffer__discard;