[PATCH bpf-next 6/7] libbpf: Update a bpf_link with another struct_ops.

[Kui-Feng Lee <kuifeng@xxxxxxxx>]

Introduce bpf_link__update_struct_ops(), which will allow you to
effortlessly transition the struct_ops map of any given bpf_link into
an alternative.

Signed-off-by: Kui-Feng Lee <kuifeng@xxxxxxxx>
---
 tools/lib/bpf/libbpf.c   | 35 +++++++++++++++++++++++++++++++++++
 tools/lib/bpf/libbpf.h   |  1 +
 tools/lib/bpf/libbpf.map |  1 +
 3 files changed, 37 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 1eff6a03ddd9..6f7c72e312d4 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -11524,6 +11524,41 @@ struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map)
 	return &link->link;
 }
 
+/*
+ * Swap the back struct_ops of a link with a new struct_ops map.
+ */
+int bpf_link__update_struct_ops(struct bpf_link *link, const struct bpf_map *map)
+{
+	struct bpf_link_struct_ops_map *st_ops_link;
+	int err, fd;
+
+	if (!bpf_map__is_struct_ops(map) || map->fd == -1)
+		return -EINVAL;
+
+	/* Ensure the type of a link is correct */
+	if (link->detach != bpf_link__detach_struct_ops)
+		return -EINVAL;
+
+	err = bpf_map__update_vdata(map);
+	if (err) {
+		err = -errno;
+		free(link);
+		return err;
+	}
+
+	fd = bpf_link_update(link->fd, map->fd, NULL);
+	if (fd < 0) {
+		err = -errno;
+		free(link);
+		return err;
+	}
+
+	st_ops_link = container_of(link, struct bpf_link_struct_ops_map, link);
+	st_ops_link->map_fd = map->fd;
+
+	return 0;
+}
+
 typedef enum bpf_perf_event_ret (*bpf_perf_event_print_t)(struct perf_event_header *hdr,
 							  void *private_data);
 
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 2efd80f6f7b9..dd25cd6759d4 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -695,6 +695,7 @@ bpf_program__attach_freplace(const struct bpf_program *prog,
 struct bpf_map;
 
 LIBBPF_API struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map);
+LIBBPF_API int bpf_link__update_struct_ops(struct bpf_link *link, const struct bpf_map *map);
 
 struct bpf_iter_attach_opts {
 	size_t sz; /* size of this struct for forward/backward compatibility */
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 11c36a3c1a9f..ca6993c744b6 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -373,6 +373,7 @@ LIBBPF_1.1.0 {
 	global:
 		bpf_btf_get_fd_by_id_opts;
 		bpf_link_get_fd_by_id_opts;
+		bpf_link__update_struct_ops;
 		bpf_map_get_fd_by_id_opts;
 		bpf_prog_get_fd_by_id_opts;
 		user_ring_buffer__discard;
-- 
2.30.2