libbpf: Update a bpf_link with another struct_ops.

Introduce bpf_link__update_map(), which allows to atomically update
underlying struct_ops implementation for given struct_ops BPF link.

Also add old_map_fd to struct bpf_link_update_opts to handle
BPF_F_REPLACE feature.

Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
Link: https://lore.kernel.org/r/20230323032405.3735486-7-kuifeng@meta.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

src/bpf.c

@@ -794,11 +794,17 @@ int bpf_link_update(int link_fd, int new_prog_fd,
 	if (!OPTS_VALID(opts, bpf_link_update_opts))
 		return libbpf_err(-EINVAL);
 
+	if (OPTS_GET(opts, old_prog_fd, 0) && OPTS_GET(opts, old_map_fd, 0))
+		return libbpf_err(-EINVAL);
+
 	memset(&attr, 0, attr_sz);
 	attr.link_update.link_fd = link_fd;
 	attr.link_update.new_prog_fd = new_prog_fd;
 	attr.link_update.flags = OPTS_GET(opts, flags, 0);
-	attr.link_update.old_prog_fd = OPTS_GET(opts, old_prog_fd, 0);
+	if (OPTS_GET(opts, old_prog_fd, 0))
+		attr.link_update.old_prog_fd = OPTS_GET(opts, old_prog_fd, 0);
+	else if (OPTS_GET(opts, old_map_fd, 0))
+		attr.link_update.old_map_fd = OPTS_GET(opts, old_map_fd, 0);
 
 	ret = sys_bpf(BPF_LINK_UPDATE, &attr, attr_sz);
 	return libbpf_err_errno(ret);

src/bpf.h

@@ -336,8 +336,9 @@ struct bpf_link_update_opts {
 	size_t sz; /* size of this struct for forward/backward compatibility */
 	__u32 flags;	   /* extra flags */
 	__u32 old_prog_fd; /* expected old program FD */
+	__u32 old_map_fd;  /* expected old map FD */
 };
-#define bpf_link_update_opts__last_field old_prog_fd
+#define bpf_link_update_opts__last_field old_map_fd
 
 LIBBPF_API int bpf_link_update(int link_fd, int new_prog_fd,
 			       const struct bpf_link_update_opts *opts);

src/libbpf.c

@@ -11677,6 +11677,41 @@ 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_map(struct bpf_link *link, const struct bpf_map *map)
+{
+	struct bpf_link_struct_ops *st_ops_link;
+	__u32 zero = 0;
+	int err;
+
+	if (!bpf_map__is_struct_ops(map) || map->fd < 0)
+		return -EINVAL;
+
+	st_ops_link = container_of(link, struct bpf_link_struct_ops, link);
+	/* Ensure the type of a link is correct */
+	if (st_ops_link->map_fd < 0)
+		return -EINVAL;
+
+	err = bpf_map_update_elem(map->fd, &zero, map->st_ops->kern_vdata, 0);
+	/* It can be EBUSY if the map has been used to create or
+	 * update a link before.  We don't allow updating the value of
+	 * a struct_ops once it is set.  That ensures that the value
+	 * never changed.  So, it is safe to skip EBUSY.
+	 */
+	if (err && err != -EBUSY)
+		return err;
+
+	err = bpf_link_update(link->fd, map->fd, NULL);
+	if (err < 0)
+		return err;
+
+	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);
 

src/libbpf.h

@@ -719,6 +719,7 @@ 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_map(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 */

src/libbpf.map

@@ -386,6 +386,7 @@ LIBBPF_1.1.0 {
 LIBBPF_1.2.0 {
 	global:
 		bpf_btf_get_info_by_fd;
+		bpf_link__update_map;
 		bpf_link_get_info_by_fd;
 		bpf_map_get_info_by_fd;
 		bpf_prog_get_info_by_fd;