[PATCH bpf-next v2 4/6] bpf: Update the struct_ops of a bpf_link.

[Kui-Feng Lee <kuifeng@meta.com>]

By improving the BPF_LINK_UPDATE command of bpf(), it should allow you
to conveniently switch between different struct_ops on a single
bpf_link. This would enable smoother transitions from one struct_ops
to another.

The struct_ops maps passing along with BPF_LINK_UPDATE should have the
BPF_F_LINK flag.

Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
---
 include/linux/bpf.h         |  1 +
 include/uapi/linux/bpf.h    |  8 +++--
 kernel/bpf/bpf_struct_ops.c | 69 +++++++++++++++++++++++++++++++++++++
 kernel/bpf/syscall.c        | 32 +++++++++++++++++
 4 files changed, 108 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 7508ca89e814..9797d9d87a3e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1409,6 +1409,7 @@ struct bpf_link_ops {
 	void (*show_fdinfo)(const struct bpf_link *link, struct seq_file *seq);
 	int (*fill_link_info)(const struct bpf_link *link,
 			      struct bpf_link_info *info);
+	int (*update_struct_ops)(struct bpf_link *link, struct bpf_map *new_map);
 };
 
 struct bpf_tramp_link {
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index cd0ff39981e8..0702e88f7c08 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1555,8 +1555,12 @@ union bpf_attr {
 
 	struct { /* struct used by BPF_LINK_UPDATE command */
 		__u32		link_fd;	/* link fd */
-		/* new program fd to update link with */
-		__u32		new_prog_fd;
+		union {
+			/* new program fd to update link with */
+			__u32		new_prog_fd;
+			/* new struct_ops map fd to update link with */
+			__u32           new_map_fd;
+		};
 		__u32		flags;		/* extra flags */
 		/* expected link's program fd; is specified only if
 		 * BPF_F_REPLACE flag is set in flags */
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index cfc69033c1b8..700dc95a6daa 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -961,11 +961,80 @@ static int bpf_struct_ops_map_link_fill_link_info(const struct bpf_link *link,
 	return 0;
 }
 
+static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map *new_map)
+{
+	struct bpf_struct_ops_value *kvalue;
+	struct bpf_struct_ops_map *st_map, *old_st_map;
+	struct bpf_struct_ops_link *st_link;
+	struct bpf_map *old_map;
+	int err = 0;
+
+	if (new_map->map_type != BPF_MAP_TYPE_STRUCT_OPS)
+		return -EINVAL;
+
+	/* Ensure that the registration of the struct_ops matches the
+	 * value of the pointer within the link.
+	 */
+	mutex_lock(&update_mutex);
+
+	st_link = container_of(link, struct bpf_struct_ops_link, link);
+
+	old_map = st_link->map;
+	if (!old_map) {
+		err = -EINVAL;
+		goto err_out;
+	}
+
+	/* The new and old struct_ops must be the same type. */
+	st_map = container_of(new_map, struct bpf_struct_ops_map, map);
+
+	if (!(new_map->map_flags & BPF_F_LINK)) {
+		err = -EINVAL;
+		goto err_out;
+	}
+
+	old_st_map = container_of(old_map, struct bpf_struct_ops_map, map);
+	if (st_map->st_ops != old_st_map->st_ops) {
+		err = -EINVAL;
+		goto err_out;
+	}
+
+	err = bpf_struct_ops_transit_state_check(st_map, BPF_STRUCT_OPS_STATE_UNREG,
+						 BPF_STRUCT_OPS_STATE_INUSE);
+	if (err)
+		goto err_out;
+
+	kvalue = &st_map->kvalue;
+
+	set_memory_rox((long)st_map->image, 1);
+	err = st_map->st_ops->update(kvalue->data, old_st_map->kvalue.data);
+	if (err) {
+		bpf_struct_ops_restore_unreg(st_map);
+
+		set_memory_nx((long)st_map->image, 1);
+		set_memory_rw((long)st_map->image, 1);
+		goto err_out;
+	}
+
+	bpf_map_inc(new_map);
+	rcu_assign_pointer(st_link->map, new_map);
+
+	bpf_struct_ops_transit_state(old_st_map, BPF_STRUCT_OPS_STATE_INUSE,
+				     BPF_STRUCT_OPS_STATE_TOBEUNREG);
+	bpf_struct_ops_put(&old_st_map->kvalue.data);
+
+err_out:
+	mutex_unlock(&update_mutex);
+
+	return err;
+}
+
 static const struct bpf_link_ops bpf_struct_ops_map_lops = {
 	.dealloc = bpf_struct_ops_map_link_dealloc,
 	.detach = bpf_struct_ops_map_link_detach,
 	.show_fdinfo = bpf_struct_ops_map_link_show_fdinfo,
 	.fill_link_info = bpf_struct_ops_map_link_fill_link_info,
+	.update_struct_ops = bpf_struct_ops_map_link_update,
 };
 
 int bpf_struct_ops_link_create(union bpf_attr *attr)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 2670de8dd0d4..423e6b7a6b41 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -4647,6 +4647,30 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
 	return ret;
 }
 
+static int link_update_struct_ops(struct bpf_link *link, union bpf_attr *attr)
+{
+	struct bpf_map *new_map;
+	int ret = 0;
+
+	new_map = bpf_map_get(attr->link_update.new_map_fd);
+	if (IS_ERR(new_map))
+		return -EINVAL;
+
+	if (new_map->map_type != BPF_MAP_TYPE_STRUCT_OPS) {
+		ret = -EINVAL;
+		goto out_put_map;
+	}
+
+	if (link->ops->update_struct_ops)
+		ret = link->ops->update_struct_ops(link, new_map);
+	else
+		ret = -EINVAL;
+
+out_put_map:
+	bpf_map_put(new_map);
+	return ret;
+}
+
 #define BPF_LINK_UPDATE_LAST_FIELD link_update.old_prog_fd
 
 static int link_update(union bpf_attr *attr)
@@ -4667,6 +4691,14 @@ static int link_update(union bpf_attr *attr)
 	if (IS_ERR(link))
 		return PTR_ERR(link);
 
+	if (link->ops->update_struct_ops) {
+		if (flags)	/* always replace the existing one */
+			ret = -EINVAL;
+		else
+			ret = link_update_struct_ops(link, attr);
+		goto out_put_link;
+	}
+
 	new_prog = bpf_prog_get(attr->link_update.new_prog_fd);
 	if (IS_ERR(new_prog)) {
 		ret = PTR_ERR(new_prog);
-- 
2.30.2