bpf: Allow struct_ops to get map id by kdata

Add bpf_struct_ops_id() to enable struct_ops implementors to use
struct_ops map id as the unique id of a struct_ops in their subsystem.
A subsystem that wishes to create a mapping between id and struct_ops
instance pointer can update the mapping accordingly during
bpf_struct_ops::reg(), unreg(), and update().

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250806162540.681679-2-ameryhung@gmail.com

include/linux/bpf.h

@@ -1985,6 +1985,7 @@ static inline void bpf_module_put(const void *data, struct module *owner)
 		module_put(owner);
 }
 int bpf_struct_ops_link_create(union bpf_attr *attr);
+u32 bpf_struct_ops_id(const void *kdata);
 
 #ifdef CONFIG_NET
 /* Define it here to avoid the use of forward declaration */

kernel/bpf/bpf_struct_ops.c

@@ -1174,6 +1174,18 @@ void bpf_struct_ops_put(const void *kdata)
 	bpf_map_put(&st_map->map);
 }
 
+u32 bpf_struct_ops_id(const void *kdata)
+{
+	struct bpf_struct_ops_value *kvalue;
+	struct bpf_struct_ops_map *st_map;
+
+	kvalue = container_of(kdata, struct bpf_struct_ops_value, data);
+	st_map = container_of(kvalue, struct bpf_struct_ops_map, kvalue);
+
+	return st_map->map.id;
+}
+EXPORT_SYMBOL_GPL(bpf_struct_ops_id);
+
 static bool bpf_struct_ops_valid_to_reg(struct bpf_map *map)
 {
 	struct bpf_struct_ops_map *st_map = (struct bpf_struct_ops_map *)map;