bpf: make bpf_key an opaque type

Since the only consumers of struct bpf_key are bpf scripts which call
the bpf kfuncs which take struct bpf_key, only the implementing
functions in bpf_trace.c should be reaching inside this structure.
Enforce this by making the structure opaque in the header with a body
that's only defined inside bpf_trace.c

Signed-off-by: James Bottomley <[email protected]>

Author: jejb

include/linux/bpf.h

@@ -3656,10 +3656,7 @@ static inline void bpf_cgroup_atype_put(int cgroup_atype) {}
 struct key;
 
 #ifdef CONFIG_KEYS
-struct bpf_key {
-	struct key *key;
-	bool has_ref;
-};
+struct bpf_key;
 #endif /* CONFIG_KEYS */
 
 static inline bool type_is_alloc(u32 type)

kernel/trace/bpf_trace.c

@@ -1242,6 +1242,11 @@ static const struct bpf_func_proto bpf_get_func_arg_cnt_proto = {
 };
 
 #ifdef CONFIG_KEYS
+struct bpf_key {
+	struct key *key;
+	bool has_ref;
+};
+
 __bpf_kfunc_start_defs();
 
 /**