netfilter: nf_tables: wrap tracing with a static key

Only needed when meta nftrace rule(s) were added.
The assumption is that no such rules are active, so the call to
nft_trace_init is "never" needed.

When nftrace rules are active, we always call the nft_trace_* functions,
but will only send netlink messages when all of the following are true:

 - traceinfo structure was initialised
 - skb->nf_trace == 1
 - at least one subscriber to trace group.

Adding an extra conditional
(static_branch ... && skb->nf_trace)
	nft_trace_init( ..)

Is possible but results in a larger nft_do_chain footprint.

Signed-off-by: Florian Westphal <[email protected]>
Acked-by: Patrick McHardy <[email protected]>
Signed-off-by: Pablo Neira Ayuso <[email protected]>

include/net/netfilter/nf_tables_core.h

@@ -57,6 +57,7 @@ struct nft_payload_set {
 };
 
 extern const struct nft_expr_ops nft_payload_fast_ops;
+extern struct static_key_false nft_trace_enabled;
 
 int nft_payload_module_init(void);
 void nft_payload_module_exit(void);

include/net/netfilter/nft_meta.h

@@ -33,4 +33,7 @@ void nft_meta_set_eval(const struct nft_expr *expr,
 		       struct nft_regs *regs,
 		       const struct nft_pktinfo *pkt);
 
+void nft_meta_set_destroy(const struct nft_ctx *ctx,
+			  const struct nft_expr *expr);
+
 #endif