netfilter: make nftables drops visible in net dropmonitor

net_dropmonitor blames core.c:nf_hook_slow.
Add NF_DROP_REASON() helper and use it in nft_do_chain().

The helper releases the skb, so exact drop location becomes
available. Calling code will observe the NF_STOLEN verdict
instead.

Adjust nf_hook_slow so we can embed an erro value wih
NF_STOLEN verdicts, just like we do for NF_DROP.

After this, drop in nftables can be pinpointed to a drop due
to a rule or the chain policy.
Signed-off-by: Florian Westphal <fw@strlen.de>

include/linux/netfilter.h

@@ -22,6 +22,16 @@ static inline int NF_DROP_GETERR(int verdict)
 	return -(verdict >> NF_VERDICT_QBITS);
 }
 
+static __always_inline int
+NF_DROP_REASON(struct sk_buff *skb, enum skb_drop_reason reason, u32 err)
+{
+	BUILD_BUG_ON(err > 0xffff);
+
+	kfree_skb_reason(skb, reason);
+
+	return ((err << 16) | NF_STOLEN);
+}
+
 static inline int nf_inet_addr_cmp(const union nf_inet_addr *a1,
 				   const union nf_inet_addr *a2)
 {

net/netfilter/core.c

@@ -639,10 +639,10 @@ int nf_hook_slow(struct sk_buff *skb, struct nf_hook_state *state,
 			if (ret == 1)
 				continue;
 			return ret;
+		case NF_STOLEN:
+			return NF_DROP_GETERR(verdict);
 		default:
-			/* Implicit handling for NF_STOLEN, as well as any other
-			 * non conventional verdicts.
-			 */
+			WARN_ON_ONCE(1);
 			return 0;
 		}
 	}

net/netfilter/nf_tables_core.c

@@ -308,10 +308,11 @@ nft_do_chain(struct nft_pktinfo *pkt, void *priv)
 
 	switch (regs.verdict.code & NF_VERDICT_MASK) {
 	case NF_ACCEPT:
-	case NF_DROP:
 	case NF_QUEUE:
 	case NF_STOLEN:
 		return regs.verdict.code;
+	case NF_DROP:
+		return NF_DROP_REASON(pkt->skb, SKB_DROP_REASON_NETFILTER_DROP, EPERM);
 	}
 
 	switch (regs.verdict.code) {
@@ -342,6 +343,9 @@ nft_do_chain(struct nft_pktinfo *pkt, void *priv)
 	if (static_branch_unlikely(&nft_counters_enabled))
 		nft_update_chain_stats(basechain, pkt);
 
+	if (nft_base_chain(basechain)->policy == NF_DROP)
+		return NF_DROP_REASON(pkt->skb, SKB_DROP_REASON_NETFILTER_DROP, EPERM);
+
 	return nft_base_chain(basechain)->policy;
 }
 EXPORT_SYMBOL_GPL(nft_do_chain);