net, sched: Add tcf_set_drop_reason for {__,}tcf_classify

Add an initial user for the newly added tcf_set_drop_reason() helper to set the
drop reason for internal errors leading to TC_ACT_SHOT inside {__,}tcf_classify().

Right now this only adds a very basic SKB_DROP_REASON_TC_ERROR as a generic
fallback indicator to mark drop locations. Where needed, such locations can be
converted to more specific codes, for example, when hitting the reclassification
limit, etc.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Victor Nogueira <victor@mojatatu.com>
Link: https://lore.kernel.org/r/20231009092655.22025-2-daniel@iogearbox.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/dropreason-core.h

@@ -80,6 +80,7 @@
 	FN(IPV6_NDISC_BAD_OPTIONS)	\
 	FN(IPV6_NDISC_NS_OTHERHOST)	\
 	FN(QUEUE_PURGE)			\
+	FN(TC_ERROR)			\
 	FNe(MAX)
 
 /**
@@ -345,6 +346,8 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_IPV6_NDISC_NS_OTHERHOST,
 	/** @SKB_DROP_REASON_QUEUE_PURGE: bulk free. */
 	SKB_DROP_REASON_QUEUE_PURGE,
+	/** @SKB_DROP_REASON_TC_ERROR: generic internal tc error. */
+	SKB_DROP_REASON_TC_ERROR,
 	/**
 	 * @SKB_DROP_REASON_MAX: the maximum of core drop reasons, which
 	 * shouldn't be used as a real 'reason' - only for tracing code gen

net/sched/cls_api.c

@@ -1681,12 +1681,16 @@ static inline int __tcf_classify(struct sk_buff *skb,
 			 * time we got here with a cookie from hardware.
 			 */
 			if (unlikely(n->tp != tp || n->tp->chain != n->chain ||
-				     !tp->ops->get_exts))
+				     !tp->ops->get_exts)) {
+				tcf_set_drop_reason(res, SKB_DROP_REASON_TC_ERROR);
 				return TC_ACT_SHOT;
+			}
 
 			exts = tp->ops->get_exts(tp, n->handle);
-			if (unlikely(!exts || n->exts != exts))
+			if (unlikely(!exts || n->exts != exts)) {
+				tcf_set_drop_reason(res, SKB_DROP_REASON_TC_ERROR);
 				return TC_ACT_SHOT;
+			}
 
 			n = NULL;
 			err = tcf_exts_exec_ex(skb, exts, act_index, res);
@@ -1712,8 +1716,10 @@ static inline int __tcf_classify(struct sk_buff *skb,
 			return err;
 	}
 
-	if (unlikely(n))
+	if (unlikely(n)) {
+		tcf_set_drop_reason(res, SKB_DROP_REASON_TC_ERROR);
 		return TC_ACT_SHOT;
+	}
 
 	return TC_ACT_UNSPEC; /* signal: continue lookup */
 #ifdef CONFIG_NET_CLS_ACT
@@ -1723,6 +1729,7 @@ static inline int __tcf_classify(struct sk_buff *skb,
 				       tp->chain->block->index,
 				       tp->prio & 0xffff,
 				       ntohs(tp->protocol));
+		tcf_set_drop_reason(res, SKB_DROP_REASON_TC_ERROR);
 		return TC_ACT_SHOT;
 	}
 
@@ -1759,8 +1766,10 @@ int tcf_classify(struct sk_buff *skb,
 			if (ext->act_miss) {
 				n = tcf_exts_miss_cookie_lookup(ext->act_miss_cookie,
 								&act_index);
-				if (!n)
+				if (!n) {
+					tcf_set_drop_reason(res, SKB_DROP_REASON_TC_ERROR);
 					return TC_ACT_SHOT;
+				}
 
 				chain = n->chain_index;
 			} else {
@@ -1768,8 +1777,10 @@ int tcf_classify(struct sk_buff *skb,
 			}
 
 			fchain = tcf_chain_lookup_rcu(block, chain);
-			if (!fchain)
+			if (!fchain) {
+				tcf_set_drop_reason(res, SKB_DROP_REASON_TC_ERROR);
 				return TC_ACT_SHOT;
+			}
 
 			/* Consume, so cloned/redirect skbs won't inherit ext */
 			skb_ext_del(skb, TC_SKB_EXT);
@@ -1788,8 +1799,11 @@ int tcf_classify(struct sk_buff *skb,
 			struct tc_skb_cb *cb = tc_skb_cb(skb);
 
 			ext = tc_skb_ext_alloc(skb);
-			if (WARN_ON_ONCE(!ext))
+			if (WARN_ON_ONCE(!ext)) {
+				tcf_set_drop_reason(res, SKB_DROP_REASON_TC_ERROR);
 				return TC_ACT_SHOT;
+			}
+
 			ext->chain = last_executed_chain;
 			ext->mru = cb->mru;
 			ext->post_ct = cb->post_ct;