net: dropreason: add SKB_DROP_REASON_FRAG_TOO_FAR

IPv4 reassembly unit can decide to drop frags based on
/proc/sys/net/ipv4/ipfrag_max_dist sysctl.

Add a specific drop reason to track this specific
and weird case.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/dropreason.h

@@ -70,6 +70,7 @@
 	FN(PKT_TOO_BIG)			\
 	FN(DUP_FRAG)			\
 	FN(FRAG_REASM_TIMEOUT)		\
+	FN(FRAG_TOO_FAR)		\
 	FNe(MAX)
 
 /**
@@ -306,6 +307,11 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_DUP_FRAG,
 	/** @SKB_DROP_REASON_FRAG_REASM_TIMEOUT: fragment reassembly timeout */
 	SKB_DROP_REASON_FRAG_REASM_TIMEOUT,
+	/**
+	 * @SKB_DROP_REASON_FRAG_TOO_FAR: ipv4 fragment too far.
+	 * (/proc/sys/net/ipv4/ipfrag_max_dist)
+	 */
+	SKB_DROP_REASON_FRAG_TOO_FAR,
 	/**
 	 * @SKB_DROP_REASON_MAX: the maximum of drop reason, which shouldn't be
 	 * used as a real 'reason'

net/ipv4/ip_fragment.c

@@ -256,7 +256,7 @@ static int ip_frag_reinit(struct ipq *qp)
 	}
 
 	sum_truesize = inet_frag_rbtree_purge(&qp->q.rb_fragments,
-					      SKB_DROP_REASON_NOT_SPECIFIED);
+					      SKB_DROP_REASON_FRAG_TOO_FAR);
 	sub_frag_mem_limit(qp->q.fqdir, sum_truesize);
 
 	qp->q.flags = 0;