ipv6: Remove dependency of ipv6_frag_thdr_truncated on ipv6 module

IPV6=m
NF_DEFRAG_IPV6=y

ld: net/ipv6/netfilter/nf_conntrack_reasm.o: in function
`nf_ct_frag6_gather':
net/ipv6/netfilter/nf_conntrack_reasm.c:462: undefined reference to
`ipv6_frag_thdr_truncated'

Netfilter is depending on ipv6 symbol ipv6_frag_thdr_truncated. This
dependency is forcing IPV6=y.

Remove this dependency by moving ipv6_frag_thdr_truncated out of ipv6. This
is the same solution as used with a similar issues: Referring to
commit 70b095c ("ipv6: remove dependency of nf_defrag_ipv6 on ipv6
module")

Fixes: 9d9e937 ("ipv6/netfilter: Discard first fragment not including all headers")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Georg Kohmann <geokohma@cisco.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Link: https://lore.kernel.org/r/20201119095833.8409-1-geokohma@cisco.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Georg Kohmann

include/net/ipv6.h

@@ -1064,8 +1064,6 @@ int ipv6_skip_exthdr(const struct sk_buff *, int start, u8 *nexthdrp,
 
 bool ipv6_ext_hdr(u8 nexthdr);
 
-bool ipv6_frag_thdr_truncated(struct sk_buff *skb, int start, u8 *nexthdrp);
-
 enum {
 	IP6_FH_F_FRAG		= (1 << 0),
 	IP6_FH_F_AUTH		= (1 << 1),

include/net/ipv6_frag.h

@@ -108,5 +108,35 @@ ip6frag_expire_frag_queue(struct net *net, struct frag_queue *fq)
 	rcu_read_unlock();
 	inet_frag_put(&fq->q);
 }
+
+/* Check if the upper layer header is truncated in the first fragment. */
+static inline bool
+ipv6frag_thdr_truncated(struct sk_buff *skb, int start, u8 *nexthdrp)
+{
+	u8 nexthdr = *nexthdrp;
+	__be16 frag_off;
+	int offset;
+
+	offset = ipv6_skip_exthdr(skb, start, &nexthdr, &frag_off);
+	if (offset < 0 || (frag_off & htons(IP6_OFFSET)))
+		return false;
+	switch (nexthdr) {
+	case NEXTHDR_TCP:
+		offset += sizeof(struct tcphdr);
+		break;
+	case NEXTHDR_UDP:
+		offset += sizeof(struct udphdr);
+		break;
+	case NEXTHDR_ICMP:
+		offset += sizeof(struct icmp6hdr);
+		break;
+	default:
+		offset += 1;
+	}
+	if (offset > skb->len)
+		return true;
+	return false;
+}
+
 #endif
 #endif

net/ipv6/netfilter/nf_conntrack_reasm.c

@@ -459,7 +459,7 @@ int nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 user)
 	/* Discard the first fragment if it does not include all headers
 	 * RFC 8200, Section 4.5
 	 */
-	if (ipv6_frag_thdr_truncated(skb, fhoff, &nexthdr)) {
+	if (ipv6frag_thdr_truncated(skb, fhoff, &nexthdr)) {
 		pr_debug("Drop incomplete fragment\n");
 		return 0;
 	}

net/ipv6/reassembly.c

@@ -318,35 +318,6 @@ static int ip6_frag_reasm(struct frag_queue *fq, struct sk_buff *skb,
 	return -1;
 }
 
-/* Check if the upper layer header is truncated in the first fragment. */
-bool ipv6_frag_thdr_truncated(struct sk_buff *skb, int start, u8 *nexthdrp)
-{
-	u8 nexthdr = *nexthdrp;
-	__be16 frag_off;
-	int offset;
-
-	offset = ipv6_skip_exthdr(skb, start, &nexthdr, &frag_off);
-	if (offset < 0 || (frag_off & htons(IP6_OFFSET)))
-		return false;
-	switch (nexthdr) {
-	case NEXTHDR_TCP:
-		offset += sizeof(struct tcphdr);
-		break;
-	case NEXTHDR_UDP:
-		offset += sizeof(struct udphdr);
-		break;
-	case NEXTHDR_ICMP:
-		offset += sizeof(struct icmp6hdr);
-		break;
-	default:
-		offset += 1;
-	}
-	if (offset > skb->len)
-		return true;
-	return false;
-}
-EXPORT_SYMBOL(ipv6_frag_thdr_truncated);
-
 static int ipv6_frag_rcv(struct sk_buff *skb)
 {
 	struct frag_hdr *fhdr;
@@ -390,7 +361,7 @@ static int ipv6_frag_rcv(struct sk_buff *skb)
 	 * the source of the fragment, with the Pointer field set to zero.
 	 */
 	nexthdr = hdr->nexthdr;
-	if (ipv6_frag_thdr_truncated(skb, skb_transport_offset(skb), &nexthdr)) {
+	if (ipv6frag_thdr_truncated(skb, skb_transport_offset(skb), &nexthdr)) {
 		__IP6_INC_STATS(net, __in6_dev_get_safely(skb->dev),
 				IPSTATS_MIB_INHDRERRORS);
 		icmpv6_param_prob(skb, ICMPV6_HDR_INCOMP, 0);