sctp: introduce sctp_mtu_payload

When given a MTU, this function calculates how much payload we can carry
on it. Without a MTU, it calculates the amount of header overhead we
have.

So that when we have extra overhead, like the one added for IP options
on SELinux patches, it is easier to handle it.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/sctp.h

@@ -607,6 +607,25 @@ static inline struct dst_entry *sctp_transport_dst_check(struct sctp_transport *
 	return t->dst;
 }
 
+/* Calculate max payload size given a MTU, or the total overhead if
+ * given MTU is zero
+ */
+static inline __u32 sctp_mtu_payload(const struct sctp_sock *sp,
+				     __u32 mtu, __u32 extra)
+{
+	__u32 overhead = sizeof(struct sctphdr) + extra;
+
+	if (sp)
+		overhead += sp->pf->af->net_header_len;
+	else
+		overhead += sizeof(struct ipv6hdr);
+
+	if (WARN_ON_ONCE(mtu && mtu <= overhead))
+		mtu = overhead;
+
+	return mtu ? mtu - overhead : overhead;
+}
+
 static inline bool sctp_transport_pmtu_check(struct sctp_transport *t)
 {
 	__u32 pmtu = max_t(size_t, SCTP_TRUNC4(dst_mtu(t->dst)),

net/sctp/output.c

@@ -90,8 +90,8 @@ void sctp_packet_config(struct sctp_packet *packet, __u32 vtag,
 {
 	struct sctp_transport *tp = packet->transport;
 	struct sctp_association *asoc = tp->asoc;
+	struct sctp_sock *sp = NULL;
 	struct sock *sk;
-	size_t overhead = sizeof(struct ipv6hdr) + sizeof(struct sctphdr);
 
 	pr_debug("%s: packet:%p vtag:0x%x\n", __func__, packet, vtag);
 	packet->vtag = vtag;
@@ -102,25 +102,20 @@ void sctp_packet_config(struct sctp_packet *packet, __u32 vtag,
 
 	/* set packet max_size with pathmtu, then calculate overhead */
 	packet->max_size = tp->pathmtu;
+
 	if (asoc) {
-		struct sctp_sock *sp = sctp_sk(asoc->base.sk);
-		struct sctp_af *af = sp->pf->af;
-
-		overhead = af->net_header_len +
-			   af->ip_options_len(asoc->base.sk);
-		overhead += sizeof(struct sctphdr);
-		packet->overhead = overhead;
-		packet->size = overhead;
-	} else {
-		packet->overhead = overhead;
-		packet->size = overhead;
-		return;
+		sk = asoc->base.sk;
+		sp = sctp_sk(sk);
 	}
+	packet->overhead = sctp_mtu_payload(sp, 0, 0);
+	packet->size = packet->overhead;
+
+	if (!asoc)
+		return;
 
 	/* update dst or transport pathmtu if in need */
-	sk = asoc->base.sk;
 	if (!sctp_transport_dst_check(tp)) {
-		sctp_transport_route(tp, NULL, sctp_sk(sk));
+		sctp_transport_route(tp, NULL, sp);
 		if (asoc->param_flags & SPP_PMTUD_ENABLE)
 			sctp_assoc_sync_pmtu(asoc);
 	} else if (!sctp_transport_pmtu_check(tp)) {

net/sctp/socket.c

@@ -3234,11 +3234,8 @@ static int sctp_setsockopt_maxseg(struct sock *sk, char __user *optval, unsigned
 	if (val) {
 		int min_len, max_len;
 
-		min_len = SCTP_DEFAULT_MINSEGMENT - af->net_header_len;
-		min_len -= af->ip_options_len(sk);
-		min_len -= sizeof(struct sctphdr) +
-			   sizeof(struct sctp_data_chunk);
-
+		min_len = sctp_mtu_payload(sp, SCTP_DEFAULT_MINSEGMENT,
+					   sizeof(struct sctp_data_chunk));
 		max_len = SCTP_MAX_CHUNK_LEN - sizeof(struct sctp_data_chunk);
 
 		if (val < min_len || val > max_len)