sctp: identify chunks that need to be fragmented at IP level

Previously, without GSO, it was easy to identify it: if the chunk didn't
fit and there was no data chunk in the packet yet, we could fragment at
IP level. So if there was an auth chunk and we were bundling a big data
chunk, it would fragment regardless of the size of the auth chunk. This
also works for the context of PMTU reductions.

But with GSO, we cannot distinguish such PMTU events anymore, as the
packet is allowed to exceed PMTU.

So we need another check: to ensure that the chunk that we are adding,
actually fits the current PMTU. If it doesn't, trigger a flush and let
it be fragmented at IP level in the next round.

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

net/sctp/output.c

@@ -878,7 +878,7 @@ static sctp_xmit_t sctp_packet_will_fit(struct sctp_packet *packet,
 					struct sctp_chunk *chunk,
 					u16 chunk_len)
 {
-	size_t psize, pmtu;
+	size_t psize, pmtu, maxsize;
 	sctp_xmit_t retval = SCTP_XMIT_OK;
 
 	psize = packet->size;
@@ -906,6 +906,17 @@ static sctp_xmit_t sctp_packet_will_fit(struct sctp_packet *packet,
 			goto out;
 		}
 
+		/* Similarly, if this chunk was built before a PMTU
+		 * reduction, we have to fragment it at IP level now. So
+		 * if the packet already contains something, we need to
+		 * flush.
+		 */
+		maxsize = pmtu - packet->overhead;
+		if (packet->auth)
+			maxsize -= WORD_ROUND(packet->auth->skb->len);
+		if (chunk_len > maxsize)
+			retval = SCTP_XMIT_PMTU_FULL;
+
 		/* It is also okay to fragment if the chunk we are
 		 * adding is a control chunk, but only if current packet
 		 * is not a GSO one otherwise it causes fragmentation of