net-timestamp: fix missing tcp fragmentation cases

Bytestream timestamps are correlated with a single byte in the skbuff,
recorded in skb_shinfo(skb)->tskey. When fragmenting skbuffs, ensure
that the tskey is set for the fragment in which the tskey falls
(seqno <= tskey < end_seqno).

The original implementation did not address fragmentation in
tcp_fragment or tso_fragment. Add code to inspect the sequence numbers
and move both tskey and the relevant tx_flags if necessary.
Reported-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -1069,6 +1069,21 @@ static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int de
 	tcp_verify_left_out(tp);
 }
 
+static void tcp_fragment_tstamp(struct sk_buff *skb, struct sk_buff *skb2)
+{
+	struct skb_shared_info *shinfo = skb_shinfo(skb);
+
+	if (unlikely(shinfo->tx_flags & SKBTX_ANY_TSTAMP) &&
+	    !before(shinfo->tskey, TCP_SKB_CB(skb2)->seq)) {
+		struct skb_shared_info *shinfo2 = skb_shinfo(skb2);
+		u8 tsflags = shinfo->tx_flags & SKBTX_ANY_TSTAMP;
+
+		shinfo->tx_flags &= ~tsflags;
+		shinfo2->tx_flags |= tsflags;
+		swap(shinfo->tskey, shinfo2->tskey);
+	}
+}
+
 /* Function to create two new TCP segments.  Shrinks the given segment
  * to the specified size and appends a new segment with the rest of the
  * packet to the list.  This won't be called frequently, I hope.
@@ -1136,6 +1151,7 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len,
 	 */
 	TCP_SKB_CB(buff)->when = TCP_SKB_CB(skb)->when;
 	buff->tstamp = skb->tstamp;
+	tcp_fragment_tstamp(skb, buff);
 
 	old_factor = tcp_skb_pcount(skb);
 
@@ -1652,6 +1668,7 @@ static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len,
 
 	buff->ip_summed = skb->ip_summed = CHECKSUM_PARTIAL;
 	skb_split(skb, buff, len);
+	tcp_fragment_tstamp(skb, buff);
 
 	/* Fix up tso_factor for both original and new SKB.  */
 	tcp_set_skb_tso_segs(sk, skb, mss_now);