net: dccp: switch rx_tstamp_last_feedback to monotonic clock

To compute delays, better not use time of the day which can
be changed by admins or malicious programs.

Also change ccid3_first_li() to use s64 type for delta variable
to avoid potential overflows.

Signed-off-by: Eric Dumazet <[e-mail]>
Cc: Gerrit Renker <[e-mail]>
Cc: [e-mail]
Signed-off-by: David S. Miller <[e-mail]>

net/dccp/ccids/ccid3.c

···
 {
 	struct ccid3_hc_rx_sock *hc = ccid3_hc_rx_sk(sk);
 	struct dccp_sock *dp = dccp_sk(sk);
-	ktime_t now = ktime_get_real();
+	ktime_t now = ktime_get();
 	s64 delta = 0;
 
 	switch (fbtype) {
···
 		return;
 	}
 
-	ccid3_pr_debug("Interval %ldusec, X_recv=%u, 1/p=%u\n", (long)delta,
+	ccid3_pr_debug("Interval %lldusec, X_recv=%u, 1/p=%u\n", delta,
 		       hc->rx_x_recv, hc->rx_pinv);
 
 	hc->rx_tstamp_last_feedback = now;
···
 static u32 ccid3_first_li(struct sock *sk)
 {
 	struct ccid3_hc_rx_sock *hc = ccid3_hc_rx_sk(sk);
-	u32 x_recv, p, delta;
+	u32 x_recv, p;
+	s64 delta;
 	u64 fval;
 
 	if (hc->rx_rtt == 0) {
···
 		hc->rx_rtt = DCCP_FALLBACK_RTT;
 	}
 
-	delta  = ktime_to_us(net_timedelta(hc->rx_tstamp_last_feedback));
+	delta = ktime_us_delta(ktime_get(), hc->rx_tstamp_last_feedback);
+	if (delta <= 0)
+		delta = 1;
 	x_recv = scaled_div32(hc->rx_bytes_recv, delta);
 	if (x_recv == 0) {		/* would also trigger divide-by-zero */
 		DCCP_WARN("X_recv==0\n");