Re: [PATCH net] tcp: fix wraparound issue in tcp_lp

On Mon, 2017-05-01 at 16:56 -0700, Stephen Hemminger wrote:

On Mon, 01 May 2017 15:29:48 -0700
Eric Dumazet [off-list ref] wrote:

quoted

From: Eric Dumazet <edumazet@google.com>

Be careful when comparing tcp_time_stamp to some u32 quantity,
otherwise result can be surprising.

Fixes: 7c106d7e782b ("[TCP]: TCP Low Priority congestion control")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/tcp_lp.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_lp.c b/net/ipv4/tcp_lp.c
index 046fd3910873306d74207615d6997e1c847ea361..d6fb6c067af4641f232b94e7c590c212648e8173 100644
--- a/net/ipv4/tcp_lp.c
+++ b/net/ipv4/tcp_lp.c

@@ -264,13 +264,15 @@ static void tcp_lp_pkts_acked(struct sock *sk, const struct ack_sample *sample)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct lp *lp = inet_csk_ca(sk);
+	u32 delta;
 
 	if (sample->rtt_us > 0)
 		tcp_lp_rtt_sample(sk, sample->rtt_us);
 
 	/* calc inference */
-	if (tcp_time_stamp > tp->rx_opt.rcv_tsecr)
-		lp->inference = 3 * (tcp_time_stamp - tp->rx_opt.rcv_tsecr);
+	delta = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
+	if ((s32)delta > 0)
+		lp->inference = 3 * delta;

Agreed time wraparound would cause problems.
But why not use existing time_after() macro here?

Simply to not perform (tcp_time_stamp - tp->rx_opt.rcv_tsecr) twice.

jiffies being volatile, this can not be optimized by the compiler.

I have a patch series (for linux-4.13) that will switch TCP stack to 1ms
TS options, regardless of CONFIG_HZ value, and when cooking it I found
this bug.