tcp: add accessors to read/set tp->snd_cwnd

We had various bugs over the years with code
breaking the assumption that tp->snd_cwnd is greater
than zero.

Lately, syzbot reported the WARN_ON_ONCE(!tp->prior_cwnd) added
in commit 8b8a321ff72c ("tcp: fix zero cwnd in tcp_cwnd_reduction")
can trigger, and without a repro we would have to spend
considerable time finding the bug.

Instead of complaining too late, we want to catch where
and when tp->snd_cwnd is set to an illegal value.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Link: https://lore.kernel.org/r/20220405233538.947344-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 9 insertions, 9 deletions

diff --git a/net/ipv4/tcp_hybla.c b/net/ipv4/tcp_hybla.c
index be39327e04e6..abd7d91807e5 100644
--- a/net/ipv4/tcp_hybla.c
+++ b/net/ipv4/tcp_hybla.c
@@ -54,7 +54,7 @@ static void hybla_init(struct sock *sk)
 	ca->rho2_7ls = 0;
 	ca->snd_cwnd_cents = 0;
 	ca->hybla_en = true;
-	tp->snd_cwnd = 2;
+	tcp_snd_cwnd_set(tp, 2);
 	tp->snd_cwnd_clamp = 65535;
 
 	/* 1st Rho measurement based on initial srtt */
@@ -62,7 +62,7 @@ static void hybla_init(struct sock *sk)
 
 	/* set minimum rtt as this is the 1st ever seen */
 	ca->minrtt_us = tp->srtt_us;
-	tp->snd_cwnd = ca->rho;
+	tcp_snd_cwnd_set(tp, ca->rho);
 }
 
 static void hybla_state(struct sock *sk, u8 ca_state)
@@ -137,31 +137,31 @@ static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 		 * as long as increment is estimated as (rho<<7)/window
 		 * it already is <<7 and we can easily count its fractions.
 		 */
-		increment = ca->rho2_7ls / tp->snd_cwnd;
+		increment = ca->rho2_7ls / tcp_snd_cwnd(tp);
 		if (increment < 128)
 			tp->snd_cwnd_cnt++;
 	}
 
 	odd = increment % 128;
-	tp->snd_cwnd += increment >> 7;
+	tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + (increment >> 7));
 	ca->snd_cwnd_cents += odd;
 
 	/* check when fractions goes >=128 and increase cwnd by 1. */
 	while (ca->snd_cwnd_cents >= 128) {
-		tp->snd_cwnd++;
+		tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
 		ca->snd_cwnd_cents -= 128;
 		tp->snd_cwnd_cnt = 0;
 	}
 	/* check when cwnd has not been incremented for a while */
-	if (increment == 0 && odd == 0 && tp->snd_cwnd_cnt >= tp->snd_cwnd) {
-		tp->snd_cwnd++;
+	if (increment == 0 && odd == 0 && tp->snd_cwnd_cnt >= tcp_snd_cwnd(tp)) {
+		tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
 		tp->snd_cwnd_cnt = 0;
 	}
 	/* clamp down slowstart cwnd to ssthresh value. */
 	if (is_slowstart)
-		tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
+		tcp_snd_cwnd_set(tp, min(tcp_snd_cwnd(tp), tp->snd_ssthresh));
 
-	tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp);
+	tcp_snd_cwnd_set(tp, min(tcp_snd_cwnd(tp), tp->snd_cwnd_clamp));
 }
 
 static struct tcp_congestion_ops tcp_hybla __read_mostly = {