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, 18 insertions, 18 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2088f93fa37b..1595b76ea2be 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -414,7 +414,7 @@ static void tcp_sndbuf_expand(struct sock *sk)
 	per_mss = roundup_pow_of_two(per_mss) +
 		  SKB_DATA_ALIGN(sizeof(struct sk_buff));
 
-	nr_segs = max_t(u32, TCP_INIT_CWND, tp->snd_cwnd);
+	nr_segs = max_t(u32, TCP_INIT_CWND, tcp_snd_cwnd(tp));
 	nr_segs = max_t(u32, nr_segs, tp->reordering + 1);
 
 	/* Fast Recovery (RFC 5681 3.2) :
@@ -909,12 +909,12 @@ static void tcp_update_pacing_rate(struct sock *sk)
 	 *	 If snd_cwnd >= (tp->snd_ssthresh / 2), we are approaching
 	 *	 end of slow start and should slow down.
 	 */
-	if (tp->snd_cwnd < tp->snd_ssthresh / 2)
+	if (tcp_snd_cwnd(tp) < tp->snd_ssthresh / 2)
 		rate *= sock_net(sk)->ipv4.sysctl_tcp_pacing_ss_ratio;
 	else
 		rate *= sock_net(sk)->ipv4.sysctl_tcp_pacing_ca_ratio;
 
-	rate *= max(tp->snd_cwnd, tp->packets_out);
+	rate *= max(tcp_snd_cwnd(tp), tp->packets_out);
 
 	if (likely(tp->srtt_us))
 		do_div(rate, tp->srtt_us);
@@ -2147,12 +2147,12 @@ void tcp_enter_loss(struct sock *sk)
 	    !after(tp->high_seq, tp->snd_una) ||
 	    (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
 		tp->prior_ssthresh = tcp_current_ssthresh(sk);
-		tp->prior_cwnd = tp->snd_cwnd;
+		tp->prior_cwnd = tcp_snd_cwnd(tp);
 		tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
 		tcp_ca_event(sk, CA_EVENT_LOSS);
 		tcp_init_undo(tp);
 	}
-	tp->snd_cwnd	   = tcp_packets_in_flight(tp) + 1;
+	tcp_snd_cwnd_set(tp, tcp_packets_in_flight(tp) + 1);
 	tp->snd_cwnd_cnt   = 0;
 	tp->snd_cwnd_stamp = tcp_jiffies32;
 
@@ -2458,7 +2458,7 @@ static void DBGUNDO(struct sock *sk, const char *msg)
 		pr_debug("Undo %s %pI4/%u c%u l%u ss%u/%u p%u\n",
 			 msg,
 			 &inet->inet_daddr, ntohs(inet->inet_dport),
-			 tp->snd_cwnd, tcp_left_out(tp),
+			 tcp_snd_cwnd(tp), tcp_left_out(tp),
 			 tp->snd_ssthresh, tp->prior_ssthresh,
 			 tp->packets_out);
 	}
@@ -2467,7 +2467,7 @@ static void DBGUNDO(struct sock *sk, const char *msg)
 		pr_debug("Undo %s %pI6/%u c%u l%u ss%u/%u p%u\n",
 			 msg,
 			 &sk->sk_v6_daddr, ntohs(inet->inet_dport),
-			 tp->snd_cwnd, tcp_left_out(tp),
+			 tcp_snd_cwnd(tp), tcp_left_out(tp),
 			 tp->snd_ssthresh, tp->prior_ssthresh,
 			 tp->packets_out);
 	}
@@ -2492,7 +2492,7 @@ static void tcp_undo_cwnd_reduction(struct sock *sk, bool unmark_loss)
 	if (tp->prior_ssthresh) {
 		const struct inet_connection_sock *icsk = inet_csk(sk);
 
-		tp->snd_cwnd = icsk->icsk_ca_ops->undo_cwnd(sk);
+		tcp_snd_cwnd_set(tp, icsk->icsk_ca_ops->undo_cwnd(sk));
 
 		if (tp->prior_ssthresh > tp->snd_ssthresh) {
 			tp->snd_ssthresh = tp->prior_ssthresh;
@@ -2599,7 +2599,7 @@ static void tcp_init_cwnd_reduction(struct sock *sk)
 	tp->high_seq = tp->snd_nxt;
 	tp->tlp_high_seq = 0;
 	tp->snd_cwnd_cnt = 0;
-	tp->prior_cwnd = tp->snd_cwnd;
+	tp->prior_cwnd = tcp_snd_cwnd(tp);
 	tp->prr_delivered = 0;
 	tp->prr_out = 0;
 	tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
@@ -2629,7 +2629,7 @@ void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int newly_lost,
 	}
 	/* Force a fast retransmit upon entering fast recovery */
 	sndcnt = max(sndcnt, (tp->prr_out ? 0 : 1));
-	tp->snd_cwnd = tcp_packets_in_flight(tp) + sndcnt;
+	tcp_snd_cwnd_set(tp, tcp_packets_in_flight(tp) + sndcnt);
 }
 
 static inline void tcp_end_cwnd_reduction(struct sock *sk)
@@ -2642,7 +2642,7 @@ static inline void tcp_end_cwnd_reduction(struct sock *sk)
 	/* Reset cwnd to ssthresh in CWR or Recovery (unless it's undone) */
 	if (tp->snd_ssthresh < TCP_INFINITE_SSTHRESH &&
 	    (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR || tp->undo_marker)) {
-		tp->snd_cwnd = tp->snd_ssthresh;
+		tcp_snd_cwnd_set(tp, tp->snd_ssthresh);
 		tp->snd_cwnd_stamp = tcp_jiffies32;
 	}
 	tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
@@ -2709,9 +2709,9 @@ static void tcp_mtup_probe_success(struct sock *sk)
 
 	/* FIXME: breaks with very large cwnd */
 	tp->prior_ssthresh = tcp_current_ssthresh(sk);
-	tp->snd_cwnd = tp->snd_cwnd *
-		       tcp_mss_to_mtu(sk, tp->mss_cache) /
-		       icsk->icsk_mtup.probe_size;
+	tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) *
+			     tcp_mss_to_mtu(sk, tp->mss_cache) /
+			     icsk->icsk_mtup.probe_size);
 	tp->snd_cwnd_cnt = 0;
 	tp->snd_cwnd_stamp = tcp_jiffies32;
 	tp->snd_ssthresh = tcp_current_ssthresh(sk);
@@ -3034,7 +3034,7 @@ static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una,
 		    tp->snd_una == tp->mtu_probe.probe_seq_start) {
 			tcp_mtup_probe_failed(sk);
 			/* Restores the reduction we did in tcp_mtup_probe() */
-			tp->snd_cwnd++;
+			tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
 			tcp_simple_retransmit(sk);
 			return;
 		}
@@ -5436,7 +5436,7 @@ static bool tcp_should_expand_sndbuf(struct sock *sk)
 		return false;
 
 	/* If we filled the congestion window, do not expand.  */
-	if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
+	if (tcp_packets_in_flight(tp) >= tcp_snd_cwnd(tp))
 		return false;
 
 	return true;
@@ -5998,9 +5998,9 @@ void tcp_init_transfer(struct sock *sk, int bpf_op, struct sk_buff *skb)
 	 * retransmission has occurred.
 	 */
 	if (tp->total_retrans > 1 && tp->undo_marker)
-		tp->snd_cwnd = 1;
+		tcp_snd_cwnd_set(tp, 1);
 	else
-		tp->snd_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
+		tcp_snd_cwnd_set(tp, tcp_init_cwnd(tp, __sk_dst_get(sk)));
 	tp->snd_cwnd_stamp = tcp_jiffies32;
 
 	bpf_skops_established(sk, bpf_op, skb);