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, 15 insertions, 4 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 70ca4a5e330a..54aa2f02bb63 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1207,9 +1207,20 @@ static inline unsigned int tcp_packets_in_flight(const struct tcp_sock *tp)
 
 #define TCP_INFINITE_SSTHRESH	0x7fffffff
 
+static inline u32 tcp_snd_cwnd(const struct tcp_sock *tp)
+{
+	return tp->snd_cwnd;
+}
+
+static inline void tcp_snd_cwnd_set(struct tcp_sock *tp, u32 val)
+{
+	WARN_ON_ONCE((int)val <= 0);
+	tp->snd_cwnd = val;
+}
+
 static inline bool tcp_in_slow_start(const struct tcp_sock *tp)
 {
-	return tp->snd_cwnd < tp->snd_ssthresh;
+	return tcp_snd_cwnd(tp) < tp->snd_ssthresh;
 }
 
 static inline bool tcp_in_initial_slowstart(const struct tcp_sock *tp)
@@ -1235,8 +1246,8 @@ static inline __u32 tcp_current_ssthresh(const struct sock *sk)
 		return tp->snd_ssthresh;
 	else
 		return max(tp->snd_ssthresh,
-			   ((tp->snd_cwnd >> 1) +
-			    (tp->snd_cwnd >> 2)));
+			   ((tcp_snd_cwnd(tp) >> 1) +
+			    (tcp_snd_cwnd(tp) >> 2)));
 }
 
 /* Use define here intentionally to get WARN_ON location shown at the caller */
@@ -1278,7 +1289,7 @@ static inline bool tcp_is_cwnd_limited(const struct sock *sk)
 
 	/* If in slow start, ensure cwnd grows to twice what was ACKed. */
 	if (tcp_in_slow_start(tp))
-		return tp->snd_cwnd < 2 * tp->max_packets_out;
+		return tcp_snd_cwnd(tp) < 2 * tp->max_packets_out;
 
 	return tp->is_cwnd_limited;
 }