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, 15 deletions

diff --git a/net/ipv4/tcp_cdg.c b/net/ipv4/tcp_cdg.c
index 709d23801823..ddc7ba0554bd 100644
--- a/net/ipv4/tcp_cdg.c
+++ b/net/ipv4/tcp_cdg.c
@@ -161,8 +161,8 @@ static void tcp_cdg_hystart_update(struct sock *sk)
 					      LINUX_MIB_TCPHYSTARTTRAINDETECT);
 				NET_ADD_STATS(sock_net(sk),
 					      LINUX_MIB_TCPHYSTARTTRAINCWND,
-					      tp->snd_cwnd);
-				tp->snd_ssthresh = tp->snd_cwnd;
+					      tcp_snd_cwnd(tp));
+				tp->snd_ssthresh = tcp_snd_cwnd(tp);
 				return;
 			}
 		}
@@ -180,8 +180,8 @@ static void tcp_cdg_hystart_update(struct sock *sk)
 					      LINUX_MIB_TCPHYSTARTDELAYDETECT);
 				NET_ADD_STATS(sock_net(sk),
 					      LINUX_MIB_TCPHYSTARTDELAYCWND,
-					      tp->snd_cwnd);
-				tp->snd_ssthresh = tp->snd_cwnd;
+					      tcp_snd_cwnd(tp));
+				tp->snd_ssthresh = tcp_snd_cwnd(tp);
 			}
 		}
 	}
@@ -252,7 +252,7 @@ static bool tcp_cdg_backoff(struct sock *sk, u32 grad)
 			return false;
 	}
 
-	ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd);
+	ca->shadow_wnd = max(ca->shadow_wnd, tcp_snd_cwnd(tp));
 	ca->state = CDG_BACKOFF;
 	tcp_enter_cwr(sk);
 	return true;
@@ -285,14 +285,14 @@ static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 	}
 
 	if (!tcp_is_cwnd_limited(sk)) {
-		ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd);
+		ca->shadow_wnd = min(ca->shadow_wnd, tcp_snd_cwnd(tp));
 		return;
 	}
 
-	prior_snd_cwnd = tp->snd_cwnd;
+	prior_snd_cwnd = tcp_snd_cwnd(tp);
 	tcp_reno_cong_avoid(sk, ack, acked);
 
-	incr = tp->snd_cwnd - prior_snd_cwnd;
+	incr = tcp_snd_cwnd(tp) - prior_snd_cwnd;
 	ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr);
 }
 
@@ -331,15 +331,15 @@ static u32 tcp_cdg_ssthresh(struct sock *sk)
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	if (ca->state == CDG_BACKOFF)
-		return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10);
+		return max(2U, (tcp_snd_cwnd(tp) * min(1024U, backoff_beta)) >> 10);
 
 	if (ca->state == CDG_NONFULL && use_tolerance)
-		return tp->snd_cwnd;
+		return tcp_snd_cwnd(tp);
 
-	ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd);
+	ca->shadow_wnd = min(ca->shadow_wnd >> 1, tcp_snd_cwnd(tp));
 	if (use_shadow)
-		return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1);
-	return max(2U, tp->snd_cwnd >> 1);
+		return max3(2U, ca->shadow_wnd, tcp_snd_cwnd(tp) >> 1);
+	return max(2U, tcp_snd_cwnd(tp) >> 1);
 }
 
 static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
@@ -357,7 +357,7 @@ static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
 
 		ca->gradients = gradients;
 		ca->rtt_seq = tp->snd_nxt;
-		ca->shadow_wnd = tp->snd_cwnd;
+		ca->shadow_wnd = tcp_snd_cwnd(tp);
 		break;
 	case CA_EVENT_COMPLETE_CWR:
 		ca->state = CDG_UNKNOWN;
@@ -380,7 +380,7 @@ static void tcp_cdg_init(struct sock *sk)
 		ca->gradients = kcalloc(window, sizeof(ca->gradients[0]),
 					GFP_NOWAIT | __GFP_NOWARN);
 	ca->rtt_seq = tp->snd_nxt;
-	ca->shadow_wnd = tp->snd_cwnd;
+	ca->shadow_wnd = tcp_snd_cwnd(tp);
 }
 
 static void tcp_cdg_release(struct sock *sk)