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 8b8a321 ("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>

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;
 }

include/trace/events/tcp.h

@@ -279,7 +279,7 @@ TRACE_EVENT(tcp_probe,
 		__entry->data_len = skb->len - __tcp_hdrlen(th);
 		__entry->snd_nxt = tp->snd_nxt;
 		__entry->snd_una = tp->snd_una;
-		__entry->snd_cwnd = tp->snd_cwnd;
+		__entry->snd_cwnd = tcp_snd_cwnd(tp);
 		__entry->snd_wnd = tp->snd_wnd;
 		__entry->rcv_wnd = tp->rcv_wnd;
 		__entry->ssthresh = tcp_current_ssthresh(sk);

net/core/filter.c

@@ -5173,7 +5173,7 @@ static int _bpf_setsockopt(struct sock *sk, int level, int optname,
 				if (val <= 0 || tp->data_segs_out > tp->syn_data)
 					ret = -EINVAL;
 				else
-					tp->snd_cwnd = val;
+					tcp_snd_cwnd_set(tp, val);
 				break;
 			case TCP_BPF_SNDCWND_CLAMP:
 				if (val <= 0) {

net/ipv4/tcp.c

@@ -429,7 +429,7 @@ void tcp_init_sock(struct sock *sk)
 	 * algorithms that we must have the following bandaid to talk
 	 * efficiently to them.  -DaveM
 	 */
-	tp->snd_cwnd = TCP_INIT_CWND;
+	tcp_snd_cwnd_set(tp, TCP_INIT_CWND);
 
 	/* There's a bubble in the pipe until at least the first ACK. */
 	tp->app_limited = ~0U;
@@ -3033,7 +3033,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 	icsk->icsk_rto_min = TCP_RTO_MIN;
 	icsk->icsk_delack_max = TCP_DELACK_MAX;
 	tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
-	tp->snd_cwnd = TCP_INIT_CWND;
+	tcp_snd_cwnd_set(tp, TCP_INIT_CWND);
 	tp->snd_cwnd_cnt = 0;
 	tp->window_clamp = 0;
 	tp->delivered = 0;
@@ -3744,7 +3744,7 @@ void tcp_get_info(struct sock *sk, struct tcp_info *info)
 	info->tcpi_max_pacing_rate = rate64;
 
 	info->tcpi_reordering = tp->reordering;
-	info->tcpi_snd_cwnd = tp->snd_cwnd;
+	info->tcpi_snd_cwnd = tcp_snd_cwnd(tp);
 
 	if (info->tcpi_state == TCP_LISTEN) {
 		/* listeners aliased fields :
@@ -3915,7 +3915,7 @@ struct sk_buff *tcp_get_timestamping_opt_stats(const struct sock *sk,
 	rate64 = tcp_compute_delivery_rate(tp);
 	nla_put_u64_64bit(stats, TCP_NLA_DELIVERY_RATE, rate64, TCP_NLA_PAD);
 
-	nla_put_u32(stats, TCP_NLA_SND_CWND, tp->snd_cwnd);
+	nla_put_u32(stats, TCP_NLA_SND_CWND, tcp_snd_cwnd(tp));
 	nla_put_u32(stats, TCP_NLA_REORDERING, tp->reordering);
 	nla_put_u32(stats, TCP_NLA_MIN_RTT, tcp_min_rtt(tp));
 

net/ipv4/tcp_bbr.c

@@ -276,7 +276,7 @@ static void bbr_init_pacing_rate_from_rtt(struct sock *sk)
 	} else {			 /* no RTT sample yet */
 		rtt_us = USEC_PER_MSEC;	 /* use nominal default RTT */
 	}
-	bw = (u64)tp->snd_cwnd * BW_UNIT;
+	bw = (u64)tcp_snd_cwnd(tp) * BW_UNIT;
 	do_div(bw, rtt_us);
 	sk->sk_pacing_rate = bbr_bw_to_pacing_rate(sk, bw, bbr_high_gain);
 }
@@ -323,9 +323,9 @@ static void bbr_save_cwnd(struct sock *sk)
 	struct bbr *bbr = inet_csk_ca(sk);
 
 	if (bbr->prev_ca_state < TCP_CA_Recovery && bbr->mode != BBR_PROBE_RTT)
-		bbr->prior_cwnd = tp->snd_cwnd;  /* this cwnd is good enough */
+		bbr->prior_cwnd = tcp_snd_cwnd(tp);  /* this cwnd is good enough */
 	else  /* loss recovery or BBR_PROBE_RTT have temporarily cut cwnd */
-		bbr->prior_cwnd = max(bbr->prior_cwnd, tp->snd_cwnd);
+		bbr->prior_cwnd = max(bbr->prior_cwnd, tcp_snd_cwnd(tp));
 }
 
 static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event)
@@ -482,7 +482,7 @@ static bool bbr_set_cwnd_to_recover_or_restore(
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct bbr *bbr = inet_csk_ca(sk);
 	u8 prev_state = bbr->prev_ca_state, state = inet_csk(sk)->icsk_ca_state;
-	u32 cwnd = tp->snd_cwnd;
+	u32 cwnd = tcp_snd_cwnd(tp);
 
 	/* An ACK for P pkts should release at most 2*P packets. We do this
 	 * in two steps. First, here we deduct the number of lost packets.
@@ -520,7 +520,7 @@ static void bbr_set_cwnd(struct sock *sk, const struct rate_sample *rs,
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct bbr *bbr = inet_csk_ca(sk);
-	u32 cwnd = tp->snd_cwnd, target_cwnd = 0;
+	u32 cwnd = tcp_snd_cwnd(tp), target_cwnd = 0;
 
 	if (!acked)
 		goto done;  /* no packet fully ACKed; just apply caps */
@@ -544,9 +544,9 @@ static void bbr_set_cwnd(struct sock *sk, const struct rate_sample *rs,
 	cwnd = max(cwnd, bbr_cwnd_min_target);
 
 done:
-	tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);	/* apply global cap */
+	tcp_snd_cwnd_set(tp, min(cwnd, tp->snd_cwnd_clamp));	/* apply global cap */
 	if (bbr->mode == BBR_PROBE_RTT)  /* drain queue, refresh min_rtt */
-		tp->snd_cwnd = min(tp->snd_cwnd, bbr_cwnd_min_target);
+		tcp_snd_cwnd_set(tp, min(tcp_snd_cwnd(tp), bbr_cwnd_min_target));
 }
 
 /* End cycle phase if it's time and/or we hit the phase's in-flight target. */
@@ -856,7 +856,7 @@ static void bbr_update_ack_aggregation(struct sock *sk,
 	bbr->ack_epoch_acked = min_t(u32, 0xFFFFF,
 				     bbr->ack_epoch_acked + rs->acked_sacked);
 	extra_acked = bbr->ack_epoch_acked - expected_acked;
-	extra_acked = min(extra_acked, tp->snd_cwnd);
+	extra_acked = min(extra_acked, tcp_snd_cwnd(tp));
 	if (extra_acked > bbr->extra_acked[bbr->extra_acked_win_idx])
 		bbr->extra_acked[bbr->extra_acked_win_idx] = extra_acked;
 }
@@ -914,7 +914,7 @@ static void bbr_check_probe_rtt_done(struct sock *sk)
 		return;
 
 	bbr->min_rtt_stamp = tcp_jiffies32;  /* wait a while until PROBE_RTT */
-	tp->snd_cwnd = max(tp->snd_cwnd, bbr->prior_cwnd);
+	tcp_snd_cwnd_set(tp, max(tcp_snd_cwnd(tp), bbr->prior_cwnd));
 	bbr_reset_mode(sk);
 }
 
@@ -1093,7 +1093,7 @@ static u32 bbr_undo_cwnd(struct sock *sk)
 	bbr->full_bw = 0;   /* spurious slow-down; reset full pipe detection */
 	bbr->full_bw_cnt = 0;
 	bbr_reset_lt_bw_sampling(sk);
-	return tcp_sk(sk)->snd_cwnd;
+	return tcp_snd_cwnd(tcp_sk(sk));
 }
 
 /* Entering loss recovery, so save cwnd for when we exit or undo recovery. */

net/ipv4/tcp_bic.c

@@ -150,7 +150,7 @@ static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 		if (!acked)
 			return;
 	}
-	bictcp_update(ca, tp->snd_cwnd);
+	bictcp_update(ca, tcp_snd_cwnd(tp));
 	tcp_cong_avoid_ai(tp, ca->cnt, acked);
 }
 
@@ -166,16 +166,16 @@ static u32 bictcp_recalc_ssthresh(struct sock *sk)
 	ca->epoch_start = 0;	/* end of epoch */
 
 	/* Wmax and fast convergence */
-	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
-		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
+	if (tcp_snd_cwnd(tp) < ca->last_max_cwnd && fast_convergence)
+		ca->last_max_cwnd = (tcp_snd_cwnd(tp) * (BICTCP_BETA_SCALE + beta))
 			/ (2 * BICTCP_BETA_SCALE);
 	else
-		ca->last_max_cwnd = tp->snd_cwnd;
+		ca->last_max_cwnd = tcp_snd_cwnd(tp);
 
-	if (tp->snd_cwnd <= low_window)
-		return max(tp->snd_cwnd >> 1U, 2U);
+	if (tcp_snd_cwnd(tp) <= low_window)
+		return max(tcp_snd_cwnd(tp) >> 1U, 2U);
 	else
-		return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
+		return max((tcp_snd_cwnd(tp) * beta) / BICTCP_BETA_SCALE, 2U);
 }
 
 static void bictcp_state(struct sock *sk, u8 new_state)

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)

net/ipv4/tcp_cong.c

@@ -393,10 +393,10 @@ int tcp_set_congestion_control(struct sock *sk, const char *name, bool load,
  */
 u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)
 {
-	u32 cwnd = min(tp->snd_cwnd + acked, tp->snd_ssthresh);
+	u32 cwnd = min(tcp_snd_cwnd(tp) + acked, tp->snd_ssthresh);
 
-	acked -= cwnd - tp->snd_cwnd;
-	tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);
+	acked -= cwnd - tcp_snd_cwnd(tp);
+	tcp_snd_cwnd_set(tp, min(cwnd, tp->snd_cwnd_clamp));
 
 	return acked;
 }
@@ -410,17 +410,17 @@ void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)
 	/* If credits accumulated at a higher w, apply them gently now. */
 	if (tp->snd_cwnd_cnt >= w) {
 		tp->snd_cwnd_cnt = 0;
-		tp->snd_cwnd++;
+		tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
 	}
 
 	tp->snd_cwnd_cnt += acked;
 	if (tp->snd_cwnd_cnt >= w) {
 		u32 delta = tp->snd_cwnd_cnt / w;
 
 		tp->snd_cwnd_cnt -= delta * w;
-		tp->snd_cwnd += delta;
+		tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + delta);
 	}
-	tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_cwnd_clamp);
+	tcp_snd_cwnd_set(tp, min(tcp_snd_cwnd(tp), tp->snd_cwnd_clamp));
 }
 EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);
 
@@ -445,7 +445,7 @@ void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 			return;
 	}
 	/* In dangerous area, increase slowly. */
-	tcp_cong_avoid_ai(tp, tp->snd_cwnd, acked);
+	tcp_cong_avoid_ai(tp, tcp_snd_cwnd(tp), acked);
 }
 EXPORT_SYMBOL_GPL(tcp_reno_cong_avoid);
 
@@ -454,15 +454,15 @@ u32 tcp_reno_ssthresh(struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 
-	return max(tp->snd_cwnd >> 1U, 2U);
+	return max(tcp_snd_cwnd(tp) >> 1U, 2U);
 }
 EXPORT_SYMBOL_GPL(tcp_reno_ssthresh);
 
 u32 tcp_reno_undo_cwnd(struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 
-	return max(tp->snd_cwnd, tp->prior_cwnd);
+	return max(tcp_snd_cwnd(tp), tp->prior_cwnd);
 }
 EXPORT_SYMBOL_GPL(tcp_reno_undo_cwnd);