tcp: add drop_reason support to tcp_disordered_ack()

Following patch is adding a new drop_reason to tcp_validate_incoming().

Change tcp_disordered_ack() to not return a boolean anymore,
but a drop reason.

Change its name to tcp_disordered_ack_check()

Refactor tcp_validate_incoming() to ease the code
review of the following patch, and reduce indentation
level.

This patch is a refactor, with no functional change.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20250113135558.3180360-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv4/tcp_input.c

@@ -4450,34 +4450,38 @@ static u32 tcp_tsval_replay(const struct sock *sk)
 	return inet_csk(sk)->icsk_rto * 1200 / HZ;
 }
 
-static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
+static enum skb_drop_reason tcp_disordered_ack_check(const struct sock *sk,
+						     const struct sk_buff *skb)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	const struct tcphdr *th = tcp_hdr(skb);
-	u32 seq = TCP_SKB_CB(skb)->seq;
+	SKB_DR_INIT(reason, TCP_RFC7323_PAWS);
 	u32 ack = TCP_SKB_CB(skb)->ack_seq;
+	u32 seq = TCP_SKB_CB(skb)->seq;
 
-	return	/* 1. Pure ACK with correct sequence number. */
-		(th->ack && seq == TCP_SKB_CB(skb)->end_seq && seq == tp->rcv_nxt) &&
+	/* 1. Is this not a pure ACK ? */
+	if (!th->ack || seq != TCP_SKB_CB(skb)->end_seq)
+		return reason;
 
-		/* 2. ... and duplicate ACK. */
-		ack == tp->snd_una &&
+	/* 2. Is its sequence not the expected one ? */
+	if (seq != tp->rcv_nxt)
+		return reason;
 
-		/* 3. ... and does not update window. */
-		!tcp_may_update_window(tp, ack, seq, ntohs(th->window) << tp->rx_opt.snd_wscale) &&
+	/* 3. Is this not a duplicate ACK ? */
+	if (ack != tp->snd_una)
+		return reason;
 
-		/* 4. ... and sits in replay window. */
-		(s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) <=
-		tcp_tsval_replay(sk);
-}
+	/* 4. Is this updating the window ? */
+	if (tcp_may_update_window(tp, ack, seq, ntohs(th->window) <<
+						tp->rx_opt.snd_wscale))
+		return reason;
 
-static inline bool tcp_paws_discard(const struct sock *sk,
-				   const struct sk_buff *skb)
-{
-	const struct tcp_sock *tp = tcp_sk(sk);
+	/* 5. Is this not in the replay window ? */
+	if ((s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) >
+	    tcp_tsval_replay(sk))
+		return reason;
 
-	return !tcp_paws_check(&tp->rx_opt, TCP_PAWS_WINDOW) &&
-	       !tcp_disordered_ack(sk, skb);
+	return 0;
 }
 
 /* Check segment sequence number for validity.
@@ -5949,23 +5953,28 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	SKB_DR(reason);
 
 	/* RFC1323: H1. Apply PAWS check first. */
-	if (tcp_fast_parse_options(sock_net(sk), skb, th, tp) &&
-	    tp->rx_opt.saw_tstamp &&
-	    tcp_paws_discard(sk, skb)) {
-		if (!th->rst) {
-			if (unlikely(th->syn))
-				goto syn_challenge;
-			NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
-			if (!tcp_oow_rate_limited(sock_net(sk), skb,
-						  LINUX_MIB_TCPACKSKIPPEDPAWS,
-						  &tp->last_oow_ack_time))
-				tcp_send_dupack(sk, skb);
-			SKB_DR_SET(reason, TCP_RFC7323_PAWS);
-			goto discard;
-		}
-		/* Reset is accepted even if it did not pass PAWS. */
-	}
-
+	if (!tcp_fast_parse_options(sock_net(sk), skb, th, tp) ||
+	    !tp->rx_opt.saw_tstamp ||
+	    tcp_paws_check(&tp->rx_opt, TCP_PAWS_WINDOW))
+		goto step1;
+
+	reason = tcp_disordered_ack_check(sk, skb);
+	if (!reason)
+		goto step1;
+	/* Reset is accepted even if it did not pass PAWS. */
+	if (th->rst)
+		goto step1;
+	if (unlikely(th->syn))
+		goto syn_challenge;
+
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
+	if (!tcp_oow_rate_limited(sock_net(sk), skb,
+				  LINUX_MIB_TCPACKSKIPPEDPAWS,
+				  &tp->last_oow_ack_time))
+		tcp_send_dupack(sk, skb);
+	goto discard;
+
+step1:
 	/* Step 1: check sequence number */
 	reason = tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);
 	if (reason) {