tcp: fix tcp_grow_skb() vs tstamps

I forgot to call tcp_skb_collapse_tstamp() in the
case we consume the second skb in write queue.

Neal suggested to create a common helper used by tcp_mtu_probe()
and tcp_grow_skb().

Fixes: 8ee602c6 ("tcp: try to send bigger TSO packets")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Link: https://lore.kernel.org/r/20240425193450.411640-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_output.c

@@ -2403,6 +2403,21 @@ static int tcp_clone_payload(struct sock *sk, struct sk_buff *to,
 	return 0;
 }
 
+/* tcp_mtu_probe() and tcp_grow_skb() can both eat an skb (src) if
+ * all its payload was moved to another one (dst).
+ * Make sure to transfer tcp_flags, eor, and tstamp.
+ */
+static void tcp_eat_one_skb(struct sock *sk,
+			    struct sk_buff *dst,
+			    struct sk_buff *src)
+{
+	TCP_SKB_CB(dst)->tcp_flags |= TCP_SKB_CB(src)->tcp_flags;
+	TCP_SKB_CB(dst)->eor = TCP_SKB_CB(src)->eor;
+	tcp_skb_collapse_tstamp(dst, src);
+	tcp_unlink_write_queue(src, sk);
+	tcp_wmem_free_skb(sk, src);
+}
+
 /* Create a new MTU probe if we are ready.
  * MTU probe is regularly attempting to increase the path MTU by
  * deliberately sending larger packets.  This discovers routing
@@ -2508,16 +2523,7 @@ static int tcp_mtu_probe(struct sock *sk)
 		copy = min_t(int, skb->len, probe_size - len);
 
 		if (skb->len <= copy) {
-			/* We've eaten all the data from this skb.
-			 * Throw it away. */
-			TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
-			/* If this is the last SKB we copy and eor is set
-			 * we need to propagate it to the new skb.
-			 */
-			TCP_SKB_CB(nskb)->eor = TCP_SKB_CB(skb)->eor;
-			tcp_skb_collapse_tstamp(nskb, skb);
-			tcp_unlink_write_queue(skb, sk);
-			tcp_wmem_free_skb(sk, skb);
+			tcp_eat_one_skb(sk, nskb, skb);
 		} else {
 			TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags &
 						   ~(TCPHDR_FIN|TCPHDR_PSH);
@@ -2705,11 +2711,10 @@ static void tcp_grow_skb(struct sock *sk, struct sk_buff *skb, int amount)
 	TCP_SKB_CB(next_skb)->seq += nlen;
 
 	if (!next_skb->len) {
+		/* In case FIN is set, we need to update end_seq */
 		TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;
-		TCP_SKB_CB(skb)->eor = TCP_SKB_CB(next_skb)->eor;
-		TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(next_skb)->tcp_flags;
-		tcp_unlink_write_queue(next_skb, sk);
-		tcp_wmem_free_skb(sk, next_skb);
+
+		tcp_eat_one_skb(sk, skb, next_skb);
 	}
 }