[PATCH v2] mptcp: do not drop partial packets

[Shardul Bankar <shardulsb08@gmail.com>]

When a packet arrives with map_seq < ack_seq < end_seq, the beginning
of the packet has already been acknowledged but the end contains new
data.  Currently the entire packet is dropped as "old data," forcing
the sender to retransmit.

Instead, skip the already-acked bytes by adjusting the skb offset and
enqueue only the new portion.  Update bytes_received and ack_seq to
reflect the new data consumed.

A previous attempt at this fix (commit 1d2ce718811a ("mptcp: do not
drop partial packets"), reverted in commit bf39160c4218 ("Revert
"mptcp: do not drop partial packets"")) also added a zero-window
check and changed rcv_wnd_sent initialization, which caused test
regressions.  This version addresses only the partial packet handling
without modifying receive window accounting.

Fixes: ab174ad8ef76 ("mptcp: move ooo skbs into msk out of order queue.")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/600
Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
---
v2: Drop the mptcp_try_coalesce() attempt for partial packets, since
    non-zero offset always prevents coalescing (Paolo).

 net/mptcp/protocol.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 614c3f583ca0..73ec6563ab2b 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -397,12 +397,24 @@ static bool __mptcp_move_skb(struct sock *sk, struct sk_buff *skb)
 		return false;
 	}
 
-	/* old data, keep it simple and drop the whole pkt, sender
-	 * will retransmit as needed, if needed.
+	/* Completely old data? */
+	if (!after64(MPTCP_SKB_CB(skb)->end_seq, msk->ack_seq)) {
+		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_DUPDATA);
+		mptcp_drop(sk, skb);
+		return false;
+	}
+
+	/* Partial packet: map_seq < ack_seq < end_seq.
+	 * Skip the already-acked bytes and enqueue the new data.
 	 */
-	MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_DUPDATA);
-	mptcp_drop(sk, skb);
-	return false;
+	copy_len = MPTCP_SKB_CB(skb)->end_seq - msk->ack_seq;
+	MPTCP_SKB_CB(skb)->offset += msk->ack_seq - MPTCP_SKB_CB(skb)->map_seq;
+	msk->bytes_received += copy_len;
+	WRITE_ONCE(msk->ack_seq, msk->ack_seq + copy_len);
+
+	skb_set_owner_r(skb, sk);
+	__skb_queue_tail(&sk->sk_receive_queue, skb);
+	return true;
 }
 
 static void mptcp_stop_rtx_timer(struct sock *sk)
-- 
2.34.1