[PATCH mptcp-net] mptcp: update window_clamp on subflows when SO_RCVBUF is set

[Gang Yan <gang.yan@linux.dev>]

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From: Gang Yan <yangang@kylinos.cn>

Add __mptcp_subflow_set_rcvbuf() helper that calls the subflow's
ops->set_rcvbuf to update window_clamp, in addition to writing
sk_rcvbuf. Use it in both mptcp_sol_socket_sync_intval() (setsockopt
path) and sync_socket_options() (new subflow creation path).

Fixes: a2cbb1603943 ("tcp: Update window clamping condition")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/619
Signed-off-by: Gang Yan <yangang@kylinos.cn>
---
Notes:

Hi Matt:

Note that I do not implement set_rcvbuf in mptcp_stream_ops for the                                                                                                                                                                                  
following reasons:

  - The callback receives the MPTCP meta socket as 'sk', but
    tcp_set_rcvbuf() must be called on each TCP subflow to update
    window_clamp. A callback implementation would need to iterate all
    subflows, duplicating the work already done by
    mptcp_sol_socket_sync_intval().

  - Subflows are grafted to the MPTCP socket via mptcp_sock_graft(),
    so ssk->sk_socket->ops points to mptcp_stream_ops, not
    inet_stream_ops. This means the TCP set_rcvbuf callback is not
    reachable through the subflow's ops either.

  - The existing subflow sync functions cover both the setsockopt path
    (mptcp_sol_socket_sync_intval) and new subflow creation path
    (sync_socket_options), making the proto_ops callback unnecessary.

WDYT？

The packetdrill tests is also implemented:
  -Link: https://github.com/multipath-tcp/packetdrill/pull/194

Thanks
Gang

---
 net/mptcp/sockopt.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index de90a2897d2d..abe63392826b 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -67,6 +67,12 @@ static int mptcp_get_int_option(struct mptcp_sock *msk, sockptr_t optval,
 	return 0;
 }
 
+static inline void __mptcp_subflow_set_rcvbuf(struct sock *ssk, int val)
+{
+	WRITE_ONCE(ssk->sk_rcvbuf, val);
+	tcp_set_rcvbuf(ssk, val);
+}
+
 static void mptcp_sol_socket_sync_intval(struct mptcp_sock *msk, int optname, int val)
 {
 	struct mptcp_subflow_context *subflow;
@@ -100,7 +106,7 @@ static void mptcp_sol_socket_sync_intval(struct mptcp_sock *msk, int optname, in
 		case SO_RCVBUF:
 		case SO_RCVBUFFORCE:
 			ssk->sk_userlocks |= SOCK_RCVBUF_LOCK;
-			WRITE_ONCE(ssk->sk_rcvbuf, sk->sk_rcvbuf);
+			__mptcp_subflow_set_rcvbuf(ssk, sk->sk_rcvbuf);
 			break;
 		case SO_MARK:
 			if (READ_ONCE(ssk->sk_mark) != sk->sk_mark) {
@@ -1556,7 +1562,7 @@ static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
 			mptcp_subflow_ctx(ssk)->cached_sndbuf = sk->sk_sndbuf;
 		}
 		if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
-			WRITE_ONCE(ssk->sk_rcvbuf, sk->sk_rcvbuf);
+			__mptcp_subflow_set_rcvbuf(ssk, sk->sk_rcvbuf);
 	}
 
 	if (sock_flag(sk, SOCK_LINGER)) {
-- 
2.43.0