tcp: update window_clamp when SO_RCVBUF is set

Commit under Fixes moved recomputing the window clamp to
tcp_measure_rcv_mss() (when scaling_ratio changes).
I suspect it missed the fact that we don't recompute the clamp
when rcvbuf is set. Until scaling_ratio changes we are
stuck with the old window clamp which may be based on
the small initial buffer. scaling_ratio may never change.

Inspired by Eric's recent commit d1361840f8 ("tcp: fix
SO_RCVLOWAT and RCVBUF autotuning") plumb the user action
thru to TCP and have it update the clamp.

A smaller fix would be to just have tcp_rcvbuf_grow()
adjust the clamp even if SOCK_RCVBUF_LOCK is set.
But IIUC this is what we were trying to get away from
in the first place.

Fixes: a2cbb16039 ("tcp: Update window clamping condition")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumaze@google.com>
Link: https://patch.msgid.link/20260408001438.129165-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/net.h

@@ -223,6 +223,7 @@ struct proto_ops {
 	int		(*sendmsg_locked)(struct sock *sk, struct msghdr *msg,
 					  size_t size);
 	int		(*set_rcvlowat)(struct sock *sk, int val);
+	void		(*set_rcvbuf)(struct sock *sk, int val);
 };
 
 #define DECLARE_SOCKADDR(type, dst, src)	\

include/net/tcp.h

@@ -516,6 +516,7 @@ void tcp_syn_ack_timeout(const struct request_sock *req);
 int tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 		int flags);
 int tcp_set_rcvlowat(struct sock *sk, int val);
+void tcp_set_rcvbuf(struct sock *sk, int val);
 int tcp_set_window_clamp(struct sock *sk, int val);
 
 static inline void

net/core/sock.c

@@ -966,6 +966,8 @@ EXPORT_SYMBOL(sock_set_keepalive);
 
 static void __sock_set_rcvbuf(struct sock *sk, int val)
 {
+	struct socket *sock = sk->sk_socket;
+
 	/* Ensure val * 2 fits into an int, to prevent max_t() from treating it
 	 * as a negative value.
 	 */
@@ -983,6 +985,13 @@ static void __sock_set_rcvbuf(struct sock *sk, int val)
 	 * we actually used in getsockopt is the most desirable behavior.
 	 */
 	WRITE_ONCE(sk->sk_rcvbuf, max_t(int, val * 2, SOCK_MIN_RCVBUF));
+
+	if (sock) {
+		const struct proto_ops *ops = READ_ONCE(sock->ops);
+
+		if (ops->set_rcvbuf)
+			ops->set_rcvbuf(sk, sk->sk_rcvbuf);
+	}
 }
 
 void sock_set_rcvbuf(struct sock *sk, int val)

net/ipv4/af_inet.c

@@ -1091,6 +1091,7 @@ const struct proto_ops inet_stream_ops = {
 	.compat_ioctl	   = inet_compat_ioctl,
 #endif
 	.set_rcvlowat	   = tcp_set_rcvlowat,
+	.set_rcvbuf	   = tcp_set_rcvbuf,
 };
 EXPORT_SYMBOL(inet_stream_ops);
 

net/ipv4/tcp.c

@@ -1858,6 +1858,11 @@ int tcp_set_rcvlowat(struct sock *sk, int val)
 	return 0;
 }
 
+void tcp_set_rcvbuf(struct sock *sk, int val)
+{
+	tcp_set_window_clamp(sk, tcp_win_from_space(sk, val));
+}
+
 #ifdef CONFIG_MMU
 static const struct vm_operations_struct tcp_vm_ops = {
 };

net/ipv6/af_inet6.c

@@ -690,6 +690,7 @@ const struct proto_ops inet6_stream_ops = {
 	.compat_ioctl	   = inet6_compat_ioctl,
 #endif
 	.set_rcvlowat	   = tcp_set_rcvlowat,
+	.set_rcvbuf	   = tcp_set_rcvbuf,
 };
 EXPORT_SYMBOL_GPL(inet6_stream_ops);