netdev - [PATCH net] tcp: fix mid stream window clamp.

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Date: Thu, 23 Nov 2023 16:24:07 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: netdev@...r.kernel.org
Cc: Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	David Ahern <dsahern@...nel.org>,
	Jakub Kicinski <kuba@...nel.org>,
	Neil Spring <ntspring@...com>,
	Paolo Abeni <pabeni@...hat.com>,
	David Gibson <david@...son.dropbear.id.au>
Subject: [PATCH net] tcp: fix mid stream window clamp.

After the blamed commit below, if the user-space application performs
window clamping when tp->rcv_wnd is 0, the TCP socket will never be
able to announce a non 0 receive window, even after completely emptying
the receive buffer and re-setting the window clamp to higher values.

Refactor tcp_set_window_clamp() to address the issue: when the user
decreases the current clamp value, set rcv_ssthresh according to the
same logic used at buffer initialization time.
When increasing the clamp value, give the rcv_ssthresh a chance to grow
according to previously implemented heuristic.

Fixes: 3aa7857fe1d7 ("tcp: enable mid stream window clamp")
Reported-by: David Gibson <david@...son.dropbear.id.au>
Reported-by: Stefano Brivio <sbrivio@...hat.com>
Reviewed-by: Stefano Brivio <sbrivio@...hat.com>
Tested-by: Stefano Brivio <sbrivio@...hat.com>
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 net/ipv4/tcp.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 53bcc17c91e4..1a9b9064e080 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3368,9 +3368,22 @@ int tcp_set_window_clamp(struct sock *sk, int val)
 			return -EINVAL;
 		tp->window_clamp = 0;
 	} else {
-		tp->window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
-			SOCK_MIN_RCVBUF / 2 : val;
-		tp->rcv_ssthresh = min(tp->rcv_wnd, tp->window_clamp);
+		u32 new_rcv_ssthresh, old_window_clamp = tp->window_clamp;
+		u32 new_window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
+						SOCK_MIN_RCVBUF / 2 : val;
+
+		if (new_window_clamp == old_window_clamp)
+			return 0;
+
+		tp->window_clamp = new_window_clamp;
+		if (new_window_clamp < old_window_clamp) {
+			tp->rcv_ssthresh = min(tp->rcv_ssthresh,
+					       new_window_clamp);
+		} else {
+			new_rcv_ssthresh = min(tp->rcv_wnd, tp->window_clamp);
+			tp->rcv_ssthresh = max(new_rcv_ssthresh,
+					       tp->rcv_ssthresh);
+		}
 	}
 	return 0;
 }
-- 
2.41.0