[PATCH net-next v3 2/3] tcp: keep scaled no-shrink window representable

[Wesley Atwell <atwellwea@gmail.com>]

In the scaled no-shrink path, __tcp_select_window() currently rounds the
raw free-space value up to the receive-window scale quantum.

When raw backed free_space sits just below the next quantum, that can
expose fresh sender-visible credit beyond the currently backed receive
space.

Fix this by keeping tp->rcv_wnd representable in scaled units: round
larger windows down to the scale quantum and preserve only the small
non-zero case that would otherwise scale away to zero.

This series intentionally leaves that smaller longstanding non-zero case
unchanged. The proven bug and the new reproducer are both in the
larger-window path where free_space is at least one scale quantum, so
changing 0 < free_space < granularity into zero would be a separate
behavior change.

That representability matters across ACK transitions too, not only on
the immediate raw-free_space-limited ACK. tcp_select_window() preserves
the currently offered window when shrinking is disallowed, so if an
earlier ACK stores a rounded-up value in tp->rcv_wnd, a later
raw-free_space-limited ACK can keep inheriting that extra unit.

Keeping tp->rcv_wnd representable throughout the scaled no-shrink path
prevents that carry-forward and makes later no-shrink decisions reason
from a right edge the peer could actually have seen on the wire.

This removes the larger-window quantization slack while preserving the
small non-zero case needed to avoid scaling away to zero.

Signed-off-by: Wesley Atwell <atwellwea@gmail.com>
---
v3:
- keep granularity in signed int space so the free_space comparison
  stays type-safe

v2:
- rename gran to granularity
- clarify why representable tp->rcv_wnd state is required across later
  no-shrink transitions
- clarify that this series still intentionally leaves the smaller
  longstanding non-zero case unchanged

 net/ipv4/tcp_output.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 35c3b0ab5a0cb714155d5720fe56888f71aecced..5fc0e0d22f10bf56ece1be536b75013768112acf 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3375,13 +3375,19 @@ u32 __tcp_select_window(struct sock *sk)
 	 * scaled window will not line up with the MSS boundary anyway.
 	 */
 	if (tp->rx_opt.rcv_wscale) {
-		window = free_space;
+		int granularity = 1 << tp->rx_opt.rcv_wscale;
 
-		/* Advertise enough space so that it won't get scaled away.
-		 * Import case: prevent zero window announcement if
-		 * 1<<rcv_wscale > mss.
+		/* Keep tp->rcv_wnd representable in scaled units so later
+		 * no-shrink decisions reason about the same right edge we
+		 * can advertise on the wire. Preserve only a small non-zero
+		 * offer that would otherwise get scaled away to zero.
 		 */
-		window = ALIGN(window, (1 << tp->rx_opt.rcv_wscale));
+		if (free_space >= granularity)
+			window = round_down(free_space, granularity);
+		else if (free_space > 0)
+			window = granularity;
+		else
+			window = 0;
 	} else {
 		window = tp->rcv_wnd;
 		/* Get the largest window that is a nice multiple of mss.
-- 
2.43.0