In the scaled no-shrink path, __tcp_select_window() currently rounds the raw free-space value up to the receive-window scale quantum.
That can expose fresh sender-visible credit beyond the currently backed free space. Fix this without changing the meaning of the stored receive-window state. Keep tp->rcv_wnd representable in scaled units by rounding larger windows down to the scale quantum and preserving only the small non-zero case that would otherwise scale away to zero. tcp_select_window() already preserves the no-shrink guarantee from the currently offered window, so later no-shrink decisions continue to reason from a right edge the peer actually saw on the wire. This removes the larger-window quantization slack from rounding free_space up, while preserving the small non-zero case needed to avoid scaling away to zero. Signed-off-by: Wesley Atwell <[email protected]> --- 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..bd3a43148a87e891bc632a47ffb5b82c475e8f6f 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; + u32 gran = 1U << 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 >= gran) + window = round_down(free_space, gran); + else if (free_space > 0) + window = gran; + else + window = 0; } else { window = tp->rcv_wnd; /* Get the largest window that is a nice multiple of mss. -- 2.43.0
