BPF_SOCK_OPS_RWND_INIT lets a sockops BPF program pick the initial TCP receive window, e.g. to advertise a larger window up front in environments where that is known to be safe. Today it is only effective for the passive (listener) side; on the active (connect) side the value is computed and then silently discarded. On the passive path tcp_openreq_init_rwin() inflates full_space when the program returns a non-zero window, so tcp_select_initial_window() can offer it: else if (full_space < rcv_wnd * mss) full_space = rcv_wnd * mss; tcp_select_initial_window() only clamps the requested window *down* to the available space, so without inflating the space first the BPF reply can never raise the offered window above tcp_full_space(sk). tcp_connect_init() calls tcp_rwnd_init_bpf() but never inflates full_space, so on connect() the requested window is clamped back to tcp_full_space(sk) (~64KB at the default rcvbuf) and the program's value is ignored. Mirror the listener-side inflation in tcp_connect_init() so both directions behave the same. tp->advmss is the mss the listener path uses as well (both are tcp_mss_clamp(tp, dst_metric_advmss(dst))). Fixes: 13d3b1ebe287 ("bpf: Support for setting initial receive window") Signed-off-by: Tejas Birajdar --- Functional test: attached a cgroup sockops BPF program that sets skops->reply for BPF_SOCK_OPS_RWND_INIT on the connecting socket, and drove it with packetdrill on the active-open (connect) path (wscale 11, advmss 1440). On the patched kernel the advertised receive window on the first post-handshake ACK now reflects the requested value; unpatched it stays clamped at ~64 KB: req_segs unpatched patched 256 ~64 KB 360 KB (256 * 1440) 1024 ~64 KB 1.41 MB (1024 * 1440) 4096 ~64 KB 5.63 MB (4096 * 1440) (The SYN window itself is unscaled/capped; the offered window appears on the first scaled post-handshake ACK.) Regression: tools/testing/selftests/net/packetdrill run under virtme-ng on this commit vs its parent produces an identical pass/fail set (no newly failing tests). net/ipv4/tcp_output.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index d7c1444b5e30..478e4951140f 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -4101,6 +4101,7 @@ static void tcp_ca_dst_init(struct sock *sk, const struct dst_entry *dst) static void tcp_connect_init(struct sock *sk) { const struct dst_entry *dst = __sk_dst_get(sk); + int full_space = tcp_full_space(sk); struct tcp_sock *tp = tcp_sk(sk); __u8 rcv_wscale; u16 user_mss; @@ -4133,14 +4134,16 @@ static void tcp_connect_init(struct sock *sk) /* limit the window selection if the user enforce a smaller rx buffer */ if (sk->sk_userlocks & SOCK_RCVBUF_LOCK && - (tp->window_clamp > tcp_full_space(sk) || tp->window_clamp == 0)) - WRITE_ONCE(tp->window_clamp, tcp_full_space(sk)); + (tp->window_clamp > full_space || tp->window_clamp == 0)) + WRITE_ONCE(tp->window_clamp, full_space); rcv_wnd = tcp_rwnd_init_bpf(sk); if (rcv_wnd == 0) rcv_wnd = dst_metric(dst, RTAX_INITRWND); + else if (full_space < rcv_wnd * tp->advmss) + full_space = rcv_wnd * tp->advmss; - tcp_select_initial_window(sk, tcp_full_space(sk), + tcp_select_initial_window(sk, full_space, tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0), &tp->rcv_wnd, &tp->window_clamp, -- 2.53.0-Meta