[PATCH] tcp: allow effective reduction of TCP's rcv-buffer via setsockopt

Via setsockopt it is possible to reduce the socket RX buffer
(SO_RCVBUF). TCP method to select the initial window and window scaling
option in tcp_select_initial_window() currently misbehaves and do not
consider a reduced RX socket buffer via setsockopt.

Even though the server's RX buffer is reduced via setsockopt() to 256
byte (Initial Window 384 byte => 256 * 2 - (256 * 2 / 4)) the window
scale option is still 7:

192.168.1.38.40676 > 78.47.222.210.5001: Flags [S], seq 2577214362, win 5840, options [mss 1460,sackOK,TS val 338417 ecr 0,nop,wscale 0], length 0
78.47.222.210.5001 > 192.168.1.38.40676: Flags [S.], seq 1570631029, ack 2577214363, win 384, options [mss 1452,sackOK,TS val 2435248895 ecr 338417,nop,wscale 7], length 0
192.168.1.38.40676 > 78.47.222.210.5001: Flags [.], ack 1, win 5840, options [nop,nop,TS val 338421 ecr 2435248895], length 0

Within tcp_select_initial_window() the original space argument - a
representation of the rx buffer size - is expanded during
tcp_select_initial_window(). Only sysctl_tcp_rmem[2], sysctl_rmem_max
and window_clamp are considered to calculate the initial window.

This patch adjust the window_clamp argument if the user explicitly
reduce the receive buffer.

Signed-off-by: Hagen Paul Pfeifer <redacted>
Cc: David S. Miller <davem@davemloft.net>
Cc: Patrick McHardy <redacted>
Cc: Eric Dumazet <redacted>
Cc: Ilpo Järvinen <redacted>
---
 net/ipv4/tcp_output.c |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index de3bd84..c605312 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c

@@ -2429,6 +2429,12 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 		__u8 rcv_wscale;
 		/* Set this up on the first call only */
 		req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
+
+		/* limit the window selection if the user enforce a smaller rx buffer */
+		if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
+				(req->window_clamp > tcp_full_space(sk) || req->window_clamp == 0))
+			req->window_clamp = tcp_full_space(sk);
+
 		/* tcp_full_space because it is guaranteed to be the first packet */
 		tcp_select_initial_window(tcp_full_space(sk),
 			mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),

@@ -2555,6 +2561,11 @@ static void tcp_connect_init(struct sock *sk)
 
 	tcp_initialize_rcv_mss(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))
+		tp->window_clamp = tcp_full_space(sk);
+
 	tcp_select_initial_window(tcp_full_space(sk),
 				  tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
 				  &tp->rcv_wnd,

-- 
1.6.6.196.g1f735.dirty

From: Hagen Paul Pfeifer <redacted>
Date: Sun, 15 Aug 2010 21:36:16 +0200

+
+		/* limit the window selection if the user enforce a smaller rx buffer */
+		if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
+				(req->window_clamp > tcp_full_space(sk) || req->window_clamp == 0))
+			req->window_clamp = tcp_full_space(sk);
+

Logically the patch looks fine, but please fix the indentation of the
second line of the two new if() statements in this patch, the
second line's first character should line up with the character
right after the openning "(" on the previous line.

Thanks.

On Wed, 18 Aug 2010 23:33:37 -0700 (PDT), David Miller wrote:

From: Hagen Paul Pfeifer <redacted>

Date: Sun, 15 Aug 2010 21:36:16 +0200

quoted

+

quoted

+		if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&

quoted

+				(req->window_clamp > tcp_full_space(sk) || req->window_clamp ==

0))

quoted

+			req->window_clamp = tcp_full_space(sk);

quoted

+

Logically the patch looks fine, but please fix the indentation of the

second line of the two new if() statements in this patch, the

second line's first character should line up with the character

right after the openning "(" on the previous line.

Sorry Dave, I played with my vim settings ... I will resubmit the patch.

;-)