Subject: [PATCH bpf-next] tcp: bpf: Add TCP_BPF_RCV_SSTHRESH for bpf_setsockopt

From: Ivan Babrou <[email-masked]>
Date: Tue, 11 Jan 2022 11:29:52 -0800

This patch adds bpf_setsockopt(TCP_BPF_RCV_SSTHRESH) to allow setting
rcv_ssthresh value for TCP connections. Combined with increased
window_clamp via tcp_rmem[1], it allows to advertise initial scaled
TCP window larger than 64k. This is useful for high BDP connections,
where it allows to push data with fewer roundtrips, reducing latency.

For active connections the larger window is advertised in the first
non-SYN ACK packet as the part of the 3 way handshake.

For passive connections the larger window is advertised whenever
there's any packet to send after the 3 way handshake.

See: https://lkml.org/lkml/2021/12/22/652

Signed-off-by: Ivan Babrou <[email-masked]>
---
 include/uapi/linux/bpf.h       | 1 +
 net/core/filter.c              | 6 ++++++
 tools/include/uapi/linux/bpf.h | 1 +
 3 files changed, 8 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 791f31dd0abe..36ebf87278bd 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -5978,6 +5978,7 @@ enum {
 	TCP_BPF_SYN		= 1005, /* Copy the TCP header */
 	TCP_BPF_SYN_IP		= 1006, /* Copy the IP[46] and TCP header */
 	TCP_BPF_SYN_MAC         = 1007, /* Copy the MAC, IP[46], and TCP header */
+	TCP_BPF_RCV_SSTHRESH	= 1008, /* Set rcv_ssthresh */
 };
 
 enum {
diff --git a/net/core/filter.c b/net/core/filter.c
index 2e32cee2c469..aafb6066b1a6 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4904,6 +4904,12 @@ static int _bpf_setsockopt(struct sock *sk, int level, int optname,
 					return -EINVAL;
 				inet_csk(sk)->icsk_rto_min = timeout;
 				break;
+			case TCP_BPF_RCV_SSTHRESH:
+				if (val <= 0)
+					ret = -EINVAL;
+				else
+					tp->rcv_ssthresh = min_t(u32, val, tp->window_clamp);
+				break;
 			case TCP_SAVE_SYN:
 				if (val < 0 || val > 1)
 					ret = -EINVAL;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 791f31dd0abe..36ebf87278bd 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -5978,6 +5978,7 @@ enum {
 	TCP_BPF_SYN		= 1005, /* Copy the TCP header */
 	TCP_BPF_SYN_IP		= 1006, /* Copy the IP[46] and TCP header */
 	TCP_BPF_SYN_MAC         = 1007, /* Copy the MAC, IP[46], and TCP header */
+	TCP_BPF_RCV_SSTHRESH	= 1008, /* Set rcv_ssthresh */
 };
 
 enum {
-- 
2.34.1