[PATCH v6 bpf-next 01/11] net: Introduce net.ipv4.tcp_migrate_req.

[Kuniyuki Iwashima]

This commit adds a new sysctl option: net.ipv4.tcp_migrate_req. If this
option is enabled or eBPF program is attached, we will be able to migrate
child sockets from a listener to another in the same reuseport group after
close() or shutdown() syscalls.

Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxxxx>
Reviewed-by: Benjamin Herrenschmidt <benh@xxxxxxxxxx>
Acked-by: Martin KaFai Lau <kafai@xxxxxx>
---
 Documentation/networking/ip-sysctl.rst | 25 +++++++++++++++++++++++++
 include/net/netns/ipv4.h               |  1 +
 net/ipv4/sysctl_net_ipv4.c             |  9 +++++++++
 3 files changed, 35 insertions(+)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index c2ecc9894fd0..22c80b0d2208 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -732,6 +732,31 @@ tcp_syncookies - INTEGER
 	network connections you can set this knob to 2 to enable
 	unconditionally generation of syncookies.
 
+tcp_migrate_req - BOOLEAN
+	The incoming connection is tied to a specific listening socket when
+	the initial SYN packet is received during the three-way handshake.
+	When a listener is closed, in-flight request sockets during the
+	handshake and established sockets in the accept queue are aborted.
+
+	If the listener has SO_REUSEPORT enabled, other listeners on the
+	same port should have been able to accept such connections. This
+	option makes it possible to migrate such child sockets to another
+	listener after close() or shutdown().
+
+	The BPF_SK_REUSEPORT_SELECT_OR_MIGRATE type of eBPF program should
+	usually be used to define the policy to pick an alive listener.
+	Otherwise, the kernel will randomly pick an alive listener only if
+	this option is enabled.
+
+	Note that migration between listeners with different settings may
+	crash applications. Let's say migration happens from listener A to
+	B, and only B has TCP_SAVE_SYN enabled. B cannot read SYN data from
+	the requests migrated from A. To avoid such a situation, cancel
+	migration by returning SK_DROP in the type of eBPF program, or
+	disable this option.
+
+	Default: 0
+
 tcp_fastopen - INTEGER
 	Enable TCP Fast Open (RFC7413) to send and accept data in the opening
 	SYN packet.
diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index f6af8d96d3c6..fd63c91addcc 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -126,6 +126,7 @@ struct netns_ipv4 {
 	u8 sysctl_tcp_syn_retries;
 	u8 sysctl_tcp_synack_retries;
 	u8 sysctl_tcp_syncookies;
+	u8 sysctl_tcp_migrate_req;
 	int sysctl_tcp_reordering;
 	u8 sysctl_tcp_retries1;
 	u8 sysctl_tcp_retries2;
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index a62934b9f15a..7bb013fcbf5f 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -940,6 +940,15 @@ static struct ctl_table ipv4_net_table[] = {
 		.proc_handler	= proc_dou8vec_minmax,
 	},
 #endif
+	{
+		.procname	= "tcp_migrate_req",
+		.data		= &init_net.ipv4.sysctl_tcp_migrate_req,
+		.maxlen		= sizeof(u8),
+		.mode		= 0644,
+		.proc_handler	= proc_dou8vec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE
+	},
 	{
 		.procname	= "tcp_reordering",
 		.data		= &init_net.ipv4.sysctl_tcp_reordering,
-- 
2.30.2