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Date:   Mon, 22 Oct 2018 08:23:59 -0700
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     sowmini.varadhan@...cle.com, netdev@...r.kernel.org
Cc:     edumazet@...gle.com, brakmo@...com
Subject: [PATCH RFC net-next 2/3] tcp: BPF_TCP_INFO_NOTIFY support

We want to be able to set up the monitoring application so that it can
be aysnchronously notified when "interesting" events happen, e.g., when
application-determined thresholds on parameters like RTT estimate, number
of retransmissions, RTO are reached.

The bpf_sock_ops infrastructure provided as part of Commit 40304b2a1567
("bpf: BPF support for sock_ops") provides an elegant way to trigger
this asynchronous notification. The BPF program can examine the
current TCP state reported in the bpf_sock_ops and conditionally
return a (new) status BPF_TCP_INFO_NOTIFY. The return status is used
by the caller to queue up a tcp_info notification for the application.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
---
 include/net/tcp.h        |   15 +++++++++++++--
 include/uapi/linux/bpf.h |    4 ++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 0d29292..df06a9f 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -47,6 +47,7 @@
 #include <linux/seq_file.h>
 #include <linux/memcontrol.h>
 #include <linux/bpf-cgroup.h>
+#include <linux/sock_diag.h>
 
 extern struct inet_hashinfo tcp_hashinfo;
 
@@ -2065,6 +2066,12 @@ struct tcp_ulp_ops {
 	__MODULE_INFO(alias, alias_userspace, name);		\
 	__MODULE_INFO(alias, alias_tcp_ulp, "tcp-ulp-" name)
 
+#define	TCPDIAG_CB(sk)							\
+do {									\
+	if (unlikely(sk->sk_net_refcnt && sock_diag_has_listeners(sk)))	\
+		sock_diag_broadcast(sk);				\
+} while (0)
+
 /* Call BPF_SOCK_OPS program that returns an int. If the return value
  * is < 0, then the BPF op failed (for example if the loaded BPF
  * program does not support the chosen operation or there is no BPF
@@ -2088,9 +2095,13 @@ static inline int tcp_call_bpf(struct sock *sk, int op, u32 nargs, u32 *args)
 		memcpy(sock_ops.args, args, nargs * sizeof(*args));
 
 	ret = BPF_CGROUP_RUN_PROG_SOCK_OPS(&sock_ops);
-	if (ret == 0)
+	if (ret == 0) {
 		ret = sock_ops.reply;
-	else
+
+		/* XXX would be nice if we could use replylong[1] here */
+		if (ret == BPF_TCP_INFO_NOTIFY)
+			TCPDIAG_CB(sk);
+	} else
 		ret = -1;
 	return ret;
 }
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index aa5ccd2..bc45e5e 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2678,6 +2678,10 @@ enum {
 	BPF_TCP_MAX_STATES	/* Leave at the end! */
 };
 
+enum {
+	BPF_TCP_INFO_NOTIFY = 2
+};
+
 #define TCP_BPF_IW		1001	/* Set TCP initial congestion window */
 #define TCP_BPF_SNDCWND_CLAMP	1002	/* Set sndcwnd_clamp */
 
-- 
1.7.1

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