[PATCH v2 net-next] tcp: socket-specific version of WARN_ON_ONCE()

[Breno Leitao <leitao@debian.org>]

There are cases where we need relevant information about the socket
during a warning, so, it could help us to find bugs that happens and do
not have an easy repro.

This patch creates a TCP-socket specific version of WARN_ON_ONCE(), which
dumps revelant information about the TCP socket when it hits rare
warnings, which is super useful for debugging purposes.

Hooking this warning tcp_snd_cwnd_set() for now, but, the intent is to
convert more TCP warnings to this helper later.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 include/net/tcp.h       |  3 ++-
 include/net/tcp_debug.h | 10 ++++++++++
 net/ipv4/tcp.c          | 30 ++++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 include/net/tcp_debug.h

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 14d45661a84d..e490af8e6fdc 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -40,6 +40,7 @@
 #include <net/inet_ecn.h>
 #include <net/dst.h>
 #include <net/mptcp.h>
+#include <net/tcp_debug.h>
 
 #include <linux/seq_file.h>
 #include <linux/memcontrol.h>
@@ -1229,7 +1230,7 @@ static inline u32 tcp_snd_cwnd(const struct tcp_sock *tp)
 
 static inline void tcp_snd_cwnd_set(struct tcp_sock *tp, u32 val)
 {
-	WARN_ON_ONCE((int)val <= 0);
+	TCP_SOCK_WARN_ON_ONCE(tp, (int)val <= 0);
 	tp->snd_cwnd = val;
 }
 
diff --git a/include/net/tcp_debug.h b/include/net/tcp_debug.h
new file mode 100644
index 000000000000..50e96d87d335
--- /dev/null
+++ b/include/net/tcp_debug.h
@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_TCP_DEBUG_H
+#define _LINUX_TCP_DEBUG_H
+
+void tcp_sock_warn(const struct tcp_sock *tp);
+
+#define TCP_SOCK_WARN_ON_ONCE(tcp_sock, condition) \
+		DO_ONCE_LITE_IF(condition, tcp_sock_warn, tcp_sock)
+
+#endif  /* _LINUX_TCP_DEBUG_H */
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 54836a6b81d6..5985ba9c4231 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -4705,6 +4705,36 @@ int tcp_abort(struct sock *sk, int err)
 }
 EXPORT_SYMBOL_GPL(tcp_abort);
 
+void tcp_sock_warn(const struct tcp_sock *tp)
+{
+	const struct sock *sk = (const struct sock *)tp;
+	struct inet_sock *inet = inet_sk(sk);
+	struct inet_connection_sock *icsk = inet_csk(sk);
+
+	WARN_ON(1);
+
+	pr_warn("Socket Info: family=%u state=%d ccname=%s cwnd=%u",
+		sk->sk_family, sk->sk_state, icsk->icsk_ca_ops->name,
+		tcp_snd_cwnd(tp));
+
+	switch (sk->sk_family) {
+	case AF_INET:
+		pr_warn("saddr=%pI4:%u daddr=%pI4:%u", &inet->inet_saddr,
+			ntohs(inet->inet_sport), &inet->inet_daddr,
+			ntohs(inet->inet_dport));
+
+		break;
+#if IS_ENABLED(CONFIG_IPV6)
+	case AF_INET6:
+		pr_warn("saddr=[%pI6]:%u daddr=[%pI6]:%u", &sk->sk_v6_rcv_saddr,
+			ntohs(inet->inet_sport), &sk->sk_v6_daddr,
+			ntohs(inet->inet_dport));
+		break;
+#endif
+	}
+}
+EXPORT_SYMBOL_GPL(tcp_sock_warn);
+
 extern struct tcp_congestion_ops tcp_reno;
 
 static __initdata unsigned long thash_entries;
-- 
2.30.2