sockstats: add validation for TCP socket stats

We can use the TCP_CONNECTION_INFO getsockopt() on Darwin to get
OS-collected tx/rx bytes for TCP sockets. Since this API is not available
for UDP sockets (or on Linux/Android), we can't rely on it for actual
stats gathering.

However, we can use it to validate the stats that we collect ourselves
using read/write hooks, so that we can be more confident in them. We
do need additional hooks from the Go standard library (added in
tailscale/go#59) to be able to collect them.

Updates tailscale/corp#9230
Updates #3363

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/sockstats/sockstats_tsgo_darwin.go

@@ -0,0 +1,30 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build tailscale_go && (darwin || ios)
+
+package sockstats
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func init() {
+	tcpConnStats = darwinTcpConnStats
+}
+
+func darwinTcpConnStats(c syscall.RawConn) (tx, rx uint64) {
+	c.Control(func(fd uintptr) {
+		if rawInfo, err := unix.GetsockoptTCPConnectionInfo(
+			int(fd),
+			unix.IPPROTO_TCP,
+			unix.TCP_CONNECTION_INFO,
+		); err == nil {
+			tx = uint64(rawInfo.Txbytes)
+			rx = uint64(rawInfo.Rxbytes)
+		}
+	})
+	return
+}