net: reflect TCP backlog size update of uint16->uint32 on Linux

The sk_max_ack_backlog was increased from uint16 to uint32 in kernel
version 4.1 and above, so adopt that change to maxListenerBacklog.

See torvalds/linux@becb74f

Fixes #41470

Change-Id: I63a142eb28f3ac3acaca57f0903c085c6cb15a6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/255898
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/net/sock_linux.go

@@ -6,6 +6,62 @@ package net
 
 import "syscall"
 
+func kernelVersion() (major int, minor int) {
+	var uname syscall.Utsname
+	if err := syscall.Uname(&uname); err != nil {
+		return
+	}
+
+	rl := uname.Release
+	var values [2]int
+	vi := 0
+	value := 0
+	for _, c := range rl {
+		if c >= '0' && c <= '9' {
+			value = (value * 10) + int(c-'0')
+		} else {
+			// Note that we're assuming N.N.N here.  If we see anything else we are likely to
+			// mis-parse it.
+			values[vi] = value
+			vi++
+			if vi >= len(values) {
+				break
+			}
+		}
+	}
+	switch vi {
+	case 0:
+		return 0, 0
+	case 1:
+		return values[0], 0
+	case 2:
+		return values[0], values[1]
+	}
+	return
+}
+
+// Linux stores the backlog as:
+//
+//  - uint16 in kernel version < 4.1,
+//  - uint32 in kernel version >= 4.1
+//
+// Truncate number to avoid wrapping.
+//
+// See issue 5030 and 41470.
+func maxAckBacklog(n int) int {
+	major, minor := kernelVersion()
+	size := 16
+	if major > 4 || (major == 4 && minor >= 1) {
+		size = 32
+	}
+
+	var max uint = 1<<size - 1
+	if uint(n) > max {
+		n = int(max)
+	}
+	return n
+}
+
 func maxListenerBacklog() int {
 	fd, err := open("/proc/sys/net/core/somaxconn")
 	if err != nil {
@@ -21,11 +77,9 @@ func maxListenerBacklog() int {
 	if n == 0 || !ok {
 		return syscall.SOMAXCONN
 	}
-	// Linux stores the backlog in a uint16.
-	// Truncate number to avoid wrapping.
-	// See issue 5030.
+
 	if n > 1<<16-1 {
-		n = 1<<16 - 1
+		return maxAckBacklog(n)
 	}
 	return n
 }

src/net/sock_linux_test.go

@@ -0,0 +1,22 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package net
+
+import (
+	"testing"
+)
+
+func TestMaxAckBacklog(t *testing.T) {
+	n := 196602
+	major, minor := kernelVersion()
+	backlog := maxAckBacklog(n)
+	expected := 1<<16 - 1
+	if major > 4 || (major == 4 && minor >= 1) {
+		expected = n
+	}
+	if backlog != expected {
+		t.Fatalf(`Kernel version: "%d.%d", sk_max_ack_backlog mismatch, got %d, want %d`, major, minor, backlog, expected)
+	}
+}