syscall: don't use 32-bit aligned access for cmsgAlignOf on dragonfly after ABI change

Use 32-bit alignment for versions before the September 2019 ABI changes
http://lists.dragonflybsd.org/pipermail/users/2019-September/358280.html

Fixes #34958

Change-Id: Iab4b23083a7c9ca7e96a737b03e75cd36d98ee24
Reviewed-on: https://go-review.googlesource.com/c/go/+/201977
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/sockcmsg_dragonfly.go

+// Copyright 2019 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 syscall
+
+// Round the length of a raw sockaddr up to align it properly.
+func cmsgAlignOf(salen int) int {
+	salign := sizeofPtr
+	if sizeofPtr == 8 && !supportsABI(_dragonflyABIChangeVersion) {
+		// 64-bit Dragonfly before the September 2019 ABI changes still requires
+		// 32-bit aligned access to network subsystem.
+		salign = 4
+	}
+	return (salen + salign - 1) & ^(salign - 1)
+}

src/syscall/sockcmsg_unix.go

@@ -9,35 +9,9 @@
 package syscall
 
 import (
-	"runtime"
 	"unsafe"
 )
 
-// Round the length of a raw sockaddr up to align it properly.
-func cmsgAlignOf(salen int) int {
-	salign := sizeofPtr
-
-	switch runtime.GOOS {
-	case "aix":
-		// There is no alignment on AIX.
-		salign = 1
-	case "darwin", "dragonfly", "illumos", "solaris":
-		// NOTE: It seems like 64-bit Darwin, DragonFly BSD and
-		// Solaris kernels still require 32-bit aligned access to
-		// network subsystem.
-		if sizeofPtr == 8 {
-			salign = 4
-		}
-	case "netbsd", "openbsd":
-		// NetBSD and OpenBSD armv7 require 64-bit alignment.
-		if runtime.GOARCH == "arm" {
-			salign = 8
-		}
-	}
-
-	return (salen + salign - 1) & ^(salign - 1)
-}
-
 // CmsgLen returns the value to store in the Len field of the Cmsghdr
 // structure, taking into account any necessary alignment.
 func CmsgLen(datalen int) int {

src/syscall/sockcmsg_unix_other.go

+// Copyright 2019 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.
+
+// +build aix darwin freebsd linux netbsd openbsd solaris
+
+package syscall
+
+import (
+	"runtime"
+)
+
+// Round the length of a raw sockaddr up to align it properly.
+func cmsgAlignOf(salen int) int {
+	salign := sizeofPtr
+
+	// dragonfly needs to check ABI version at runtime, see cmsgAlignOf in
+	// sockcmsg_dragonfly.go
+	switch runtime.GOOS {
+	case "aix":
+		// There is no alignment on AIX.
+		salign = 1
+	case "darwin", "illumos", "solaris":
+		// NOTE: It seems like 64-bit Darwin, Illumos and Solaris
+		// kernels still require 32-bit aligned access to network
+		// subsystem.
+		if sizeofPtr == 8 {
+			salign = 4
+		}
+	case "netbsd", "openbsd":
+		// NetBSD and OpenBSD armv7 require 64-bit alignment.
+		if runtime.GOARCH == "arm" {
+			salign = 8
+		}
+	}
+
+	return (salen + salign - 1) & ^(salign - 1)
+}

src/syscall/syscall_dragonfly.go

@@ -12,7 +12,25 @@
 
 package syscall
 
-import "unsafe"
+import (
+	"sync"
+	"unsafe"
+)
+
+// See version list in https://github.com/DragonFlyBSD/DragonFlyBSD/blob/master/sys/sys/param.h
+var (
+	osreldateOnce sync.Once
+	osreldate     uint32
+)
+
+// First __DragonFly_version after September 2019 ABI changes
+// http://lists.dragonflybsd.org/pipermail/users/2019-September/358280.html
+const _dragonflyABIChangeVersion = 500705
+
+func supportsABI(ver uint32) bool {
+	osreldateOnce.Do(func() { osreldate, _ = SysctlUint32("kern.osreldate") })
+	return osreldate >= ver
+}
 
 type SockaddrDatalink struct {
 	Len    uint8