tun: windows: check alignment in unit test

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/alignment_windows_test.go

@@ -0,0 +1,67 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2017-2021 WireGuard LLC. All Rights Reserved.
+ */
+
+package tun
+
+import (
+	"reflect"
+	"testing"
+	"unsafe"
+)
+
+func checkAlignment(t *testing.T, name string, offset uintptr) {
+	t.Helper()
+	if offset%8 != 0 {
+		t.Errorf("offset of %q within struct is %d bytes, which does not align to 64-bit word boundaries (missing %d bytes). Atomic operations will crash on 32-bit systems.", name, offset, 8-(offset%8))
+	}
+}
+
+// TestRateJugglerAlignment checks that atomically-accessed fields are
+// aligned to 64-bit boundaries, as required by the atomic package.
+//
+// Unfortunately, violating this rule on 32-bit platforms results in a
+// hard segfault at runtime.
+func TestRateJugglerAlignment(t *testing.T) {
+	var r rateJuggler
+
+	typ := reflect.TypeOf(&r).Elem()
+	t.Logf("Peer type size: %d, with fields:", typ.Size())
+	for i := 0; i < typ.NumField(); i++ {
+		field := typ.Field(i)
+		t.Logf("\t%30s\toffset=%3v\t(type size=%3d, align=%d)",
+			field.Name,
+			field.Offset,
+			field.Type.Size(),
+			field.Type.Align(),
+		)
+	}
+
+	checkAlignment(t, "rateJuggler.current", unsafe.Offsetof(r.current))
+	checkAlignment(t, "rateJuggler.nextByteCount", unsafe.Offsetof(r.nextByteCount))
+	checkAlignment(t, "rateJuggler.nextStartTime", unsafe.Offsetof(r.nextStartTime))
+}
+
+// TestNativeTunAlignment checks that atomically-accessed fields are
+// aligned to 64-bit boundaries, as required by the atomic package.
+//
+// Unfortunately, violating this rule on 32-bit platforms results in a
+// hard segfault at runtime.
+func TestNativeTunAlignment(t *testing.T) {
+	var tun NativeTun
+
+	typ := reflect.TypeOf(&tun).Elem()
+	t.Logf("Peer type size: %d, with fields:", typ.Size())
+	for i := 0; i < typ.NumField(); i++ {
+		field := typ.Field(i)
+		t.Logf("\t%30s\toffset=%3v\t(type size=%3d, align=%d)",
+			field.Name,
+			field.Offset,
+			field.Type.Size(),
+			field.Type.Align(),
+		)
+	}
+
+	checkAlignment(t, "NativeTun.rate", unsafe.Offsetof(tun.rate))
+}