util/deephash: fix unexported time.Time hashing

Updates tailscale/corp#6311

Change-Id: I33cd7e4040966261c2f2eb3d32f29936aeb7f632
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/deephash/deephash.go

@@ -544,9 +544,11 @@ func (h *hasher) hashComplex128v(v reflect.Value) bool {
 func (h *hasher) hashTimev(v reflect.Value) bool {
 	var t time.Time
 	if v.CanAddr() {
-		t = *(v.Addr().Interface().(*time.Time))
-	} else {
+		t = *(*time.Time)(v.Addr().UnsafePointer())
+	} else if v.CanInterface() {
 		t = v.Interface().(time.Time)
+	} else {
+		return false
 	}
 	b := t.AppendFormat(h.scratch[:1], time.RFC3339Nano)
 	b[0] = byte(len(b) - 1) // more than sufficient width; if not, good enough.

util/deephash/deephash_test.go

@@ -28,6 +28,7 @@ import (
 	"tailscale.com/types/ipproto"
 	"tailscale.com/types/key"
 	"tailscale.com/types/structs"
+	"tailscale.com/util/deephash/testtype"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/version"
 	"tailscale.com/wgengine/filter"
@@ -516,6 +517,21 @@ func TestGetTypeHasher(t *testing.T) {
 			val:  time.Unix(0, 0).In(time.UTC),
 			out:  "\x141970-01-01T00:00:00Z",
 		},
+		{
+			name: "time_ptr", // addressable, as opposed to "time" test above
+			val:  ptrTo(time.Unix(0, 0).In(time.UTC)),
+			out:  "\x01\x141970-01-01T00:00:00Z",
+		},
+		{
+			name: "time_ptr_via_unexported",
+			val:  testtype.NewUnexportedAddressableTime(time.Unix(0, 0).In(time.UTC)),
+			out:  "\x01\x141970-01-01T00:00:00Z",
+		},
+		{
+			name: "time_ptr_via_unexported_value",
+			val:  *testtype.NewUnexportedAddressableTime(time.Unix(0, 0).In(time.UTC)),
+			want: false, // neither addressable nor interface-able
+		},
 		{
 			name: "time_custom_zone",
 			val:  time.Unix(1655311822, 0).In(time.FixedZone("FOO", -60*60)),
@@ -817,6 +833,26 @@ func TestArrayAllocs(t *testing.T) {
 	}
 }
 
+// Test for http://go/corp/6311 issue.
+func TestHashThroughView(t *testing.T) {
+	type sshPolicyOut struct {
+		Rules []tailcfg.SSHRuleView
+	}
+	type mapResponseOut struct {
+		SSHPolicy *sshPolicyOut
+	}
+	// Just test we don't panic:
+	_ = Hash(&mapResponseOut{
+		SSHPolicy: &sshPolicyOut{
+			Rules: []tailcfg.SSHRuleView{
+				(&tailcfg.SSHRule{
+					RuleExpires: ptrTo(time.Unix(123, 0)),
+				}).View(),
+			},
+		},
+	})
+}
+
 func BenchmarkHashArray(b *testing.B) {
 	b.ReportAllocs()
 	type T struct {

util/deephash/testtype/testtype.go

@@ -0,0 +1,16 @@
+// Copyright (c) 2022 Tailscale Inc & 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 testtype contains types for testing deephash.
+package testtype
+
+import "time"
+
+type UnexportedAddressableTime struct {
+	t time.Time
+}
+
+func NewUnexportedAddressableTime(t time.Time) *UnexportedAddressableTime {
+	return &UnexportedAddressableTime{t: t}
+}