reflect: fix pointer past-the-end in Call with zero-sized return value

If a function with nonzero frame but zero-sized return value is
Call'd, we may write a past-the-end pointer in preparing the
return Values. Fix by return the zero value for zero-sized
return value.

Fixes #21717.

Change-Id: I5351cd86d898467170a888b4c3fc9392f0e7aa3b
Reviewed-on: https://go-review.googlesource.com/60811
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/reflect/all_test.go

@@ -19,6 +19,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 	"unicode"
@@ -1629,6 +1630,30 @@ func TestCallWithStruct(t *testing.T) {
 	}
 }
 
+func TestCallReturnsEmpty(t *testing.T) {
+	// Issue 21717: past-the-end pointer write in Call with
+	// nonzero-sized frame and zero-sized return value.
+	runtime.GC()
+	var finalized uint32
+	f := func() (emptyStruct, *int) {
+		i := new(int)
+		runtime.SetFinalizer(i, func(*int) { atomic.StoreUint32(&finalized, 1) })
+		return emptyStruct{}, i
+	}
+	v := ValueOf(f).Call(nil)[0] // out[0] should not alias out[1]'s memory, so the finalizer should run.
+	timeout := time.After(5 * time.Second)
+	for atomic.LoadUint32(&finalized) == 0 {
+		select {
+		case <-timeout:
+			t.Fatal("finalizer did not run")
+		default:
+		}
+		runtime.Gosched()
+		runtime.GC()
+	}
+	runtime.KeepAlive(v)
+}
+
 func BenchmarkCall(b *testing.B) {
 	fv := ValueOf(func(a, b string) {})
 	b.ReportAllocs()

src/reflect/value.go

@@ -455,8 +455,14 @@ func (v Value) call(op string, in []Value) []Value {
 			tv := t.Out(i)
 			a := uintptr(tv.Align())
 			off = (off + a - 1) &^ (a - 1)
-			fl := flagIndir | flag(tv.Kind())
+			if tv.Size() != 0 {
-			ret[i] = Value{tv.common(), unsafe.Pointer(uintptr(args) + off), fl}
+				fl := flagIndir | flag(tv.Kind())
+				ret[i] = Value{tv.common(), unsafe.Pointer(uintptr(args) + off), fl}
+			} else {
+				// For zero-sized return value, args+off may point to the next object.
+				// In this case, return the zero value instead.
+				ret[i] = Zero(tv)
+			}
 			off += tv.Size()
 		}
 	}