cmd/gc: pad structs which end in zero-sized fields

For a non-zero-sized struct with a final zero-sized field,
add a byte to the size (before rounding to alignment).  This
change ensures that taking the address of the zero-sized field
will not incorrectly leak the following object in memory.

reflect.funcLayout also needs this treatment.

Fixes #9401

Change-Id: I1dc503dc5af4ca22c8f8c048fb7b4541cc957e0f
Reviewed-on: https://go-review.googlesource.com/2452
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/runtime_test.go

@@ -247,3 +247,44 @@ func TestEqString(t *testing.T) {
 		}
 	}
 }
+
+func TestTrailingZero(t *testing.T) {
+	// make sure we add padding for structs with trailing zero-sized fields
+	type T1 struct {
+		n int32
+		z [0]byte
+	}
+	if unsafe.Sizeof(T1{}) != 8 {
+		t.Errorf("sizeof(%#v)==%d, want 8", T1{}, unsafe.Sizeof(T1{}))
+	}
+	type T2 struct {
+		n int64
+		z struct{}
+	}
+	if unsafe.Sizeof(T2{}) != 16 {
+		t.Errorf("sizeof(%#v)==%d, want 16", T2{}, unsafe.Sizeof(T2{}))
+	}
+	type T3 struct {
+		n byte
+		z [4]struct{}
+	}
+	if unsafe.Sizeof(T3{}) != 2 {
+		t.Errorf("sizeof(%#v)==%d, want 2", T3{}, unsafe.Sizeof(T3{}))
+	}
+	// make sure padding can double for both zerosize and alignment
+	type T4 struct {
+		a int32
+		b int16
+		c int8
+		z struct{}
+	}
+	if unsafe.Sizeof(T4{}) != 8 {
+		t.Errorf("sizeof(%#v)==%d, want 8", T4{}, unsafe.Sizeof(T4{}))
+	}
+	// make sure we don't pad a zero-sized thing
+	type T5 struct {
+	}
+	if unsafe.Sizeof(T5{}) != 0 {
+		t.Errorf("sizeof(%#v)==%d, want 0", T5{}, unsafe.Sizeof(T5{}))
+	}
+}