cmd/link: add a test to test RODATA is indeed read-only

Updates #38830.

Change-Id: Ie1f6ccef40a773f038aac587dfc26bf70a1a8536
Reviewed-on: https://go-review.googlesource.com/c/go/+/253921
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/link_test.go

@@ -800,3 +800,17 @@ func TestContentAddressableSymbols(t *testing.T) {
 		t.Errorf("command %s failed: %v\n%s", cmd, err, out)
 	}
 }
+
+func TestReadOnly(t *testing.T) {
+	// Test that read-only data is indeed read-only.
+	testenv.MustHaveGoBuild(t)
+
+	t.Parallel()
+
+	src := filepath.Join("testdata", "testRO", "x.go")
+	cmd := exec.Command(testenv.GoToolPath(t), "run", src)
+	out, err := cmd.CombinedOutput()
+	if err == nil {
+		t.Errorf("running test program did not fail. output:\n%s", out)
+	}
+}

src/cmd/link/testdata/testRO/x.go

@@ -0,0 +1,22 @@
+// Copyright 2020 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.
+
+// Test that read-only data is indeed read-only. This
+// program attempts to modify read-only data, and it
+// should fail.
+
+package main
+
+import "unsafe"
+
+var s = "hello"
+
+func main() {
+	println(s)
+	*(*struct {
+		p *byte
+		l int
+	})(unsafe.Pointer(&s)).p = 'H'
+	println(s)
+}