[dev.link] cmd/link: fix mmapping OutBuf on windows

We got the permissions wrong on the mmapped region.

Change-Id: Ica6372fd9d9a787ab20a763e5785fb9fb34ff623
Reviewed-on: https://go-review.googlesource.com/c/go/+/226366
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/ld/outbuf_test.go

@@ -0,0 +1,30 @@
+// 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.
+
+package ld
+
+import (
+	"os"
+	"runtime"
+	"testing"
+)
+
+// TestMMap ensures that we can actually mmap on every supported platform.
+func TestMMap(t *testing.T) {
+	switch runtime.GOOS {
+	default:
+		t.Skip("unsupported OS")
+	case "darwin", "dragonfly", "freebsd", "linux", "openbsd", "windows":
+	}
+	filename := "foo.out"
+	ob := NewOutBuf(nil)
+	if err := ob.Open(filename); err != nil {
+		t.Errorf("error opening file: %v", err)
+	}
+	defer os.RemoveAll(filename)
+	defer ob.Close()
+	if err := ob.Mmap(1 << 20); err != nil {
+		t.Errorf("error mmapping file %v", err)
+	}
+}

src/cmd/link/internal/ld/outbuf_windows.go

@@ -17,7 +17,7 @@ func (out *OutBuf) Mmap(filesize uint64) error {
 	}
 
 	low, high := uint32(filesize), uint32(filesize>>32)
-	fmap, err := syscall.CreateFileMapping(syscall.Handle(out.f.Fd()), nil, syscall.PAGE_READONLY, high, low, nil)
+	fmap, err := syscall.CreateFileMapping(syscall.Handle(out.f.Fd()), nil, syscall.PAGE_READWRITE, high, low, nil)
 	if err != nil {
 		return err
 	}