Open file as read-write when mount points to a file

This is to allow files mapped directly, like /etc/hosts, to be writable.
Closes #40

PiperOrigin-RevId: 196155920
Change-Id: Id2027e421cef5f94a0951c3e18b398a77c285bbd

runsc/fsgofer/fsgofer.go

@@ -98,7 +98,17 @@ func (a *attachPoint) Attach(appPath string) (p9.File, error) {
 	}
 
 	root := filepath.Join(a.prefix, appPath)
-	f, err := os.OpenFile(root, openFlags|syscall.O_RDONLY, 0)
+	fi, err := os.Stat(root)
+	if err != nil {
+		return nil, err
+	}
+
+	mode := syscall.O_RDWR
+	if a.conf.ROMount || fi.IsDir() {
+		mode = syscall.O_RDONLY
+	}
+
+	f, err := os.OpenFile(root, mode|openFlags, 0)
 	if err != nil {
 		return nil, fmt.Errorf("unable to open file %q, err: %v", root, err)
 	}

runsc/fsgofer/fsgofer_test.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
+	"path"
 	"syscall"
 	"testing"
 
@@ -102,7 +103,7 @@ func setup(ft fileType) (string, string, error) {
 		return "", "", fmt.Errorf("ioutil.TempDir() failed, err: %v", err)
 	}
 
-	// First attach with writable configuiration to setup tree.
+	// First attach with writable configuration to setup tree.
 	a := NewAttachPoint(path, Config{})
 	root, err := a.Attach("/")
 	if err != nil {
@@ -574,3 +575,50 @@ func TestReaddir(t *testing.T) {
 		}
 	})
 }
+
+// Test that attach point can be written to when it points to a file, e.g.
+// /etc/hosts.
+func TestAttachFile(t *testing.T) {
+	conf := Config{ROMount: false}
+	dir, err := ioutil.TempDir("", "root-")
+	if err != nil {
+		t.Fatalf("ioutil.TempDir() failed, err: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	path := path.Join(dir, "test")
+	if _, err := os.Create(path); err != nil {
+		t.Fatalf("os.Create(%q) failed, err: %v", path, err)
+	}
+
+	a := NewAttachPoint(path, conf)
+	root, err := a.Attach("/")
+	if err != nil {
+		t.Fatalf("Attach(%q) failed, err: %v", "/", err)
+	}
+
+	if _, _, _, err := root.Open(p9.ReadWrite); err != nil {
+		t.Fatalf("Open(ReadWrite) failed, err: %v", err)
+	}
+	defer root.Close()
+
+	b := []byte("foobar")
+	w, err := root.WriteAt(b, 0)
+	if err != nil {
+		t.Fatalf("Write() failed, err: %v", err)
+	}
+	if w != len(b) {
+		t.Fatalf("Write() was partial, got: %d, expected: %d", w, len(b))
+	}
+	rBuf := make([]byte, len(b))
+	r, err := root.ReadAt(rBuf, 0)
+	if err != nil {
+		t.Fatalf("ReadAt() failed, err: %v", err)
+	}
+	if r != len(rBuf) {
+		t.Fatalf("ReadAt() was partial, got: %d, expected: %d", r, len(rBuf))
+	}
+	if string(rBuf) != "foobar" {
+		t.Fatalf("ReadAt() wrong data, got: %s, expected: %s", string(rBuf), "foobar")
+	}
+}