mount: handle loopback mount

If a mount has specified `loop` option, we need to handle it on our
own instead of passing it to the kernel. In such case, create a
loopback device, attach the mount source to it, and mount the loopback
device rather than the mount source.

Signed-off-by: Peng Tao <bergwolf@hyper.sh>

mount/losetup_linux_test.go

@@ -0,0 +1,117 @@
+// +build linux
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package mount
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"github.com/containerd/continuity/testutil"
+)
+
+func TestSetupLoop(t *testing.T) {
+	testutil.RequiresRoot(t)
+	const randomdata = "randomdata"
+
+	/* Non-existing loop */
+	backingFile := "setup-loop-test-no-such-file"
+	_, err := setupLoop(backingFile, LoopParams{})
+	if err == nil {
+		t.Fatalf("setupLoop with non-existing file should fail")
+	}
+
+	f, err := ioutil.TempFile("", "losetup")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err = f.Truncate(512); err != nil {
+		t.Fatal(err)
+	}
+	backingFile = f.Name()
+	f.Close()
+	defer func() {
+		if err := os.Remove(backingFile); err != nil {
+			t.Fatal(err)
+		}
+	}()
+
+	/* RO loop */
+	f, err = setupLoop(backingFile, LoopParams{Readonly: true, Autoclear: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	ff, err := os.OpenFile(f.Name(), os.O_RDWR, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err = ff.Write([]byte(randomdata)); err == nil {
+		t.Fatalf("writing to readonly loop device should fail")
+	}
+	if err = ff.Close(); err != nil {
+		t.Fatal(err)
+	}
+	if err = f.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	/* RW loop */
+	f, err = setupLoop(backingFile, LoopParams{Autoclear: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	ff, err = os.OpenFile(f.Name(), os.O_RDWR, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err = ff.Write([]byte(randomdata)); err != nil {
+		t.Fatal(err)
+	}
+	if err = ff.Close(); err != nil {
+		t.Fatal(err)
+	}
+	if err = f.Close(); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestAttachDetachLoopDevice(t *testing.T) {
+	testutil.RequiresRoot(t)
+	f, err := ioutil.TempFile("", "losetup")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err = f.Truncate(512); err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+	defer func() {
+		if err := os.Remove(f.Name()); err != nil {
+			t.Fatal(err)
+		}
+	}()
+
+	dev, err := AttachLoopDevice(f.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err = DetachLoopDevice(dev); err != nil {
+		t.Fatal(err)
+	}
+}