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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 <[email protected]>
1 parent e2e40e1 commit 37c9ddb

2 files changed

Lines changed: 199 additions & 7 deletions

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‎mount/losetup_linux.go‎

Lines changed: 178 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,178 @@
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/*
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Copyright The containerd Authors.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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package mount
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import (
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"fmt"
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"math/rand"
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"os"
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"strings"
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"syscall"
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"time"
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"unsafe"
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"github.com/pkg/errors"
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)
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const (
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loopControlPath = "/dev/loop-control"
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loopDevFormat = "/dev/loop%d"
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// According to util-linux/include/loopdev.h
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ioctlSetFd = 0x4C00
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ioctlClrFd = 0x4C01
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ioctlSetStatus64 = 0x4C04
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ioctlGetFree = 0x4C82
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loFlagsReadonly = 1
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//loFlagsUseAops = 2
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loFlagsAutoclear = 4
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//loFlagsPartScan = 8
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loFlagsDirectIO = 16
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ebusyString = "device or resource busy"
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)
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// struct loop_info64 in util-linux/include/loopdev.h
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type loopInfo struct {
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/*
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device uint64
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inode uint64
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rdevice uint64
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offset uint64
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sizelimit uint64
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number uint32
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encryptType uint32
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encryptKeySize uint32
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*/
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_ [13]uint32
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flags uint32
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fileName [64]byte
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/*
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cryptName [64]byte
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encryptKey [32]byte
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init [2]uint64
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*/
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_ [112]byte
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}
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func ioctl(fd, req, args uintptr) (uintptr, uintptr, error) {
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r1, r2, errno := syscall.Syscall(syscall.SYS_IOCTL, fd, req, args)
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if errno != 0 {
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return 0, 0, errno
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}
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return r1, r2, nil
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}
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func getFreeLoopDev() (uint32, error) {
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ctrl, err := os.OpenFile(loopControlPath, os.O_RDWR, 0)
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if err != nil {
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return 0, errors.Errorf("could not open %v: %v", loopControlPath, err)
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}
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defer ctrl.Close()
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num, _, err := ioctl(ctrl.Fd(), ioctlGetFree, 0)
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if err != nil {
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return 0, errors.Wrap(err, "could not get free loop device")
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}
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return uint32(num), nil
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}
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func setupLoopDev(backingFile, loopDev string, ro bool) (devFile *os.File, err error) {
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// 1. Open backing file and loop device
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oflags := os.O_RDWR
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if ro {
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oflags = os.O_RDONLY
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}
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back, err := os.OpenFile(backingFile, oflags, 0)
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if err != nil {
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return nil, errors.Errorf("could not open backing file: %v", err)
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}
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defer back.Close()
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loopFile, err := os.OpenFile(loopDev, oflags, 0)
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if err != nil {
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return nil, errors.Errorf("could not open loop device: %v", err)
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}
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defer func() {
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if err != nil {
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loopFile.Close()
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}
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}()
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// 2. Set FD
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if _, _, err = ioctl(loopFile.Fd(), ioctlSetFd, back.Fd()); err != nil {
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return nil, errors.Errorf("could not set loop fd: %v", err)
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}
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// 3. Set Info
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info := loopInfo{}
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copy(info.fileName[:], []byte(backingFile))
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// Always set autoclear flag so that the device goes away upon umount.
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// Always set direct IO flag to avoid double caching.
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info.flags = loFlagsAutoclear | loFlagsDirectIO
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if ro {
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info.flags |= loFlagsReadonly
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}
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if _, _, err := ioctl(loopFile.Fd(), ioctlSetStatus64, uintptr(unsafe.Pointer(&info))); err != nil {
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// Retry w/o direct IO flag in case kernel does not support it. The downside is that
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// it will suffer from double cache problem.
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info.flags &= ^(uint32(loFlagsDirectIO))
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if _, _, err := ioctl(loopFile.Fd(), ioctlSetStatus64, uintptr(unsafe.Pointer(&info))); err != nil {
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ioctl(loopFile.Fd(), ioctlClrFd, 0)
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return nil, errors.Errorf("cannot set loop info:%v", err)
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}
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}
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return loopFile, nil
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}
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// setupLoop looks for (and possibly creates) a free loop device, and
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// then attaches backingFile to it.
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//
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// Upon success, the file handle to the loop device is also returned.
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// Caller should take care to close it when done with the loop device
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// The loop device file handle keeps loFlagsAutoclear in effect and we
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// rely on it to clean up the loop device. If caller closes the file
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// handle after mounting the device, kernel will clear the loop device
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// after it is umounted. Otherwise the loop device is cleared when the
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// file handle is closed.
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func setupLoop(backingFile string, ro bool) (string, *os.File, error) {
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var loopDev string
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for retry := 1; ; retry++ {
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num, err := getFreeLoopDev()
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if err != nil {
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return "", nil, err
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}
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loopDev = fmt.Sprintf(loopDevFormat, num)
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loopFile, err := setupLoopDev(backingFile, loopDev, ro)
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if err != nil {
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// Per util-linux/sys-utils/losetup.c:create_loop(),
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// free loop device can race and we end up failing
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// with EBUSY when trying to set it up.
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if strings.Contains(err.Error(), ebusyString) {
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// Fallback a bit to avoid live lock
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time.Sleep(time.Millisecond * time.Duration(rand.Intn(retry*10)))
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continue
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}
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return "", nil, err
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}
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return loopDev, loopFile, nil
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}
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}

‎mount/mount_linux.go‎

Lines changed: 21 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -42,7 +42,7 @@ func init() {
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//
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// If m.Type starts with "fuse." or "fuse3.", "mount.fuse" or "mount.fuse3"
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// helper binary is called.
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func (m *Mount) Mount(target string) error {
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func (m *Mount) Mount(target string) (err error) {
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for _, helperBinary := range allowedHelperBinaries {
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// helperBinary = "mount.fuse", typePrefix = "fuse."
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typePrefix := strings.TrimPrefix(helperBinary, "mount.") + "."
@@ -62,7 +62,7 @@ func (m *Mount) Mount(target string) error {
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chdir, options = compactLowerdirOption(options)
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}
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flags, data := parseMountOptions(options)
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flags, data, losetup := parseMountOptions(options)
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if len(data) > pagesize {
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return errors.Errorf("mount options is too long")
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}
@@ -77,7 +77,17 @@ func (m *Mount) Mount(target string) error {
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if flags&unix.MS_REMOUNT == 0 || data != "" {
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// Initial call applying all non-propagation flags for mount
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// or remount with changed data
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if err := mountAt(chdir, m.Source, target, m.Type, uintptr(oflags), data); err != nil {
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source := m.Source
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if losetup {
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dev, devFile, err := setupLoop(m.Source, oflags&unix.MS_RDONLY == unix.MS_RDONLY)
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if err != nil {
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return err
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}
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defer devFile.Close()
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// Mount the loop device instead
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source = dev
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}
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if err := mountAt(chdir, source, target, m.Type, uintptr(oflags|unix.MS_MGC_VAL), data); err != nil {
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return err
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}
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}
@@ -175,11 +185,13 @@ func UnmountAll(mount string, flags int) error {
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// parseMountOptions takes fstab style mount options and parses them for
177187
// use with a standard mount() syscall
178-
func parseMountOptions(options []string) (int, string) {
188+
func parseMountOptions(options []string) (int, string, bool) {
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var (
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flag int
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data []string
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flag int
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losetup bool
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data []string
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)
194+
loopOpt := "loop"
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flags := map[string]struct {
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clear bool
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flag int
@@ -220,11 +232,13 @@ func parseMountOptions(options []string) (int, string) {
220232
} else {
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flag |= f.flag
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}
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} else if o == loopOpt {
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losetup = true
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} else {
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data = append(data, o)
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}
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}
227-
return flag, strings.Join(data, ",")
241+
return flag, strings.Join(data, ","), losetup
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}
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// compactLowerdirOption updates overlay lowdir option and returns the common

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