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//go:build linux
// +build linux
package loopback // import "github.com/docker/docker/pkg/loopback"
import (
"errors"
"fmt"
"os"
"github.com/sirupsen/logrus"
"golang.org/x/sys/unix"
)
// Loopback related errors
var (
ErrAttachLoopbackDevice = errors.New("loopback attach failed")
ErrGetLoopbackBackingFile = errors.New("unable to get loopback backing file")
ErrSetCapacity = errors.New("unable set loopback capacity")
)
func stringToLoopName(src string) [unix.LO_NAME_SIZE]uint8 {
var dst [unix.LO_NAME_SIZE]uint8
copy(dst[:], src[:])
return dst
}
func getNextFreeLoopbackIndex() (int, error) {
f, err := os.OpenFile("/dev/loop-control", os.O_RDONLY, 0644)
if err != nil {
return 0, err
}
defer f.Close()
return unix.IoctlRetInt(int(f.Fd()), unix.LOOP_CTL_GET_FREE)
}
func openNextAvailableLoopback(index int, sparseFile *os.File) (loopFile *os.File, err error) {
// Start looking for a free /dev/loop
for {
target := fmt.Sprintf("/dev/loop%d", index)
index++
fi, err := os.Stat(target)
if err != nil {
if os.IsNotExist(err) {
logrus.Error("There are no more loopback devices available.")
}
return nil, ErrAttachLoopbackDevice
}
if fi.Mode()&os.ModeDevice != os.ModeDevice {
logrus.Errorf("Loopback device %s is not a block device.", target)
continue
}
// OpenFile adds O_CLOEXEC
loopFile, err = os.OpenFile(target, os.O_RDWR, 0644)
if err != nil {
logrus.Errorf("Error opening loopback device: %s", err)
return nil, ErrAttachLoopbackDevice
}
// Try to attach to the loop file
if err = unix.IoctlSetInt(int(loopFile.Fd()), unix.LOOP_SET_FD, int(sparseFile.Fd())); err != nil {
loopFile.Close()
// If the error is EBUSY, then try the next loopback
if err != unix.EBUSY {
logrus.Errorf("Cannot set up loopback device %s: %s", target, err)
return nil, ErrAttachLoopbackDevice
}
// Otherwise, we keep going with the loop
continue
}
// In case of success, we finished. Break the loop.
break
}
// This can't happen, but let's be sure
if loopFile == nil {
logrus.Errorf("Unreachable code reached! Error attaching %s to a loopback device.", sparseFile.Name())
return nil, ErrAttachLoopbackDevice
}
return loopFile, nil
}
// AttachLoopDevice attaches the given sparse file to the next
// available loopback device. It returns an opened *os.File.
func AttachLoopDevice(sparseName string) (loop *os.File, err error) {
// Try to retrieve the next available loopback device via syscall.
// If it fails, we discard error and start looping for a
// loopback from index 0.
startIndex, err := getNextFreeLoopbackIndex()
if err != nil {
logrus.Debugf("Error retrieving the next available loopback: %s", err)
}
// OpenFile adds O_CLOEXEC
sparseFile, err := os.OpenFile(sparseName, os.O_RDWR, 0644)
if err != nil {
logrus.Errorf("Error opening sparse file %s: %s", sparseName, err)
return nil, ErrAttachLoopbackDevice
}
defer sparseFile.Close()
loopFile, err := openNextAvailableLoopback(startIndex, sparseFile)
if err != nil {
return nil, err
}
// Set the status of the loopback device
loopInfo := &unix.LoopInfo64{
File_name: stringToLoopName(loopFile.Name()),
Offset: 0,
Flags: unix.LO_FLAGS_AUTOCLEAR,
}
if err = unix.IoctlLoopSetStatus64(int(loopFile.Fd()), loopInfo); err != nil {
logrus.Errorf("Cannot set up loopback device info: %s", err)
// If the call failed, then free the loopback device
if err = unix.IoctlSetInt(int(loopFile.Fd()), unix.LOOP_CLR_FD, 0); err != nil {
logrus.Error("Error while cleaning up the loopback device")
}
loopFile.Close()
return nil, ErrAttachLoopbackDevice
}
return loopFile, nil
}
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