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//go:build linux
package loopback
import (
"errors"
"fmt"
"io/fs"
"os"
"syscall"
"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) [LoNameSize]uint8 {
var dst [LoNameSize]uint8
copy(dst[:], src[:])
return dst
}
func getNextFreeLoopbackIndex() (int, error) {
f, err := os.OpenFile("/dev/loop-control", os.O_RDONLY, 0o644)
if err != nil {
return 0, err
}
defer f.Close()
index, err := ioctlLoopCtlGetFree(f.Fd())
if index < 0 {
index = 0
}
return index, err
}
func openNextAvailableLoopback(sparseName string, sparseFile *os.File) (*os.File, error) {
// Read information about the loopback file.
var st syscall.Stat_t
if err := syscall.Fstat(int(sparseFile.Fd()), &st); err != nil {
logrus.Errorf("Reading information about loopback file %s: %v", sparseName, err)
return nil, ErrAttachLoopbackDevice
}
// upper bound to avoid infinite loop
remaining := 1000
// Start looking for a free /dev/loop
for {
if remaining == 0 {
logrus.Errorf("No free loopback devices available")
return nil, ErrAttachLoopbackDevice
}
remaining--
index, err := getNextFreeLoopbackIndex()
if err != nil {
logrus.Debugf("Error retrieving the next available loopback: %s", err)
return nil, err
}
target := fmt.Sprintf("/dev/loop%d", index)
// OpenFile adds O_CLOEXEC
loopFile, err := os.OpenFile(target, os.O_RDWR, 0o644)
if err != nil {
// The kernel returns ENXIO when opening a device that is in the "deleting" or "rundown" state, so
// just treat ENXIO as if the device does not exist.
if errors.Is(err, fs.ErrNotExist) || errors.Is(err, unix.ENXIO) {
// Another process could have taken the loopback device in the meantime. So repeat
// the process with the next loopback device.
continue
}
logrus.Errorf("Opening loopback device: %s", err)
return nil, ErrAttachLoopbackDevice
}
fi, err := loopFile.Stat()
if err != nil {
loopFile.Close()
logrus.Errorf("Stat loopback device: %s", err)
return nil, ErrAttachLoopbackDevice
}
if fi.Mode()&os.ModeDevice != os.ModeDevice {
loopFile.Close()
logrus.Errorf("Loopback device %s is not a block device.", target)
continue
}
// Try to attach to the loop file
if err := ioctlLoopSetFd(loopFile.Fd(), sparseFile.Fd()); err != nil {
loopFile.Close()
// If the error is EBUSY, then try the next loopback
if err == syscall.EBUSY {
continue
}
logrus.Errorf("Cannot set up loopback device %s: %s", target, err)
return nil, ErrAttachLoopbackDevice
}
// Check if the loopback driver and underlying filesystem agree on the loopback file's
// device and inode numbers.
dev, ino, err := getLoopbackBackingFile(loopFile)
if err != nil {
logrus.Errorf("Getting loopback backing file: %s", err)
return nil, ErrGetLoopbackBackingFile
}
if dev != uint64(st.Dev) || ino != st.Ino { //nolint:unconvert
logrus.Errorf("Loopback device and filesystem disagree on device/inode for %q: %#x(%d):%#x(%d) vs %#x(%d):%#x(%d)", sparseName, dev, dev, ino, ino, st.Dev, st.Dev, st.Ino, st.Ino)
}
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) {
return attachLoopDevice(sparseName, false)
}
// AttachLoopDeviceRO attaches the given sparse file opened read-only to
// the next available loopback device. It returns an opened *os.File.
func AttachLoopDeviceRO(sparseName string) (loop *os.File, err error) {
return attachLoopDevice(sparseName, true)
}
func attachLoopDevice(sparseName string, readonly bool) (loop *os.File, err error) {
var sparseFile *os.File
// OpenFile adds O_CLOEXEC
if readonly {
sparseFile, err = os.OpenFile(sparseName, os.O_RDONLY, 0o644)
} else {
sparseFile, err = os.OpenFile(sparseName, os.O_RDWR, 0o644)
}
if err != nil {
logrus.Errorf("Opening sparse file: %v", err)
return nil, ErrAttachLoopbackDevice
}
defer sparseFile.Close()
loopFile, err := openNextAvailableLoopback(sparseName, sparseFile)
if err != nil {
return nil, err
}
// Set the status of the loopback device
loopInfo := &loopInfo64{
loFileName: stringToLoopName(loopFile.Name()),
loOffset: 0,
loFlags: LoFlagsAutoClear,
}
if err := ioctlLoopSetStatus64(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 := ioctlLoopClrFd(loopFile.Fd()); err != nil {
logrus.Error("While cleaning up the loopback device")
}
loopFile.Close()
return nil, ErrAttachLoopbackDevice
}
return loopFile, nil
}
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