loopback: separate loop logic from devicemapper

The loopback logic is not technically exclusive to the devicemapper
driver. This reorganizes the code such that the loopback code is usable
outside of the devicemapper package and driver.

Signed-off-by: Vincent Batts <vbatts@redhat.com>

loopback/attach_loopback.go

@@ -0,0 +1,137 @@
+// +build linux
+
+package loopback
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"syscall"
+
+	"github.com/Sirupsen/logrus"
+)
+
+// 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, 0644)
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+
+	index, err := ioctlLoopCtlGetFree(f.Fd())
+	if index < 0 {
+		index = 0
+	}
+	return index, err
+}
+
+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.Errorf("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 := ioctlLoopSetFd(loopFile.Fd(), sparseFile.Fd()); err != nil {
+			loopFile.Close()
+
+			// If the error is EBUSY, then try the next loopback
+			if err != syscall.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 := &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.Errorf("Error while cleaning up the loopback device")
+		}
+		loopFile.Close()
+		return nil, ErrAttachLoopbackDevice
+	}
+
+	return loopFile, nil
+}

loopback/ioctl.go

@@ -0,0 +1,53 @@
+// +build linux
+
+package loopback
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func ioctlLoopCtlGetFree(fd uintptr) (int, error) {
+	index, _, err := syscall.Syscall(syscall.SYS_IOCTL, fd, LoopCtlGetFree, 0)
+	if err != 0 {
+		return 0, err
+	}
+	return int(index), nil
+}
+
+func ioctlLoopSetFd(loopFd, sparseFd uintptr) error {
+	if _, _, err := syscall.Syscall(syscall.SYS_IOCTL, loopFd, LoopSetFd, sparseFd); err != 0 {
+		return err
+	}
+	return nil
+}
+
+func ioctlLoopSetStatus64(loopFd uintptr, loopInfo *loopInfo64) error {
+	if _, _, err := syscall.Syscall(syscall.SYS_IOCTL, loopFd, LoopSetStatus64, uintptr(unsafe.Pointer(loopInfo))); err != 0 {
+		return err
+	}
+	return nil
+}
+
+func ioctlLoopClrFd(loopFd uintptr) error {
+	if _, _, err := syscall.Syscall(syscall.SYS_IOCTL, loopFd, LoopClrFd, 0); err != 0 {
+		return err
+	}
+	return nil
+}
+
+func ioctlLoopGetStatus64(loopFd uintptr) (*loopInfo64, error) {
+	loopInfo := &loopInfo64{}
+
+	if _, _, err := syscall.Syscall(syscall.SYS_IOCTL, loopFd, LoopGetStatus64, uintptr(unsafe.Pointer(loopInfo))); err != 0 {
+		return nil, err
+	}
+	return loopInfo, nil
+}
+
+func ioctlLoopSetCapacity(loopFd uintptr, value int) error {
+	if _, _, err := syscall.Syscall(syscall.SYS_IOCTL, loopFd, LoopSetCapacity, uintptr(value)); err != 0 {
+		return err
+	}
+	return nil
+}

loopback/loop_wrapper.go

@@ -0,0 +1,52 @@
+// +build linux
+
+package loopback
+
+/*
+#include <linux/loop.h> // FIXME: present only for defines, maybe we can remove it?
+
+#ifndef LOOP_CTL_GET_FREE
+  #define LOOP_CTL_GET_FREE 0x4C82
+#endif
+
+#ifndef LO_FLAGS_PARTSCAN
+  #define LO_FLAGS_PARTSCAN 8
+#endif
+
+*/
+import "C"
+
+type loopInfo64 struct {
+	loDevice         uint64 /* ioctl r/o */
+	loInode          uint64 /* ioctl r/o */
+	loRdevice        uint64 /* ioctl r/o */
+	loOffset         uint64
+	loSizelimit      uint64 /* bytes, 0 == max available */
+	loNumber         uint32 /* ioctl r/o */
+	loEncryptType    uint32
+	loEncryptKeySize uint32 /* ioctl w/o */
+	loFlags          uint32 /* ioctl r/o */
+	loFileName       [LoNameSize]uint8
+	loCryptName      [LoNameSize]uint8
+	loEncryptKey     [LoKeySize]uint8 /* ioctl w/o */
+	loInit           [2]uint64
+}
+
+// IOCTL consts
+const (
+	LoopSetFd       = C.LOOP_SET_FD
+	LoopCtlGetFree  = C.LOOP_CTL_GET_FREE
+	LoopGetStatus64 = C.LOOP_GET_STATUS64
+	LoopSetStatus64 = C.LOOP_SET_STATUS64
+	LoopClrFd       = C.LOOP_CLR_FD
+	LoopSetCapacity = C.LOOP_SET_CAPACITY
+)
+
+// LOOP consts.
+const (
+	LoFlagsAutoClear = C.LO_FLAGS_AUTOCLEAR
+	LoFlagsReadOnly  = C.LO_FLAGS_READ_ONLY
+	LoFlagsPartScan  = C.LO_FLAGS_PARTSCAN
+	LoKeySize        = C.LO_KEY_SIZE
+	LoNameSize       = C.LO_NAME_SIZE
+)

loopback/loopback.go

@@ -0,0 +1,63 @@
+// +build linux
+
+package loopback
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+
+	"github.com/Sirupsen/logrus"
+)
+
+func getLoopbackBackingFile(file *os.File) (uint64, uint64, error) {
+	loopInfo, err := ioctlLoopGetStatus64(file.Fd())
+	if err != nil {
+		logrus.Errorf("Error get loopback backing file: %s", err)
+		return 0, 0, ErrGetLoopbackBackingFile
+	}
+	return loopInfo.loDevice, loopInfo.loInode, nil
+}
+
+// SetCapacity reloads the size for the loopback device.
+func SetCapacity(file *os.File) error {
+	if err := ioctlLoopSetCapacity(file.Fd(), 0); err != nil {
+		logrus.Errorf("Error loopbackSetCapacity: %s", err)
+		return ErrSetCapacity
+	}
+	return nil
+}
+
+// FindLoopDeviceFor returns a loopback device file for the specified file which
+// is backing file of a loop back device.
+func FindLoopDeviceFor(file *os.File) *os.File {
+	stat, err := file.Stat()
+	if err != nil {
+		return nil
+	}
+	targetInode := stat.Sys().(*syscall.Stat_t).Ino
+	targetDevice := stat.Sys().(*syscall.Stat_t).Dev
+
+	for i := 0; true; i++ {
+		path := fmt.Sprintf("/dev/loop%d", i)
+
+		file, err := os.OpenFile(path, os.O_RDWR, 0)
+		if err != nil {
+			if os.IsNotExist(err) {
+				return nil
+			}
+
+			// Ignore all errors until the first not-exist
+			// we want to continue looking for the file
+			continue
+		}
+
+		dev, inode, err := getLoopbackBackingFile(file)
+		if err == nil && dev == targetDevice && inode == targetInode {
+			return file
+		}
+		file.Close()
+	}
+
+	return nil
+}