Minimize shelling out (#439)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Dimitris Karakasilis <dimitris@karakasilis.me>

pkg/utils/loop/loopback.go

@@ -0,0 +1,112 @@
+package loop
+
+import (
+	"fmt"
+	"github.com/kairos-io/kairos-agent/v2/pkg/config"
+	"os"
+	"syscall"
+	"unsafe"
+
+	v1 "github.com/kairos-io/kairos-agent/v2/pkg/types/v1"
+	"golang.org/x/sys/unix"
+)
+
+// syscalls will return an errno type (which implements error) for all calls,
+// including success (errno 0) so we need to check its value to know if its an actual error or not
+func errnoIsErr(err error) error {
+	if err != nil && err.(syscall.Errno) != 0 {
+		return err
+	}
+
+	return nil
+}
+
+// Loop will setup a /dev/loopX device linked to the image file by using syscalls directly to set it
+func Loop(img *v1.Image, cfg *config.Config) (loopDevice string, err error) {
+	log := cfg.Logger
+	log.Debugf("Opening loop control device")
+	fd, err := cfg.Fs.OpenFile("/dev/loop-control", os.O_RDONLY, 0o644)
+	if err != nil {
+		log.Error("failed to open /dev/loop-control")
+		return loopDevice, err
+	}
+
+	defer fd.Close()
+	log.Debugf("Getting free loop device")
+	loopInt, _, err := cfg.Syscall.Syscall(syscall.SYS_IOCTL, fd.Fd(), unix.LOOP_CTL_GET_FREE, 0)
+	if errnoIsErr(err) != nil {
+		log.Error("failed to get loop device")
+		return loopDevice, err
+	}
+
+	loopDevice = fmt.Sprintf("/dev/loop%d", loopInt)
+	log.Logger.Debug().Str("device", loopDevice).Msg("Opening loop device")
+	loopFile, err := cfg.Fs.OpenFile(loopDevice, os.O_RDWR, 0)
+	if err != nil {
+		log.Error("failed to open loop device")
+		return loopDevice, err
+	}
+	log.Logger.Debug().Str("image", img.File).Msg("Opening img file")
+	imageFile, err := cfg.Fs.OpenFile(img.File, os.O_RDWR, os.ModePerm)
+	if err != nil {
+		log.Error("failed to open image file")
+		return loopDevice, err
+	}
+	defer loopFile.Close()
+	defer imageFile.Close()
+
+	log.Debugf("Setting loop device")
+	_, _, err = cfg.Syscall.Syscall(
+		syscall.SYS_IOCTL,
+		loopFile.Fd(),
+		unix.LOOP_SET_FD,
+		imageFile.Fd(),
+	)
+	if errnoIsErr(err) != nil {
+		log.Error("failed to set loop device")
+		return loopDevice, err
+	}
+
+	// Force kernel to scan partition table on loop device
+	status := &unix.LoopInfo64{
+		Flags: unix.LO_FLAGS_PARTSCAN,
+	}
+	// Dont set read only flag
+	status.Flags &= ^uint32(unix.LO_FLAGS_READ_ONLY)
+
+	log.Debugf("Setting loop flags")
+	_, _, err = cfg.Syscall.Syscall(
+		syscall.SYS_IOCTL,
+		loopFile.Fd(),
+		unix.LOOP_SET_STATUS64,
+		uintptr(unsafe.Pointer(status)),
+	)
+
+	if errnoIsErr(err) != nil {
+		log.Error("failed to set loop device status")
+		return loopDevice, err
+	}
+
+	return loopDevice, nil
+}
+
+// Unloop will clear a loop device and free the underlying image linked to it
+func Unloop(loopDevice string, cfg *config.Config) error {
+	log := cfg.Logger
+	log.Logger.Debug().Str("device", loopDevice).Msg("Opening loop device")
+	fd, err := cfg.Fs.OpenFile(loopDevice, os.O_RDONLY, 0o644)
+	if err != nil {
+		log.Error("failed to set open loop device")
+		return err
+	}
+	defer fd.Close()
+	log.Debugf("Clearing loop device")
+	_, _, err = cfg.Syscall.Syscall(syscall.SYS_IOCTL, fd.Fd(), unix.LOOP_CLR_FD, 0)
+
+	if errnoIsErr(err) != nil {
+		log.Error("failed to set loop device status")
+		return err
+	}
+
+	return nil
+}