loopback: use fstat on the open file descriptor

move the stat call later after the file is already opened so it is
less vulnerable to the file being removed between the stat and the
open syscall.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/loopback/attach_loopback.go

@@ -6,6 +6,7 @@ package loopback
 import (
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"syscall"
 
@@ -53,26 +54,28 @@ func openNextAvailableLoopback(index int, sparseName string, sparseFile *os.File
 		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, 0o644)
 		if err != nil {
+			if errors.Is(err, fs.ErrNotExist) {
+				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()