format ESP using mformat

Dockerfile

@@ -1,7 +1,7 @@
 from novit.tech/direktil/dkl:v1.2.0 as dkl
 
 # ------------------------------------------------------------------------
-from golang:1.26.1-trixie as build
+from golang:1.26.4-trixie as build
 
 run apt-get update && apt-get install -y git
 
@@ -27,11 +27,11 @@ from debian:trixie
 entrypoint ["/bin/dkl-local-server"]
 
 env _uncache=1
-run apt update \
+run --mount=type=cache,id=debian-trixie-apt,target=/var/cache/apt \
+ apt update \
  && yes |apt install -y genisoimage gdisk dosfstools util-linux udev binutils systemd \
  grub2 grub-pc-bin grub-efi-amd64-bin ca-certificates curl openssh-client qemu-utils wireguard-tools \
- erofs-utils erofsfuse cryptsetup systemd-boot-efi \
- && apt clean
+ erofs-utils erofsfuse cryptsetup systemd-boot-efi mtools
 
 copy --from=dkl   /bin/dkl /bin/dls /bin/
 copy --from=build /src/dist/ /bin/

cmd/dkl-local-server/boot-img.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"archive/tar"
+	"bytes"
 	"compress/gzip"
 	"flag"
 	"fmt"
@@ -93,47 +94,11 @@ func qemuImgBootImg(format string, uki bool, cmdline string) func(out io.Writer,
 	}
 }
 
-var grubSupportVersion = flag.String("grub-support", "1.1.0", "GRUB support version")
-
 func setupBootImage(bootImg *os.File, ctx *renderContext, uki bool, cmdline string) (err error) {
 	if uki {
-		err = bootImg.Truncate(192 << 20)
-		if err != nil {
-			return
-		}
-		err = bootImg.Sync()
-		if err != nil {
-			return
-		}
-
-		err = run("sgdisk", bootImg.Name(), "--clear", "--largest-new=1", "--typecode=1:EF00", "--change-name=1:ESP")
-		if err != nil {
-			return
-		}
-
+		err = baseDiskUki(bootImg, 192)
 	} else {
-		err = func() (err error) {
-			path, err := distFetch("grub-support", *grubSupportVersion)
-			if err != nil {
-				return
-			}
-
-			baseImage, err := os.Open(path)
-			if err != nil {
-				return
-			}
-
-			defer baseImage.Close()
-
-			baseImageGz, err := gzip.NewReader(baseImage)
-			if err != nil {
-				return
-			}
-
-			defer baseImageGz.Close()
-			_, err = io.Copy(bootImg, baseImageGz)
-			return
-		}()
+		err = baseDiskGrub(bootImg)
 	}
 	if err != nil {
 		return err
@@ -170,13 +135,6 @@ func setupBootImage(bootImg *os.File, ctx *renderContext, uki bool, cmdline stri
 
 	devp1 := dev + "p1"
 
-	if uki {
-		err = run("mkfs.vfat", "-F", "32", devp1)
-		if err != nil {
-			return fmt.Errorf("mkfs.vfat: %w", err)
-		}
-	}
-
 	err = syscall.Mount(devp1, tempDir, "vfat", 0, "")
 	if err != nil {
 		return fmt.Errorf("failed to mount %s to %s: %v", devp1, tempDir, err)
@@ -231,6 +189,74 @@ func setupBootImage(bootImg *os.File, ctx *renderContext, uki bool, cmdline stri
 	return
 }
 
+func baseDiskUki(image *os.File, sizeMB int16) (err error) {
+	err = image.Truncate(0)
+	if err != nil {
+		return
+	}
+
+	size := int64(sizeMB) << 20
+
+	err = image.Truncate(size)
+	if err != nil {
+		return
+	}
+	err = image.Sync()
+	if err != nil {
+		return
+	}
+
+	err = run("sgdisk", "--zap", "--new=1:2048:", "--typecode=1:EF00", "--change-name=1:ESP", image.Name())
+	if err != nil {
+		return
+	}
+
+	const (
+		espOffset        = 2048 * 512
+		gptBackupSectors = 34 // real world data
+	)
+	espSectors := (size-espOffset)/512 - gptBackupSectors
+
+	cfg := fmt.Sprintf("drive d: file=%q offset=%d\n", image.Name(), espOffset)
+	err = mtool(cfg, "mformat", "-F", "-T", fmt.Sprint(espSectors), "d:")
+
+	return
+}
+
+var grubSupportVersion = flag.String("grub-support", "1.1.1", "GRUB support version")
+
+func baseDiskGrub(image *os.File) (err error) {
+	path, err := distFetch("grub-support", *grubSupportVersion)
+	if err != nil {
+		return
+	}
+
+	baseImage, err := os.Open(path)
+	if err != nil {
+		return
+	}
+
+	defer baseImage.Close()
+
+	baseImageGz, err := gzip.NewReader(baseImage)
+	if err != nil {
+		return
+	}
+
+	defer baseImageGz.Close()
+	_, err = io.Copy(image, baseImageGz)
+	return
+}
+
+func mtool(config string, program string, args ...string) (err error) {
+	cmd := exec.Command(program, args...)
+	cmd.Env = append(os.Environ(), "MTOOLSRC=/dev/stdin")
+	cmd.Stdin = bytes.NewBufferString(config)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	return cmd.Run()
+}
+
 func run(program string, args ...string) (err error) {
 	cmd := exec.Command(program, args...)
 	cmd.Stdout = os.Stdout