cmd: Add scaffolding to decompress the kernel

Add the '-vmlinux' flag to build and pass it all
the way to the kernel filter.

Note, this commit only adds the flag but does not
yet perform the decompression. This will be added
with the next commit.

Signed-off-by: Rolf Neugebauer <rn@rneugeba.io>

src/cmd/linuxkit/build.go

@@ -49,6 +49,7 @@ func build(args []string) {
 	buildSize := buildCmd.String("size", "1024M", "Size for output image, if supported and fixed size")
 	buildPull := buildCmd.Bool("pull", false, "Always pull images")
 	buildDisableTrust := buildCmd.Bool("disable-content-trust", false, "Skip image trust verification specified in trust section of config (default false)")
+	buildDecompressKernel := buildCmd.Bool("decompress-kernel", false, "Decompress the Linux kernel (default false)")
 	buildCmd.Var(&buildFormats, "format", "Formats to create [ "+strings.Join(outputTypes, " ")+" ]")
 
 	if err := buildCmd.Parse(args); err != nil {
@@ -203,7 +204,7 @@ func build(args []string) {
 	if moby.Streamable(buildFormats[0]) {
 		tp = buildFormats[0]
 	}
-	err = moby.Build(m, w, *buildPull, tp)
+	err = moby.Build(m, w, *buildPull, tp, *buildDecompressKernel)
 	if err != nil {
 		log.Fatalf("%v", err)
 	}

src/cmd/linuxkit/moby/build.go

@@ -142,7 +142,7 @@ func outputImage(image *Image, section string, prefix string, m Moby, idMap map[
 }
 
 // Build performs the actual build process
-func Build(m Moby, w io.Writer, pull bool, tp string) error {
+func Build(m Moby, w io.Writer, pull bool, tp string, decompressKernel bool) error {
 	if MobyDir == "" {
 		MobyDir = defaultMobyConfigDir()
 	}
@@ -179,7 +179,7 @@ func Build(m Moby, w io.Writer, pull bool, tp string) error {
 	if m.Kernel.ref != nil {
 		// get kernel and initrd tarball and ucode cpio archive from container
 		log.Infof("Extract kernel image: %s", m.Kernel.ref)
-		kf := newKernelFilter(iw, m.Kernel.Cmdline, m.Kernel.Binary, m.Kernel.Tar, m.Kernel.UCode)
+		kf := newKernelFilter(iw, m.Kernel.Cmdline, m.Kernel.Binary, m.Kernel.Tar, m.Kernel.UCode, decompressKernel)
 		err := ImageTar(m.Kernel.ref, "", kf, enforceContentTrust(m.Kernel.ref.String(), &m.Trust), pull, "")
 		if err != nil {
 			return fmt.Errorf("Failed to extract kernel image and tarball: %v", err)
@@ -255,20 +255,21 @@ func Build(m Moby, w io.Writer, pull bool, tp string) error {
 
 // kernelFilter is a tar.Writer that transforms a kernel image into the output we want on underlying tar writer
 type kernelFilter struct {
-	tw          *tar.Writer
-	buffer      *bytes.Buffer
-	hdr         *tar.Header
-	cmdline     string
-	kernel      string
-	tar         string
-	ucode       string
-	discard     bool
-	foundKernel bool
-	foundKTar   bool
-	foundUCode  bool
+	tw               *tar.Writer
+	buffer           *bytes.Buffer
+	hdr              *tar.Header
+	cmdline          string
+	kernel           string
+	tar              string
+	ucode            string
+	decompressKernel bool
+	discard          bool
+	foundKernel      bool
+	foundKTar        bool
+	foundUCode       bool
 }
 
-func newKernelFilter(tw *tar.Writer, cmdline string, kernel string, tar, ucode *string) *kernelFilter {
+func newKernelFilter(tw *tar.Writer, cmdline string, kernel string, tar, ucode *string, decompressKernel bool) *kernelFilter {
 	tarName, kernelName, ucodeName := "kernel.tar", "kernel", ""
 	if tar != nil {
 		tarName = *tar
@@ -282,7 +283,7 @@ func newKernelFilter(tw *tar.Writer, cmdline string, kernel string, tar, ucode *
 	if ucode != nil {
 		ucodeName = *ucode
 	}
-	return &kernelFilter{tw: tw, cmdline: cmdline, kernel: kernelName, tar: tarName, ucode: ucodeName}
+	return &kernelFilter{tw: tw, cmdline: cmdline, kernel: kernelName, tar: tarName, ucode: ucodeName, decompressKernel: decompressKernel}
 }
 
 func (k *kernelFilter) finishTar() error {

src/cmd/linuxkit/moby/linuxkit.go

@@ -62,7 +62,7 @@ func ensureLinuxkitImage(name string) error {
 		return err
 	}
 	defer os.Remove(tf.Name())
-	Build(m, tf, false, "")
+	Build(m, tf, false, "", false)
 	if err := tf.Close(); err != nil {
 		return err
 	}