Allow specifying the kernel and tarball names, or omitting tarball

fix #113

Use `tar: none` or `tar: ""` to omit the tarball.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

docs/yaml.md

@@ -44,6 +44,9 @@ which should contain a `kernel` file that will be booted (eg a `bzImage` for `am
 called `kernel.tar` which is a tarball that is unpacked into the root, which should usually
 contain a kernel modules directory. `cmdline` specifies the kernel command line options if required.
 
+To override the names, you can specify the kernel image name with `binary: bzImage` and the tar image
+with `tar: kernel.tar` or the empty string or `none` if you do not want to use a tarball at all.
+
 ## `init`
 
 The `init` section is a list of images that are used for the `init` system and are unpacked directly

src/moby/build.go

@@ -140,7 +140,7 @@ func Build(m Moby, w io.Writer, pull bool, tp string) error {
 	if m.Kernel.Image != "" {
 		// get kernel and initrd tarball from container
 		log.Infof("Extract kernel image: %s", m.Kernel.Image)
-		kf := newKernelFilter(iw, m.Kernel.Cmdline)
+		kf := newKernelFilter(iw, m.Kernel.Cmdline, m.Kernel.Binary, m.Kernel.Tar)
 		err := ImageTar(m.Kernel.Image, "", kf, enforceContentTrust(m.Kernel.Image, &m.Trust), pull)
 		if err != nil {
 			return fmt.Errorf("Failed to extract kernel image and tarball: %v", err)
@@ -225,13 +225,25 @@ type kernelFilter struct {
 	tw          *tar.Writer
 	buffer      *bytes.Buffer
 	cmdline     string
+	kernel      string
+	tar         string
 	discard     bool
 	foundKernel bool
 	foundKTar   bool
 }
 
-func newKernelFilter(tw *tar.Writer, cmdline string) *kernelFilter {
+func newKernelFilter(tw *tar.Writer, cmdline string, kernel string, tar *string) *kernelFilter {
-	return &kernelFilter{tw: tw, cmdline: cmdline}
+	tarName, kernelName := "kernel.tar", "kernel"
+	if tar != nil {
+		tarName = *tar
+		if tarName == "none" {
+			tarName = ""
+		}
+	}
+	if kernel != "" {
+		kernelName = kernel
+	}
+	return &kernelFilter{tw: tw, cmdline: cmdline, kernel: kernelName, tar: tarName}
 }
 
 func (k *kernelFilter) finishTar() error {
@@ -248,8 +260,8 @@ func (k *kernelFilter) Close() error {
 	if !k.foundKernel {
 		return errors.New("did not find kernel in kernel image")
 	}
-	if !k.foundKTar {
+	if !k.foundKTar && k.tar != "" {
-		return errors.New("did not find kernel.tar in kernel image")
+		return errors.New("did not find kernel tar in kernel image")
 	}
 	return k.finishTar()
 }
@@ -279,7 +291,7 @@ func (k *kernelFilter) WriteHeader(hdr *tar.Header) error {
 	}
 	tw := k.tw
 	switch hdr.Name {
-	case "kernel":
+	case k.kernel:
 		if k.foundKernel {
 			return errors.New("found more than one possible kernel image")
 		}
@@ -315,7 +327,7 @@ func (k *kernelFilter) WriteHeader(hdr *tar.Header) error {
 		if err := tw.WriteHeader(whdr); err != nil {
 			return err
 		}
-	case "kernel.tar":
+	case k.tar:
 		k.foundKTar = true
 		k.discard = false
 		k.buffer = new(bytes.Buffer)

src/moby/config.go

@@ -19,10 +19,7 @@ import (
 
 // Moby is the type of a Moby config file
 type Moby struct {
-	Kernel struct {
+	Kernel   KernelConfig
-		Image   string
-		Cmdline string
-	}
 	Init     []string
 	Onboot   []Image
 	Services []Image
@@ -30,6 +27,14 @@ type Moby struct {
 	Files    []File
 }
 
+// KernelConfig is the type of the config for a kernel
+type KernelConfig struct {
+	Image   string
+	Cmdline string
+	Binary  string
+	Tar     *string
+}
+
 // TrustConfig is the type of a content trust config
 type TrustConfig struct {
 	Image []string
@@ -168,6 +173,12 @@ func AppendConfig(m0, m1 Moby) (Moby, error) {
 	if m1.Kernel.Cmdline != "" {
 		moby.Kernel.Cmdline = m1.Kernel.Cmdline
 	}
+	if m1.Kernel.Binary != "" {
+		moby.Kernel.Binary = m1.Kernel.Binary
+	}
+	if m1.Kernel.Tar != nil {
+		moby.Kernel.Tar = m1.Kernel.Tar
+	}
 	moby.Init = append(moby.Init, m1.Init...)
 	moby.Onboot = append(moby.Onboot, m1.Onboot...)
 	moby.Services = append(moby.Services, m1.Services...)

src/moby/schema.go

@@ -10,8 +10,10 @@ var schema = string(`
       "type": "object",
       "additionalProperties": false,
       "properties": {
-        "image": { "type": "string"},
+        "image": {"type": "string"},
-        "cmdline": { "type": "string"}
+        "cmdline": {"type": "string"},
+        "binary": {"type": "string"},
+        "tar": {"type": "string"}
       }
     },
     "file": {