build: Fix the ModTime for files created during build

When creating files for the "intermediate" tar ball,
fix the ModTime. This reduces the difference between
LinuxKit images build from identical inputs.

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

src/cmd/linuxkit/moby/build.go

@@ -54,6 +54,7 @@ var additions = map[string]addFun{
 			Name:    "Dockerfile",
 			Mode:    0644,
 			Size:    int64(len(dockerfile)),
+			ModTime: defaultModTime,
 			Format:  tar.FormatPAX,
 		}
 		if err := tw.WriteHeader(hdr); err != nil {
@@ -368,6 +369,7 @@ func (k *kernelFilter) WriteHeader(hdr *tar.Header) error {
 				Name:     "boot",
 				Mode:     0755,
 				Typeflag: tar.TypeDir,
+				ModTime:  defaultModTime,
 				Format:   tar.FormatPAX,
 			}
 			if err := tw.WriteHeader(whdr); err != nil {
@@ -379,6 +381,7 @@ func (k *kernelFilter) WriteHeader(hdr *tar.Header) error {
 			Name:    "boot/cmdline",
 			Mode:    0644,
 			Size:    int64(len(k.cmdline)),
+			ModTime: defaultModTime,
 			Format:  tar.FormatPAX,
 		}
 		if err := tw.WriteHeader(whdr); err != nil {
@@ -394,6 +397,7 @@ func (k *kernelFilter) WriteHeader(hdr *tar.Header) error {
 			Name:    "boot/kernel",
 			Mode:    hdr.Mode,
 			Size:    hdr.Size,
+			ModTime: defaultModTime,
 			Format:  tar.FormatPAX,
 		}
 		k.buffer = new(bytes.Buffer)
@@ -410,6 +414,7 @@ func (k *kernelFilter) WriteHeader(hdr *tar.Header) error {
 				Name:     "boot",
 				Mode:     0755,
 				Typeflag: tar.TypeDir,
+				ModTime:  defaultModTime,
 				Format:   tar.FormatPAX,
 			}
 			if err := tw.WriteHeader(whdr); err != nil {
@@ -420,6 +425,7 @@ func (k *kernelFilter) WriteHeader(hdr *tar.Header) error {
 			Name:    "boot/ucode.cpio",
 			Mode:    hdr.Mode,
 			Size:    hdr.Size,
+			ModTime: defaultModTime,
 			Format:  tar.FormatPAX,
 		}
 		if err := tw.WriteHeader(whdr); err != nil {
@@ -653,6 +659,7 @@ func filesystem(m Moby, tw *tar.Writer, idMap map[string]uint32) error {
 					Name:     root,
 					Typeflag: tar.TypeDir,
 					Mode:     dirMode,
+					ModTime:  defaultModTime,
 					Uid:      int(uid),
 					Gid:      int(gid),
 					Format:   tar.FormatPAX,
@@ -668,6 +675,7 @@ func filesystem(m Moby, tw *tar.Writer, idMap map[string]uint32) error {
 		hdr := &tar.Header{
 			Name:    f.Path,
 			Mode:    mode,
+			ModTime: defaultModTime,
 			Uid:     int(uid),
 			Gid:     int(gid),
 			Format:  tar.FormatPAX,

src/cmd/linuxkit/moby/image.go

@@ -71,6 +71,7 @@ func tarPrefix(path string, tw tarWriter) error {
 		hdr := &tar.Header{
 			Name:     mkdir,
 			Mode:     0755,
+			ModTime:  defaultModTime,
 			Typeflag: tar.TypeDir,
 			Format:   tar.FormatPAX,
 		}
@@ -154,6 +155,7 @@ func ImageTar(ref *reference.Spec, prefix string, tw tarWriter, trust bool, pull
 				contents := replace[hdr.Name]
 				hdr.Size = int64(len(contents))
 				hdr.Name = prefix + hdr.Name
+				hdr.ModTime = defaultModTime
 				log.Debugf("image tar: %s %s add %s", ref, prefix, hdr.Name)
 				if err := tw.WriteHeader(hdr); err != nil {
 					return err
@@ -169,6 +171,7 @@ func ImageTar(ref *reference.Spec, prefix string, tw tarWriter, trust bool, pull
 				hdr.Size = 0
 				hdr.Typeflag = tar.TypeSymlink
 				hdr.Linkname = resolv
+				hdr.ModTime = defaultModTime
 				log.Debugf("image tar: %s %s add resolv symlink /etc/resolv.conf -> %s", ref, prefix, resolv)
 				if err := tw.WriteHeader(hdr); err != nil {
 					return err
@@ -224,6 +227,7 @@ func ImageBundle(prefix string, ref *reference.Spec, config []byte, runtime Runt
 		Name:    path.Join(prefix, "config.json"),
 		Mode:    0644,
 		Size:    int64(len(config)),
+		ModTime: defaultModTime,
 		Format:  tar.FormatPAX,
 	}
 	if err := tw.WriteHeader(hdr); err != nil {
@@ -242,6 +246,7 @@ func ImageBundle(prefix string, ref *reference.Spec, config []byte, runtime Runt
 			Name:     tmp,
 			Mode:     0755,
 			Typeflag: tar.TypeDir,
+			ModTime:  defaultModTime,
 			Format:   tar.FormatPAX,
 		}
 		if err := tw.WriteHeader(hdr); err != nil {
@@ -252,6 +257,7 @@ func ImageBundle(prefix string, ref *reference.Spec, config []byte, runtime Runt
 			Name:     path.Join(prefix, "rootfs"),
 			Mode:     0755,
 			Typeflag: tar.TypeDir,
+			ModTime:  defaultModTime,
 			Format:   tar.FormatPAX,
 		}
 		if err := tw.WriteHeader(hdr); err != nil {
@@ -271,6 +277,7 @@ func ImageBundle(prefix string, ref *reference.Spec, config []byte, runtime Runt
 				Name:     path.Join(prefix, "rootfs"),
 				Mode:     0755,
 				Typeflag: tar.TypeDir,
+				ModTime:  defaultModTime,
 				Format:   tar.FormatPAX,
 			}
 			if err := tw.WriteHeader(hdr); err != nil {
@@ -297,6 +304,7 @@ func ImageBundle(prefix string, ref *reference.Spec, config []byte, runtime Runt
 		Name:    path.Join(prefix, "runtime.json"),
 		Mode:    0644,
 		Size:    int64(len(runtimeConfig)),
+		ModTime: defaultModTime,
 		Format:  tar.FormatPAX,
 	}
 	if err := tw.WriteHeader(hdr); err != nil {

src/cmd/linuxkit/moby/output.go

@@ -284,6 +284,7 @@ func tarInitrdKernel(kernel, initrd []byte, cmdline string) (*bytes.Buffer, erro
 		Name:    "kernel",
 		Mode:    0600,
 		Size:    int64(len(kernel)),
+		ModTime: defaultModTime,
 		Format:  tar.FormatPAX,
 	}
 	err := tw.WriteHeader(hdr)
@@ -298,6 +299,7 @@ func tarInitrdKernel(kernel, initrd []byte, cmdline string) (*bytes.Buffer, erro
 		Name:    "initrd.img",
 		Mode:    0600,
 		Size:    int64(len(initrd)),
+		ModTime: defaultModTime,
 		Format:  tar.FormatPAX,
 	}
 	err = tw.WriteHeader(hdr)
@@ -312,6 +314,7 @@ func tarInitrdKernel(kernel, initrd []byte, cmdline string) (*bytes.Buffer, erro
 		Name:    "cmdline",
 		Mode:    0600,
 		Size:    int64(len(cmdline)),
+		ModTime: defaultModTime,
 		Format:  tar.FormatPAX,
 	}
 	err = tw.WriteHeader(hdr)
@@ -413,6 +416,7 @@ func outputKernelInitrdTarball(base string, kernel []byte, initrd []byte, cmdlin
 			Name:    "kernel",
 			Mode:    0644,
 			Size:    int64(len(kernel)),
+			ModTime: defaultModTime,
 			Format:  tar.FormatPAX,
 		}
 		if err := tw.WriteHeader(hdr); err != nil {
@@ -427,6 +431,7 @@ func outputKernelInitrdTarball(base string, kernel []byte, initrd []byte, cmdlin
 			Name:    "initrd.img",
 			Mode:    0644,
 			Size:    int64(len(initrd)),
+			ModTime: defaultModTime,
 			Format:  tar.FormatPAX,
 		}
 		if err := tw.WriteHeader(hdr); err != nil {
@@ -441,6 +446,7 @@ func outputKernelInitrdTarball(base string, kernel []byte, initrd []byte, cmdlin
 			Name:    "cmdline",
 			Mode:    0644,
 			Size:    int64(len(cmdline)),
+			ModTime: defaultModTime,
 			Format:  tar.FormatPAX,
 		}
 		if err := tw.WriteHeader(hdr); err != nil {
@@ -455,6 +461,7 @@ func outputKernelInitrdTarball(base string, kernel []byte, initrd []byte, cmdlin
 			Name:    "ucode.cpio",
 			Mode:    0644,
 			Size:    int64(len(ucode)),
+			ModTime: defaultModTime,
 			Format:  tar.FormatPAX,
 		}
 		if err := tw.WriteHeader(hdr); err != nil {

src/cmd/linuxkit/moby/util.go

@@ -2,11 +2,14 @@ package moby
 
 import (
 	"path/filepath"
+	"time"
 )
 
 var (
 	// MobyDir is the location of the cache directory, defaults to ~/.moby
 	MobyDir string
+	// Default ModTime for files created during build. Roughly the time LinuxKit got open sourced.
+	defaultModTime = time.Date(2017, time.April, 18, 16, 30, 0, 0, time.UTC)
 )
 
 func defaultMobyConfigDir() string {