more robust manifest <-> tarball handling

rootfs/rootfs.go

@@ -15,13 +15,13 @@ import (
 
 const (
 	_manifestJSON = "manifest.json"
-	_layerSuffix  = "/layer.tar"
+	_tarSuffix    = ".tar"
 	_whReaddir    = ".wh..wh..opq"
 	_whPrefix     = ".wh."
 )
 
 var (
-	errBadManifest = errors.New("bad or missing manifest.json")
+	errBadManifest = errors.New("bad manifest.json")
 )
 
 type (
@@ -63,7 +63,7 @@ func Flatten(rd io.ReadSeeker, w io.Writer) (err error) {
 			if err := dec.Decode(&manifest); err != nil {
 				return fmt.Errorf("decode %s: %w", _manifestJSON, err)
 			}
-		case strings.HasSuffix(hdr.Name, _layerSuffix):
+		case strings.HasSuffix(hdr.Name, _tarSuffix):
 			here, err := rd.Seek(0, io.SeekCurrent)
 			if err != nil {
 				return err
@@ -72,8 +72,8 @@ func Flatten(rd io.ReadSeeker, w io.Writer) (err error) {
 		}
 	}
 
-	if len(manifest) == 0 || len(layerOffsets) != len(manifest[0].Layers) {
-		return errBadManifest
+	if err := validateManifest(layerOffsets, manifest); err != nil {
+		return err
 	}
 
 	// enumerate layers the way they would be laid down in the image
@@ -228,6 +228,21 @@ func whiteoutDirs(whreaddir map[string]int, nlayers int) []*tree {
 	return ret
 }
 
+// validateManifest
+func validateManifest(layerOffsets map[string]int64, manifest dockerManifestJSON) error {
+	if len(manifest) == 0 {
+		return fmt.Errorf("empty or missing manifest")
+	}
+
+	for _, layer := range manifest[0].Layers {
+		if _, ok := layerOffsets[layer]; !ok {
+			return fmt.Errorf("%s defined in manifest, missing in tarball", layer)
+		}
+	}
+
+	return nil
+}
+
 // openTargz creates a tar reader from a targzip or tar.
 //
 // It will try to open a gzip stream, and, if that fails, silently fall back to

rootfs/rootfs_test.go

@@ -44,10 +44,15 @@ func TestRootFS(t *testing.T) {
 			image: tarball{manifest{}},
 			want:  []extractable{},
 		},
+		{
+			name:    "no manifest",
+			image:   tarball{},
+			wantErr: "empty or missing manifest",
+		},
 		{
 			name:    "missing layer",
 			image:   tarball{manifest{"layer0/layer.tar"}},
-			wantErr: "bad or missing manifest.json",
+			wantErr: "layer0/layer.tar defined in manifest, missing in tarball",
 		},
 		{
 			name: "basic file overwrite, layer order mixed",