fix: handle overlay xattr opaque bit

Current behavior determines if a path is a whiteout if a overlay char
dev is present.

Additionally, also check the extended attrs.

For files, this is not important: if a file /foo/bar exists in
some two layers, then the higher layer's version will replace
the lower layer's version whether or not the opaque bit is set.

However, directories behave differently.  If /foo exists in two
layers and no opaque bit is set, then we see the union of their
contents.  But if the opaque bit is set on the higher layer's
version, then we should not see the lower layer's version.

So, in that case, create a whiteout /.wh_foo.

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

oci/layer/generate.go

@@ -88,7 +88,7 @@ func GenerateLayer(path string, deltas []mtree.InodeDelta, opt *RepackOptions) (
 						return errors.Wrapf(err, "couldn't determine overlay whiteout for %s", fullPath)
 					}
 
-					whiteout, err := isOverlayWhiteout(fi)
+					whiteout, err := isOverlayWhiteout(fi, fullPath, tg.fsEval)
 					if err != nil {
 						return err
 					}
@@ -166,7 +166,7 @@ func GenerateInsertLayer(root string, target string, opaque bool, opt *RepackOpt
 			}
 
 			pathInTar := path.Join(target, curPath[len(root):])
-			whiteout, err := isOverlayWhiteout(info)
+			whiteout, err := isOverlayWhiteout(info, curPath, tg.fsEval)
 			if err != nil {
 				return err
 			}

oci/layer/tar_extract_linux_test.go

@@ -96,7 +96,7 @@ func TestUnpackEntryOverlayFSWhiteout(t *testing.T) {
 		t.Fatalf("failed to stat `file`: %v", err)
 	}
 
-	whiteout, err := isOverlayWhiteout(fi)
+	whiteout, err := isOverlayWhiteout(fi, filepath.Join(dir, "file"), te.fsEval)
 	if err != nil {
 		t.Fatalf("failed to check overlay whiteout: %v", err)
 	}

oci/layer/utils.go

@@ -25,6 +25,7 @@ import (
 
 	"github.com/apex/log"
 	rspec "github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/opencontainers/umoci/pkg/fseval"
 	"github.com/opencontainers/umoci/pkg/idtools"
 	"github.com/pkg/errors"
 	rootlesscontainers "github.com/rootless-containers/proto/go-proto"
@@ -230,7 +231,7 @@ func InnerErrno(err error) error {
 
 // isOverlayWhiteout returns true if the FileInfo represents an overlayfs style
 // whiteout (i.e. mknod c 0 0) and false otherwise.
-func isOverlayWhiteout(info os.FileInfo) (bool, error) {
+func isOverlayWhiteout(info os.FileInfo, fullPath string, fsEval fseval.FsEval) (bool, error) {
 	var major, minor uint32
 	switch stat := info.Sys().(type) {
 	case *unix.Stat_t:
@@ -243,6 +244,25 @@ func isOverlayWhiteout(info os.FileInfo) (bool, error) {
 		return false, errors.Errorf("[internal error] unknown stat info type %T", info.Sys())
 	}
 
-	return major == 0 && minor == 0 &&
-		info.Mode()&os.ModeCharDevice != 0, nil
+	if major == 0 && minor == 0 &&
+		info.Mode()&os.ModeCharDevice != 0 {
+		return true, nil
+	}
+
+	// also evaluate xattrs which may have opaque value set
+	attr, err := fsEval.Lgetxattr(fullPath, "user.overlay.opaque")
+	if err != nil {
+		v := errors.Cause(err)
+		if !errors.Is(err, os.ErrNotExist) && v != unix.EOPNOTSUPP && v != unix.ENODATA && v != unix.EPERM && v != unix.EACCES {
+			return false, errors.Errorf("[internal error] unknown stat info type %T", info.Sys())
+		}
+
+		return false, nil
+	}
+
+	if string(attr) == "y" {
+		return true, nil
+	}
+
+	return false, nil
 }