Get absolute path of file before checking whitelist

Issue 291 pointed out that symlink "../proc/self/mounts" in the fedora image wasn't being extracted properly and kaniko was erroring out.
This is because the file path wasn't absolute so kaniko wasn't recognizing it as a whitelisted path.
With this change, we first resolve a path to it's absolute path before checking the whitelist.

integration/dockerfiles/Dockerfile_test_multistage

@@ -8,6 +8,10 @@ COPY --from=0 $foopath context/b* /foo/
 FROM second
 COPY --from=base /context/foo /new/foo
 
+# This base image contains symlinks with relative paths to whitelisted directories
+# We need to test they're extracted correctly
+FROM fedora@sha256:c4cc32b09c6ae3f1353e7e33a8dda93dc41676b923d6d89afa996b421cc5aa48
+
 FROM base
 ARG file
 COPY --from=second /foo ${file}

pkg/snapshot/snapshot.go

@@ -95,7 +95,11 @@ func (s *Snapshotter) snapshotFiles(f io.Writer, files []string) (bool, error) {
 		if val, ok := snapshottedFiles[file]; ok && val {
 			continue
 		}
-		if util.CheckWhitelist(file) && !isBuildFile(file) {
+		whitelisted, err := util.CheckWhitelist(file)
+		if err != nil {
+			return false, err
+		}
+		if whitelisted && !isBuildFile(file) {
 			logrus.Infof("Not adding %s to layer, as it's whitelisted", file)
 			continue
 		}
@@ -168,7 +172,11 @@ func (s *Snapshotter) snapShotFS(f io.Writer) (bool, error) {
 
 	// Now create the tar.
 	for path, info := range memFs {
-		if util.CheckWhitelist(path) {
+		whitelisted, err := util.CheckWhitelist(path)
+		if err != nil {
+			return false, err
+		}
+		if whitelisted {
 			logrus.Debugf("Not adding %s to layer, as it's whitelisted", path)
 			continue
 		}

pkg/util/fs_util.go

@@ -90,13 +90,20 @@ func GetFSFromImage(root string, img v1.Image) error {
 				logrus.Infof("Not adding %s because it was added by a prior layer", path)
 				continue
 			}
-
-			if CheckWhitelist(path) && !checkWhitelistRoot(root) {
+			whitelisted, err := CheckWhitelist(path)
+			if err != nil {
+				return err
+			}
+			if whitelisted && !checkWhitelistRoot(root) {
 				logrus.Infof("Not adding %s because it is whitelisted", path)
 				continue
 			}
 			if hdr.Typeflag == tar.TypeSymlink {
-				if CheckWhitelist(hdr.Linkname) {
+				whitelisted, err := CheckWhitelist(hdr.Linkname)
+				if err != nil {
+					return err
+				}
+				if whitelisted {
 					logrus.Debugf("skipping symlink from %s to %s because %s is whitelisted", hdr.Linkname, path, hdr.Linkname)
 					continue
 				}
@@ -115,7 +122,11 @@ func GetFSFromImage(root string, img v1.Image) error {
 func DeleteFilesystem() error {
 	logrus.Info("Deleting filesystem...")
 	err := filepath.Walk(constants.RootDir, func(path string, info os.FileInfo, err error) error {
-		if CheckWhitelist(path) || ChildDirInWhitelist(path, constants.RootDir) {
+		whitelisted, err := CheckWhitelist(path)
+		if err != nil {
+			return err
+		}
+		if whitelisted || ChildDirInWhitelist(path, constants.RootDir) {
 			logrus.Debugf("Not deleting %s, as it's whitelisted", path)
 			return nil
 		}
@@ -247,13 +258,18 @@ func checkWhiteouts(path string, whiteouts map[string]struct{}) bool {
 	return false
 }
 
-func CheckWhitelist(path string) bool {
+func CheckWhitelist(path string) (bool, error) {
+	abs, err := filepath.Abs(path)
+	if err != nil {
+		logrus.Infof("unable to get absolute path for %s", path)
+		return false, err
+	}
 	for _, wl := range whitelist {
-		if HasFilepathPrefix(path, wl) {
-			return true
+		if HasFilepathPrefix(abs, wl) {
+			return true, nil
 		}
 	}
-	return false
+	return false, nil
 }
 
 func checkWhitelistRoot(root string) bool {
@@ -313,7 +329,11 @@ func RelativeFiles(fp string, root string) ([]string, error) {
 	fullPath := filepath.Join(root, fp)
 	logrus.Debugf("Getting files and contents at root %s", fullPath)
 	err := filepath.Walk(fullPath, func(path string, info os.FileInfo, err error) error {
-		if CheckWhitelist(path) && !HasFilepathPrefix(path, root) {
+		whitelisted, err := CheckWhitelist(path)
+		if err != nil {
+			return err
+		}
+		if whitelisted && !HasFilepathPrefix(path, root) {
 			return nil
 		}
 		if err != nil {
@@ -334,7 +354,11 @@ func Files(root string) ([]string, error) {
 	var files []string
 	logrus.Debugf("Getting files and contents at root %s", root)
 	err := filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
-		if CheckWhitelist(path) {
+		whitelisted, err := CheckWhitelist(path)
+		if err != nil {
+			return err
+		}
+		if whitelisted {
 			return nil
 		}
 		files = append(files, path)

pkg/util/fs_util_test.go

@@ -266,7 +266,11 @@ func Test_CheckWhitelist(t *testing.T) {
 				whitelist = original
 			}()
 			whitelist = tt.args.whitelist
-			if got := CheckWhitelist(tt.args.path); got != tt.want {
+			got, err := CheckWhitelist(tt.args.path)
+			if err != nil {
+				t.Fatalf("error checking whitelist: %v", err)
+			}
+			if got != tt.want {
 				t.Errorf("CheckWhitelist() = %v, want %v", got, tt.want)
 			}
 		})