Confine published diagnostic files

internal/stage/publish.go

@@ -978,7 +978,7 @@ func collectPublishPreviousFiles(ctx context.Context, previousDir string) ([]pub
 func openPublishRunFiles(runRoot string, files []publishUploadFile) ([]*os.File, error) {
 	sources := make([]*os.File, 0, len(files))
 	for _, candidate := range files {
-		file, err := openPublishOutputSource(filepath.Join(runRoot, filepath.FromSlash(candidate.SourceRelativePath)))
+		file, err := openPublishSourceUnderRoot(runRoot, candidate.SourceRelativePath)
 		if err != nil {
 			closePublishSources(sources)
 			return nil, err
@@ -993,6 +993,56 @@ func openPublishRunFiles(runRoot string, files []publishUploadFile) ([]*os.File,
 	return sources, nil
 }
 
+func openPublishSourceUnderRoot(rootPath, relativePath string) (*os.File, error) {
+	relativePath, err := pathsafe.NormalizeRelativeDestination(relativePath)
+	if err != nil {
+		return nil, fmt.Errorf("validate publish source path %q: %w", relativePath, err)
+	}
+
+	root, err := os.OpenRoot(rootPath)
+	if err != nil {
+		return nil, fmt.Errorf("open publish source root %q: %w", rootPath, err)
+	}
+	openedRoots := []*os.Root{root}
+	defer func() {
+		for index := len(openedRoots) - 1; index >= 0; index-- {
+			_ = openedRoots[index].Close()
+		}
+	}()
+
+	parts := strings.Split(relativePath, "/")
+	current := root
+	for _, part := range parts[:len(parts)-1] {
+		info, err := current.Lstat(part)
+		if err != nil {
+			return nil, fmt.Errorf("inspect publish source ancestor %q: %w", part, err)
+		}
+		if info.Mode()&os.ModeSymlink != 0 || !info.IsDir() {
+			return nil, fmt.Errorf("publish source ancestor %q is not a directory without symlinks", part)
+		}
+		next, err := current.OpenRoot(part)
+		if err != nil {
+			return nil, fmt.Errorf("open publish source ancestor %q: %w", part, err)
+		}
+		openedRoots = append(openedRoots, next)
+		current = next
+	}
+
+	leaf := parts[len(parts)-1]
+	info, err := current.Lstat(leaf)
+	if err != nil {
+		return nil, fmt.Errorf("inspect publish source %q: %w", relativePath, err)
+	}
+	if info.Mode()&os.ModeSymlink != 0 || !info.Mode().IsRegular() {
+		return nil, fmt.Errorf("publish source %q is not a regular file without symlinks", relativePath)
+	}
+	file, err := current.Open(leaf)
+	if err != nil {
+		return nil, fmt.Errorf("open publish source %q: %w", relativePath, err)
+	}
+	return file, nil
+}
+
 func verifyPublishSourceChecksum(file *os.File, relativePath, expected string) error {
 	if strings.TrimSpace(expected) == "" {
 		return nil