From: "ko1 (Koichi Sasada)" <redmine@...>
Date: 2013-12-09T15:35:21+09:00
Subject: [ruby-core:58990] [ruby-trunk - Feature #8977] String#frozen that	takes advantage of the deduping


Issue #8977 has been updated by ko1 (Koichi Sasada).


Now, I have one concern about security concern.

This kind of method can be used widely and easily.

And if this method is used with external string getting from IO,
fstring table can be grow and grow easily.

I'm afraid about such kind of security risk:
(1) DoS attack
(2) Side channel attack (observe from outside)

But I'm not a security expert. So I want to ask experts.

Note that this problem has not impact than Symbol related DoS attack
because these keys are collected.

-----

I think multiple tables support solve kind of issues.

To solve such issue (and continue to discuss this issue to 2.2),
Ruby level implementation and gem is reasonable alternative, I believe.

However, in ruby-level, we can't do that same thing.

Therefor nobu made a patch for WeakHash, a variant of WeakMap (we will make another ticket for it).

WeakMap is object_id -> Object map.
WeakHash is Object -> Object map.

With this class, we can make fstring technique
with multiple tables easily.

class FrozenStringTable
  def initialize
    @table = {} # WeakHash.new
  end

  def get str
    raise TypeError unless str.kind_of?(String)

    unless @table.has_key? str
      str.freeze
      @table[str] = str
    end
    @table[str]
  end
end

F1 = FrozenStringTable.new
p F1.get('foo').object_id #=> 8274120
p F1.get('foo').object_id #=> 8274120

----

In this comment, I show (1) security concern, and (2) alternative approach.


----------------------------------------
Feature #8977: String#frozen that takes advantage of the deduping 
https://bugs.ruby-lang.org/issues/8977#change-43544

Author: sam.saffron (Sam Saffron)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: 
Target version: current: 2.1.0


During memory profiling I noticed that a large amount of string duplication is generated from non pre-determined strings.

Take this report for example https://gist.github.com/SamSaffron/6789005 (generated using the memory_profiler gem that works against head) 

">=" x 4953
    /Users/sam/.rbenv/versions/2.1.0-dev/lib/ruby/2.1.0/rubygems/requirement.rb:93 x 4535

This string is most likely extracted from a version. 

Or 

"/Users/sam/.rbenv/versions/2.1.0-dev/lib/ruby/gems" x 5808
    /Users/sam/.rbenv/versions/2.1.0-dev/lib/ruby/gems/2.1.0/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251 x 3894

A string that can not be pre-determined. 

---- 


It would be nice to have 

"hello,world".split(",")[0].frozen.object_id == "hello"f.object_id 

Adding #frozen will give library builders a way of using the de-duping. It also could be implemented using weak refs in 2.0 and stubbed with a .dup.freeze in 1.9.3 . 

Thoughts ?  





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