[ruby-core:78504] [Ruby trunk Bug#13002] Hash calculations no longer using universal hashing

Issue #13002 has been updated by Martin D端rst.

Assignee set to Yui NARUSE
ruby -v set to ruby 2.4.0dev (2016-12-02 trunk 56965) [x86_64-cygwin]

Hello Victor,

Thanks for clearing up my confusing re. Bug #9381. It's too bad that that means we can't switch hashing methods.

Nobu - One thing I don't understand is why there wasn't any test failure on CI because there is a test that checks for bug #9318.

Yui - I have assigned this bug to you because it's related to the release of 2.4. Please feel free to reassign it to somebody else (not me :-).

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Bug #13002: Hash calculations no longer using universal hashing
https://bugs.ruby-lang.org/issues/13002#change-61885

* Author: Martin D端rst
* Status: Open
* Priority: Normal
* Assignee: Yui NARUSE
* ruby -v: ruby 2.4.0dev (2016-12-02 trunk 56965) [x86_64-cygwin]
* Backport: 2.1: DONTNEED, 2.2: DONTNEED, 2.3: DONTNEED
----------------------------------------
When preparing for my lecture on hash tables last week, I found that Ruby trunk doesn't do universal hashing anymore. See http://events.ccc.de/congress/2011/Fahrplan/attachments/2007_28C3_Effective_DoS_on_web_application_platforms.pdf for background.

I contacted [e-mail], but was told by Shugo that because trunk is not a published version, we can talk about it publicly.

Shugo also said that the change was introduced in r56650.

Following is some output from two different versions of Ruby that show the problem:

On Ruby 2.2.3, different hash value for the same number every time Ruby is restarted:

C:\Users\duerst>ruby -v
ruby 2.2.3p173 (2015-08-18 revision 51636) [i386-mingw32]

C:\Users\duerst>ruby -e 'puts 12345678.hash'
611647260

C:\Users\duerst>ruby -e 'puts 12345678.hash'
-844752827

C:\Users\duerst>ruby -e 'puts 12345678.hash'
387106497

On Ruby trunk, always the same value:

duerst@Arnisee /cygdrive/c/Data/ruby
$ ruby -v
ruby 2.4.0dev (2016-12-02 trunk 56965) [x86_64-cygwin]

duerst@Arnisee /cygdrive/c/Data/ruby
$ ruby -e 'puts 12345678.hash'
1846311797112760547

duerst@Arnisee /cygdrive/c/Data/ruby
$ ruby -e 'puts 12345678.hash'
1846311797112760547

duerst@Arnisee /cygdrive/c/Data/ruby
$ ruby -e 'puts 12345678.hash'
1846311797112760547


---Files--------------------------------
switching_hash_removal.patch (9.21 KB)


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