Bug #14218: Addition of negative Object#hash values can overflow

• Status: Closed
• Assignee: usa (Usaku NAKAMURA)
• Target version: -
• ruby -v: ruby 2.4.3p205 (2017-12-14 revision 61247) [x64-mingw32]
• Backport: 2.3: REQUIRED, 2.4: DONE

[honnza (John Dvořák)]

Script to reproduce:

p 1_000_000.times.count{a=Object.new.hash; b=Object.new.hash; a < 0 && b < 0 && a + b > 0}
p 1_000_000.times.count{a=Object.new.hash; b=Object.new.hash; 0 + a + b != 0 + b + a}

Expected value: As hash values are regular integers according to the documentation, the above script should produce a pair of zeroes.

Actual value: there are consistently about 58 000 cases out of a million (5.8%) in which the first loop detects a discrepancy. The second loop detects a discrepancy in about 78 000 cases out of a million.

Testing environment:

• ruby 2.4.1p111 (2017-03-22 revision 58053) [x64-mingw32] - originally discovered on and reproduced
• ruby 2.4.3p205 (2017-12-14 revision 61247) [x64-mingw32] - reproduced
• Ruby 2.4.3 i386-mingw32, same OS / hardware - NOT reproduced, IIRC

Linux seems to consistently not reproduce:

• ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-linux] - Ubuntu for Windows, same hardware
• ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-linux] - different machine, same as for the three cases below it
• ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]
• ruby 1.8.7 (2014-01-28 patchlevel 376) [x86_64-linux],
• ruby 1.9.3p551 (2014-11-13 revision 48407) [x86_64-linux]

Both irb and ruby reproduce the issue.

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Further observations:

The cases that produce the discrepancy are always pairs of negative int32s such that 32-bit addition overflows. The errorneous results are always positive and correct modulo 2**32.

Changing Object.new.hash to Object.new.hash.to_i does not change the observed behavior. Changing it to Object.new.hash + 0 does remove the inconsistencies, as does retyping the hash values into the interpreter. Removing 0+ from both sides of the comparison also removes the second inconsistency. No inconsistency observed with integer multiplication.

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Relevant discussion on StackOverflow chat begins here: https://chat.stackoverflow.com/transcript/message/40546901#40546901

[usa (Usaku NAKAMURA)]

• Status changed from Open to Assigned
• Assignee set to usa (Usaku NAKAMURA)
• Backport changed from 2.3: UNKNOWN, 2.4: UNKNOWN to 2.3: REQUIRED, 2.4: REQUIRED

[usa (Usaku NAKAMURA)]

• Status changed from Assigned to Closed

Applied in changeset trunk|r61413.

---

force hash values fixable

• include/ruby/ruby.h (RB_ST2FIX): force fixable on LLP64 environment.

• hash.c (any_hash): ditto.
  [ruby-core:84395] [Bug #14218]

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.3: REQUIRED, 2.4: REQUIRED to 2.3: REQUIRED, 2.4: DONE

ruby_2_4 r62849 merged revision(s) 61413.