From: "mrkn (Kenta Murata)" <muraken@...> Date: 2012-11-04T03:04:46+09:00 Subject: [ruby-core:48816] [ruby-trunk - Bug #7242] Bignum mathematical accuracy regression in r31695 Issue #7242 has been updated by mrkn (Kenta Murata). Category set to core Although I'm trying to fix it for about two days, I found I need more time to find the direct causes of the bug. I decide to disable Toom3 method until the bug is fixed. ---------------------------------------- Bug #7242: Bignum mathematical accuracy regression in r31695 https://bugs.ruby-lang.org/issues/7242#change-32305 Author: mhall (Matthew Hall) Status: Assigned Priority: Urgent Assignee: mrkn (Kenta Murata) Category: core Target version: 2.0.0 ruby -v: ruby 1.9.3dev (2011-05-22 trunk 31695) [x86_64-linux] We have some pure Ruby code which performs a DH (Diffie Hellman) cryptographic handshake. If I upgrade to using the Ruby 1.9.3 interpreter instead of 1.9.2, the unit tests around the code fail, because the client and server can no longer compute matching secret keys. I would expect they could agree on matching keys as the DH algorithm expects. The code worked fine since May 2007 on Ruby 1.8 and 1.9 until the commit below was introduced. Using svn-bisect, I was able to find that this commit from ruby-trunk in between Ruby 1.9.2 (27656) and Ruby 1.9.3 (32500) causes the problem: r31695 | mrkn | 2011-05-22 08:37:00 -0700 (Sun, 22 May 2011) | 4 lines * bignum.c (dump_bignum, bigmul1_balance, big_split, biglsh_bang, bigrsh_bang, big_split3, bigmul1_toom3, bigmul0): implement Toom3 (Toom-Cook) multiplication. * include/ruby/defines.h: add format prefixes for BDIGIT and BDIGIT_DBL. Attached to this bug I have a simplified example test case which will pass without this commit, and fail with this commit present. Since this commit covers some relatively complex mathematical algorithms I'd appreciate some assistance in finding the root cause from the experts on how bignum.c works. I think the bug is relatively serious because it could cause inaccurate output for other mathematical code using Bignums since it was introduced in mid-2011. This ruby -v comes from ruby trunk SVN at revision 31695, where the failure begins to happen. I hand-re-compiled at the bisection points on trunk from 27655 through 32501 to identify the one which caused the problem. $ ruby -v ruby 1.9.3dev (2011-05-22 trunk 31695) [x86_64-linux] C Compiler: gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3 configure params: ./configure \ --with-static-linked-ext \ --prefix=/usr/local/ruby192 \ --enable-shared \ --with-ruby-version=full -- http://bugs.ruby-lang.org/