From: Tim Pease <tim.pease@...>
Date: 2006-09-07T04:23:51+09:00
Subject: mmap causing a bus error

$ cat mmap-abort.rb

require 'mmap'
require 'fileutils'

FileUtils.touch 'output.txt'
File.open('mmap.bin', 'w') {|f| f.truncate 16} unless File.exist? 'mmap.bin'

m = Mmap.new 'output.txt', 'rw'

(0...4).each do |x|
  fork do
    mmap = Mmap.new 'mmap.bin', 'rw'
    range = (x*4)...(x*4+4)
    val = mmap[range].unpack('N')[0]
    mmap[range] = [val+x].pack('N')
    mmap.unmap
  end
end

m << "Done!\n"

# EOF
$ ruby mmap-abort.rb
mmap-abort.rb:20: [BUG] Bus Error
ruby 1.8.4 (2005-12-24) [powerpc-linux]

Aborted (core dumped)
$

The line at the end -- m << "Done!\n" -- is causing the abort.  I'm
seeing this behavior on all the Unix platforms I have access to ...

SunOS ravenclaw 5.9 Generic_112233-12 sun4u sparc SUNW,Sun-Blade-1500

Linux panic 2.6.17-1.2157_FC5 #1 SMP Tue Jul 11 23:03:20 EDT 2006
ppc64 ppc64 ppc64 GNU/Linux

Linux pong 2.6.17-1.2142_FC4 #1 Tue Jul 11 22:41:14 EDT 2006 i686 i686
i386 GNU/Linux


So, the interesting part is that if I open the "output.txt" file and
add a single character to it, the problem disappears ...

$ echo 'x' > output.txt
$ ruby mmap-abort.rb
$ cat output.txt
x
Done!
$

I can also make the problem go away by creating the mmap to output.txt
after the each/fork block.

I can also make the problem go away by commenting out the "mmap.unmap"
line at the end of the fork block.

The bus error is being caused by a call to memcpy in the "mm_cat"
method of the mmap source code -- line 1480 of mmap.c

Any patches out there?