From: Pit Capitain <pit@...>
Date: 2006-06-19T17:44:34+09:00
Subject: Re: Why does this code traversing iv_tbl using dl/struct via evil.rb not work?

Rob Pitt schrieb:
> I seem to have misunderstood some finer point of how the instance 
> variable tables work. (...) Could someone please point out where I'm 
> going wrong by making this tiny test case work?

Rob, I added the necessary changes to your code below:

> require 'evil'
> require 'test/unit'

   module RubyInternal
     ST_TABLE_BIN = struct [
       "ST_TABLE_ENTRY *entry"
     ]
   end

> class A;end
>   
> class TestSHStructs < Test::Unit::TestCase
>   include RubyInternal
>   
>   def test_iv_tbl_traverse
>     a = A.new
>     located_keys = []
>     created_keys = ['@foo', '@bar', '@baz', '@dave', '@jess', '@sally']
>     created_keys.each do |key|
>       a.instance_variable_set(key,1)
>     end
>     r_object = RObject.new(a.internal_ptr)
>     table = ST_TABLE.new(r_object.iv_tbl)
>     chain = 0
>     while chain < table.num_bins

replace the following line:

>       table_entry = ST_TABLE_ENTRY.new(table.bins + (chain * DL::sizeof('P')))

with:

         table_bin = ST_TABLE_BIN.new(table.bins + (chain * DL::sizeof('P')))
         table_entry = table_bin.entry && ST_TABLE_ENTRY.new(table_bin.entry)

>       while table_entry
>         sym = table_entry.key.to_sym 
>         located_keys << sym.to_s if sym && sym.to_s[0] == ?@
>         table_entry = table_entry.next && ST_TABLE_ENTRY.new(table_entry.next)
>       end
>       chain += 1
>     end
>     assert_equal(created_keys.sort,located_keys.sort)
>   end
> end

"table.bins" points to an array of "table.num_bins" pointers to table entries. So

   table.bins + (chain * DL::sizeof('P'))

isn't the address of a table entry, but the address of a pointer to a table 
entry. In order to dereference these pointers, I added the ST_TABLE_BIN struct 
to your code.

Regards,
Pit