长生不老药用毒药解码

Question

I'm getting this string as query result from my database:

"%Sample.Struct{list: [], total: \"0.00\", day: 6, id: \"8vfts6\"}"

Is there any way to convert this one back to map? I'm getting this error decoding it with poison

** (Poison.SyntaxError) Unexpected token: %
(poison) lib/poison/parser.ex:56: Poison.Parser.parse!/2
(poison) lib/poison.ex:83: Poison.decode!/2

I can't fix the way data is being added to database, i must find a proper way for a key/value route to easily retrive data from that. (this is just a sample for a more complex result)

Answer 1

As it was mentioned in comments, you should not use Code.eval_string . But, there is a way to safely convert your code to Elixir struct, using Code module:

ex(1)> encoded = "%Sample.Struct{list: [], total: \"0.00\", day: 6, id: \"8vfts6\"}"
"%Sample.Struct{list: [], total: \"0.00\", day: 6, id: \"8vfts6\"}"

First, get the AST from the string, but use the pattern matching to ensure it is a struct you are looking for ( {:__aliases__, _, [:Sample, :Struct]} ). All other (potentially malicious) code will fail this match:

iex(2)> {:ok, {:%, _, [{:__aliases__, _, [:Sample, :Struct]}, {:%{}, _, keymap}]} = ast} = Code.string_to_quoted(encoded)
{:ok,
 {:%, [line: 1],
  [{:__aliases__, [line: 1], [:Sample, :Struct]},
   {:%{}, [line: 1], [list: [], total: "0.00", day: 6, id: "8vfts6"]}]}}

Here you have the full ast for you struct, and the keymap . You may now be tempted to use eval_quoted with the AST, to get the struct you needed:

iex(3)> {struct, _} = Code.eval_quoted(ast)
{%Sample.Struct{day: 6, id: "8vfts6", list: [], total: "0.00"}, []}
iex(4)> struct
%Sample.Struct{day: 6, id: "8vfts6", list: [], total: "0.00"}

But it is still not safe! Someone may put a function causing side effect into the string, like "%Sample.Struct{list: IO.puts \\"Something\\"}" , which will be executed during the evaluation. So you will need to check the keymap firsts, if it contain safe data .

Or you may just use keymap directly, without evaluating anyting:

iex(5)> struct(Sample.Struct, keymap)                                                                                    
%Sample.Struct{day: 6, id: "8vfts6", list: [], total: "0.00"}