From: Brian Candler <B.Candler@...>
Date: 2003-05-30T18:07:01+09:00
Subject: Re: output redirection within mod_ruby

On Fri, May 30, 2003 at 05:32:13PM +0900, Ian Macdonald wrote:
> I have a script that runs under mod_ruby with Apache 2.0.40. At a
> certain point, it needs to invoke Kernel.system. For reasons of
> security, I pass this method multiple arguments, so that it doesn't hand
> off the whole thing to the shell, as then I'd need to worry about
> sanitising the tainted data.
> 
> Unfortunately, the output of the binary called by system does not get
> sent to the browser, even though the binary sends its output to stdout.
> Presumably, mod_ruby is discarding the output.
> 
> Is there any way around this phenomenon?

I think that you have to capture the output of the command yourself. Your
Unix stdout cannot be captured by Apache unless your applicaton is a
separate process (which is true of a CGI or FastCGI application, but not
true of mod_ruby where your code is in the same process as the Apache
worker)

There was a thread about capturing the output of system() a few days ago.
Here is a better version: system2() is like system() but returns three items
(stdout string, stderr string, exit status), and system3() has an extra
initial parameter for a string to pass to stdin. It's based very much on
'open3.rb' from the Ruby standard library.

module Kernel
  private
  def system3(inp_data, cmd, *args)
    pw = IO::pipe
    pr = IO::pipe
    pe = IO::pipe 
    pid = fork do
      pw[1].close
      pr[0].close
      pe[0].close
      STDIN.reopen(pw[0])
      STDOUT.reopen(pr[1])
      STDERR.reopen(pe[1])
      exec(cmd, *args)
    end
    pw[0].close
    pr[1].close
    pe[1].close
    out_data = err_data = nil
    t1 = Thread.new do
      pw[1].write inp_data if inp_data
      pw[1].close
    end
    t2 = Thread.new do
      out_data = pr[0].read
      pr[0].close
    end
    t3 = Thread.new do
      err_data = pe[0].read
      pe[0].close
    end
    wpid, wstatus = Process.waitpid2(pid)
    t1.join
    t2.join
    t3.join
    return out_data, err_data, wstatus
  end
  def system2(cmd, *args)
    system3(nil, cmd, *args)
  end
end

# Example:
# a, b, c = system2("ls","/dev/null","/nonexistent")
# p a,b,c
# >> "/dev/null\n"
# >> "ls: /nonexistent: No such file or directory\n"
# >> 256     (= exit code 1 from ls)