From: Jeff Moore <lists@...>
Date: 2012-10-29T11:56:00+09:00
Subject: Re: File.readable? and /proc

Sam Duncan wrote in post #1081722:
> On 10/29/2012 09:18 AM, Jeff Moore wrote:
>>> Well, that's how Unix works. I have two objections:
>>>
>>> (1) You should not be using superuser account for regular work!
>>> (2) You have no business running irb in /proc filesystem!
>>>
>>> If you do either of these things, you better not be surprised, ever!
>> And if you 'regular work' involves interrogating the /proc file system,
>> then what?
>>
> You could use something like this? You'd need to flesh out some error/
> input handling, and these constants (sadly not exposed by ruby that I
> can find) will be platform specific. I think the 'problem' is that root
> can do pretty much anything it wants (make a file owned root:root, chmod
> 000, and enjoy echo'ing stuff into it). The stdlib seems to use
> 'eaccess( path, R_OK )' which I guess gives a 0 for euid 0.
>
> require 'etc'
> S_IFMT   = 0170000
> S_IFLNK    0120000
> S_IRUSR  = 00400
> S_IRGRP  = 00040
> S_IROTH  = 00004
>
> def readable?( path, follow=false )
>      st = File.lstat( path )
>      if follow && ( S_IFLNK == ( st.mode & S_IFMT ) )
>          st = File.stat( path )
>      end
>      uid = Process.euid
>      if ( st.mode & S_IROTH ) > 0
>          print 'Other read'
>          true
>      elsif( ( ( st.mode & S_IRUSR ) > 0 ) && ( st.uid == uid ) )
>          print 'Owner read'
>          true
>      elsif ( st.mode & S_IRGRP ) > 0
>          username = Etc.getpwuid( uid ).name
>          if Etc.getgrgid( st.gid ).mem.include?( username )
>              print 'Group read'
>              true
>          else
>              false
>          end
>      else
>          false
>      end
> end
>
>
> Sam

Sam, Thanks for this.

It wasn't exactly what I expected but it focused on all the right issues
and that has lead to a conclusion.

As you pointed out, eaccess is the key.

If the stdlib implements eaccess with the same behavior as the 
implementation in file.c (line 1034), root is simply granted
read and write permission without ever hitting stat.

'ls -al', on the other hand, relies on stat.

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