From: Sam Duncan <sduncan@...>
Date: 2012-10-29T06:06:04+09:00
Subject: Re: File.readable? and /proc

On 10/29/2012 09:18 AM, Jeff Moore wrote:
> Igor Pirnovar wrote in post #1081714:
>> Jeff Moore wrote in post #1081712:
>>
>>> It's the inconsistency with:
>>>
>>> root@nail:/projects/proc_fs# ls -al /proc/sys/fs/binfmt_misc/register
>>> --w------- 1 root root 0 2012-10-28 08:24
>>> /proc/sys/fs/binfmt_misc/register
>>>
>>> that I find mostly surprising.
>> 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