linux-kernel - Re: how to get right euid?

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Message-ID: <20100102151928.GB401@unpythonic.net>
Date:	Sat, 2 Jan 2010 09:19:28 -0600
From:	Jeff Epler <jepler@...ythonic.net>
To:	Kevin Qu <rofail@...il.com>
Cc:	dhowells@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: how to get right euid?

On Sat, Jan 02, 2010 at 09:16:42PM +0800, Kevin Qu wrote:
> sudo echo "some thing" > /proc/my_file
> 
> It denied. (But it works when I su to superuser and do so.)
> 
> So I checked the current_euid(),
> but it returns 1000 (not 0),Why?

This may be due to a misunderstanding of how shell redirects work.
Probably current_euid() is returning exactly the right thing.

When you execute
    some command line > somefile
the shell opens somefile for writing and makes it be fd 1 (stdout) using
dup2.  Then it execs 'some' with the argument array being
['some', 'command', 'line'].

So consider your sudo:  The shell, running as user 1000, opens
'somefile' for writing, then execs 'sudo' which happens to be setuid
root.

Jeff
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