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From: ebiederm@xmission.com (Eric W. Biederman)
To: Dave Hansen <haveblue@us.ibm.com>
Cc: "Serge E. Hallyn" <serue@us.ibm.com>, Cedric Le Goater <clg@fr.ibm.com>,
       linux-kernel@vger.kernel.org, Andrew Morton <akpm@osdl.org>,
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       Herbert Poetzl <herbert@13thfloor.at>,
       Sam Vilain <sam.vilain@catalyst.net.nz>
Subject: Re: [PATCH -mm 5/7] add user namespace
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Date: Fri, 14 Jul 2006 13:07:14 -0600
In-Reply-To: <1152902522.314.47.camel@localhost.localdomain> (Dave Hansen's
	message of "Fri, 14 Jul 2006 11:42:02 -0700")
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Dave Hansen <haveblue@us.ibm.com> writes:

> On Fri, 2006-07-14 at 12:06 -0600, Eric W. Biederman wrote:
>> > On Fri, 2006-07-14 at 11:18 -0600, Eric W. Biederman wrote:
>> >> /proc/<pid>/fd/...
>> >> /proc/<pid>/exe
>> >> /proc/<pid>/cwd
>> >> 
>> >> It isn't quite the same as you are actually opening a second
>> >> copy of the file descriptor but the essence is the same. 
>> >
>> > Last I checked, those were symlinks and didn't work for things like
>> > deleted files.  Am I wrong?
>> 
>> Yes.  They are not really symlinks.
>> 
>> Wanting to have an executable that was deleted after it was done
>> executing.  I wrote it to a file. opened it, unlinked it, set close
>> on exec, and the exec'd it with /proc/self/fd/N.
>
> Well, on one hand, it makes checkpoints with deleted files easier ;)
>
> Now that I'm actually looking at the code, isn't
> proc_fd_access_allowed()'s permission just derived from ptrace
> permissions?  It doesn't seem to involve uids directly at all!

You still have to actually open the file and from there you get to permission().

Eric