linux-kernel - Re: [PATCH] procfs: make /proc style symlinks behave like "normal" symlinks

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Date:	Thu, 19 Nov 2009 10:57:08 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeff Layton <jlayton@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	pavel@....cz
Subject: Re: [PATCH] procfs: make /proc style symlinks behave like "normal" symlinks

Jeff Layton <jlayton@...hat.com> writes:

> On Thu, 19 Nov 2009 09:07:16 -0800
> ebiederm@...ssion.com (Eric W. Biederman) wrote:
>
>> 
>> Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>> 
>> This is broken.  If the referenced file is in a different mount namespace
>> the path returned could point to a completely different path in your
>> own mount namespace.  Even in your own mount namespace this makes the
>> proc symlinks racy and not guaranteed to return the file of interest.
>> 
>> I don't see any hope of this approach ever working.
>> 
>> Eric
>> 
>
> Then is proc_pid_readlink broken in the same way?

proc_pid_readlink has the same deficiencies.  The race is fundamental
to all readlink operations, the difference is that for normal symlinks
it is a don't care, and for proc it is incorrect behavior if you follow
the symlink to the wrong file.   If you are dealing with a file in a
different namespace or a socket what you get back doesn't actually
work as a file in your local namespace but that is the best we can do
with a pathname, and if you know the context of what is going on readlink
is still useful.

Adding all of the short comings to followlink that readlink has is a problem,
especially as followlink does much better now.

At a practical level I think your changes are much easier to exploit than
Pavels contrived example.

I really don't have any problems with your first patch to proc to add the
missing revalidate.

Eric
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