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Date: Thu, 26 Sep 2013 04:02:54 +0100
From: Al Viro <viro@...IV.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Djalal Harouni <tixxdz@...ndz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
	tixxdz@...il.com
Subject: Re: [PATCH 04/12] seq_file: Make seq_file able to access the file's
 opener cred

On Wed, Sep 25, 2013 at 05:22:51PM -0700, Linus Torvalds wrote:
> On Wed, Sep 25, 2013 at 1:14 PM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> >
> > Therefor add the f_cred field to the seq_file struct and a helper
> > seq_f_cred() to return it.
> 
> I hate how you've split up this patch from the next one that actually
> _initializes_ the new field.
> 
> The two patches should have been one.
> 
> I think the patch should also remove the 'user_ns' member, since it's
> now available as f_cred->user_ns.
> 
> I also suspect that it would be better to just make the the new
> seq_file member point to the 'struct file' instead. Sure, it's an
> extra level of indirection, but the lifetime of f_cred is not as clear
> otherwise. You don't increment the reference count, which is correct
> *only* because 'seq_file' has the same lifetime as 'struct file', and
> thus the reference count from struct file for the f_cred is
> sufficient.

That's better than f_cred (or user_ns, for that matter), but... I'm
afraid that it'll get abused very soon.  And I don't understand the
argument about the lifetime rules - what makes struct file ones
different from struct cred ones in that respect?  Except that in this
case it's really obvious that we can't grab a reference, that is...

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