linux-kernel - Re: [PATCH 3/7] seccomp_filter: Enable ftrace-based system call filtering

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Message-ID: <1304017616.18763.204.camel@gandalf.stny.rr.com>
Date:	Thu, 28 Apr 2011 15:06:56 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Will Drewry <wad@...omium.org>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>, linux-kernel@...r.kernel.org,
	kees.cook@...onical.com, eparis@...hat.com, agl@...omium.org,
	mingo@...e.hu, jmorris@...ei.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Michal Marek <mmarek@...e.cz>,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jiri Slaby <jslaby@...e.cz>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 3/7] seccomp_filter: Enable ftrace-based system call
 filtering

On Thu, 2011-04-28 at 13:34 -0500, Will Drewry wrote:

> > Perhaps, but those free standing functions (the ones that are exported
> > to modules) seem like they can destroy state.
> 
> My intent was to make them available for use by seccomp.c during state
> teardown/dup.  I don't think there's benefit to exposing them outside
> of that.  Would dropping the export, and adding an local seccomp.h
> with the shared functions in them resolve that more cleanly?

Yes, having a local seccomp.h in the same directory as seccomp.c is a
way of saying "these are internal functions, don't use them directly".
Adding a function to EXPORT_SYMBOL*() is saying "here you go, have a
field day with it!".


> 
> > Your code would have been correct if you could call kzalloc under
> > rcu_read_lock() (which you can on some kernel configurations but not
> > all). The issue is that you need to pull out that allocation from the
> > rcu_read_lock() because rcu_read_lock assumes you can't preempt, and
> > that allocation can schedule out. The access to the filters must be done
> > under rcu_read_lock(), other than that, you're fine.
> 
> That makes sense.  I think I'd prefer to not share those functions
> rather than guard the list just in case a future consumer of the
> interface comes along.  Would that make sense to you?  Since I don't
> see any other users right now other than seccomp.c, it might make
> sense to tackle the impact when an actual need arises.
> 
> I'll go whichever way pointed on this, though.
> thanks again,

I'm fine either way. If you make them local internal functions, then you
don't need the locking if they are safe in their current context.

-- Steve


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