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

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Date:	Thu, 28 Apr 2011 13:54:16 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Will Drewry <wad@...omium.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, 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

Quoting Will Drewry (wad@...omium.org):
> 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?

And add a clear comment explaining :)

> > 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.

Complicating the locking for nonexistent users doesn't seem like the
right way.

-serge
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