linux-kernel - Re: [PATCH v3 03/13] seccomp_filters: new mode with configurable syscall filters

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Date:	Thu, 2 Jun 2011 14:12:28 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	kees.cook@...onical.com, torvalds@...ux-foundation.org,
	tglx@...utronix.de, mingo@...e.hu, jmorris@...ei.org,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-security-module@...r.kernel.org
Subject: Re: [PATCH v3 03/13] seccomp_filters: new mode with configurable
 syscall filters

On Thu, Jun 02, 2011 at 04:46:07PM -0400, Steven Rostedt wrote:
> On Thu, 2011-06-02 at 15:28 -0500, Will Drewry wrote:
> 
> [ Snipped 860 lines of non relevant text ]
> 
> Seriously guys, Please trim your replies. These last few messages were
> ridicules. I spent more than 30 seconds searching for what the email was
> about. That's too much wasted time.

Because every time I do trim the messages, I get a response from the
reviewee of the form "Oh, I take care of that in function foo()."
And of course function foo() will be in the part I trimmed.  So I then
have to find the earlier message, copy the function back in, and by
that time something else has distracted me.

							Thanx, Paul

> -- Steve
> 
> 
> > >> Ah, I spaced on rcu_dereference().  The goal was to make the
> > >> assignment and replacement of the seccomp_filters pointer
> > >> RCU-protected (in seccomp_state) so there's no concern over it being
> > >> replaced partial on platforms where pointer assignments are non-atomic
> > >> - such as via /proc/<pid>/seccomp_filters access or a call via the
> > >> exported symbols.  Object lifetime is managed by reference counting so
> > >> that I don't have to worry about extending the RCU read-side critical
> > >> section by much or deal with pre-allocations.
> > >>
> > >> I'll add rcu_dereference() to all the get_seccomp_filters() uses where
> > >> it makes sense, so that it is called safely.  Just to make sure, does
> > >> it make sense to continue to rcu protect the specific pointer?
> > >
> > > It might.  The usual other options is to use a lock outside of the element
> > > containing the reference count to protect reference-count manipulation.
> > > If there is some convenient lock, especially if it is already held where
> > > needed, then locking is more straightforward.  Otherwise, RCU is usually
> > > a reasonable option.
> > 
> > I was concerned about the overhead a lock would have at each system
> > call entry, but I didn't benchmark it to see.  I'll add the
> > rcu_dereference right away, then look into seeing whether there's a
> > cleaner approach.  I was trying to be overly protective of mutating
> > any data internal to the filters through complete replacement on any
> > change.  I'll take a step back and see if
> > 
> 
> 
> --
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