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

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Date:	Thu, 28 Apr 2011 11:20:15 -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>,
	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):
> >>  void __secure_computing(int this_syscall)
> >>  {
> >> -     int mode = current->seccomp.mode;
> >> +     int mode = -1;
> >>       int * syscall;
> >> -
> >> +     /* Do we need an RCU read lock to access current's state? */
> >
> > I'm actually confused to why you are using RCU. What are you protecting.
> > Currently, I see the state is always accessed from current->seccomp. But
> > current should not be fighting with itself.
> >
> > Maybe I'm missing something.
> 
> I'm sure it's me that's missing something.  I believe the seccomp
> pointer can be accessed from:
> - current
> - via /proc/<pid>/seccomp_filter (read-only)
> 
> Given those cases, would it make sense to ditch the RCU interface for it?

ISTM you need them to protect the reader.

-serge
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