linux-kernel - Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model

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Date:	Fri, 6 May 2016 07:44:10 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
	Miroslav Benes <mbenes@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-s390@...r.kernel.org, Vojtech Pavlik <vojtech@...e.com>,
	Jiri Slaby <jslaby@...e.cz>,
	Chris J Arges <chris.j.arges@...onical.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency
 model

On Fri, May 06, 2016 at 01:33:01PM +0200, Petr Mladek wrote:
> On Thu 2016-04-28 15:44:48, Josh Poimboeuf wrote:
> > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> > index 782fbb5..b3b8639 100644
> > --- a/kernel/livepatch/patch.c
> > +++ b/kernel/livepatch/patch.c
> > @@ -29,6 +29,7 @@
> >  #include <linux/bug.h>
> >  #include <linux/printk.h>
> >  #include "patch.h"
> > +#include "transition.h"
> >  
> >  static LIST_HEAD(klp_ops);
> >  
> > @@ -58,11 +59,42 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> >  	ops = container_of(fops, struct klp_ops, fops);
> >  
> >  	rcu_read_lock();
> > +
> >  	func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
> >  				      stack_node);
> > -	if (WARN_ON_ONCE(!func))
> > +
> > +	if (!func)
> >  		goto unlock;
> >  
> > +	/*
> > +	 * See the comment for the 2nd smp_wmb() in klp_init_transition() for
> > +	 * an explanation of why this read barrier is needed.
> > +	 */
> > +	smp_rmb();
> > +
> > +	if (unlikely(func->transition)) {
> > +
> > +		/*
> > +		 * See the comment for the 1st smp_wmb() in
> > +		 * klp_init_transition() for an explanation of why this read
> > +		 * barrier is needed.
> > +		 */
> > +		smp_rmb();
> 
> I would add here:
> 
> 		WARN_ON_ONCE(current->patch_state == KLP_UNDEFINED);
> 
> We do not know in which context this is called, so the printk's are
> not ideal. But it will get triggered only if there is a bug in
> the livepatch implementation. It should happen on random locations
> and rather early when a bug is introduced.
> 
> Anyway, better to die and catch the bug that let the system running
> in an undefined state and produce cryptic errors later on.

Ok, makes sense.

> > +		if (current->patch_state == KLP_UNPATCHED) {
> > +			/*
> > +			 * Use the previously patched version of the function.
> > +			 * If no previous patches exist, use the original
> > +			 * function.
> > +			 */
> > +			func = list_entry_rcu(func->stack_node.next,
> > +					      struct klp_func, stack_node);
> > +
> > +			if (&func->stack_node == &ops->func_stack)
> > +				goto unlock;
> > +		}
> > +	}
> 
> I am staring into the code for too long now. I need to step back for a
> while. I'll do another look when you send the next version. Anyway,
> you did a great work. I speak mainly for the livepatch part and
> I like it.

Thanks for the helpful reviews!  I'll be on vacation again next week so
I get a break too :-)

-- 
Josh