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

Josh Poimboeuf
2016-05-06T12:44:15Z

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