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

Petr Mladek
2016-05-06T11:33:10Z

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.

> + 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.

Best Regards,
Petr