Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

Miroslav Benes
2015-02-12T10:45:15Z

On Wed, 11 Feb 2015, Josh Poimboeuf wrote:

> On Wed, Feb 11, 2015 at 11:21:51AM +0100, Miroslav Benes wrote:
> >
> > On Mon, 9 Feb 2015, Josh Poimboeuf wrote:
> >
> > [...]
> >
> > > @@ -38,14 +39,34 @@ 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);
> > > - rcu_read_unlock();
> > >
> > > if (WARN_ON_ONCE(!func))
> > > - return;
> > > + goto unlock;
> > > +
> > > + if (unlikely(func->transition)) {
> > > + /* corresponding smp_wmb() is in klp_init_transition() */
> > > + smp_rmb();
> > > +
> > > + if (current->klp_universe == KLP_UNIVERSE_OLD) {
> > > + /*
> > > + * 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;
> > > + }
> > > + }
> > >
> > > klp_arch_set_pc(regs, (unsigned long)func->new_func);
> > > +unlock:
> > > + rcu_read_unlock();
> > > }
> >
> > I decided to understand the code more before answering the email about the
> > race and found another problem. I think.
> >
> > Imagine we patched some function foo() with foo_1() from patch_1 and now
> > we'd like to patch it again with foo_2() in patch_2. __klp_enable_patch
> > calls klp_init_transition which sets klp_universe for all processes to
> > KLP_UNIVERSE_OLD and marks the foo_2() for transition (it is gonna be 1).
> > Then __klp_enable_patch adds foo_2() to the RCU-protected list for foo().
> > BUT what if somebody calls foo() right between klp_init_transition and
> > the loop in __klp_enable_patch? The ftrace handler first returns the
> > first entry in the list which is foo_1() (foo_2() is still not present),
> > then it checks for func->transition. It is 1.
>
> No, actually foo_1()'s func->transition will be 0. Only foo_2()'s
> func->transition will be 1.

Ah, you're right in both cases. Sorry for the noise.

Miroslav

>
> > It checks for
> > current->klp_universe which is KLP_UNIVERSE_OLD and so the next entry is
> > retrieved. There is no such and therefore foo() is called. This is
> > obviously wrong because foo_1() was expected.
> >
> > Everything would work fine if one would call foo() before
> > klp_start_transition and after the loop in __klp_enable_patch. The
> > solution might be to move the setting of func->transition to
> > klp_start_transition, but this could break something different. I don't
> > know yet.
> >
> > Am I wrong?
> >
> > Miroslav
>
> --
> Josh
>