Re: [PATCH v4 13/15] livepatch: change to a per-task consistency model

Josh Poimboeuf
2017-02-08T16:46:43Z

On Wed, Feb 08, 2017 at 04:47:50PM +0100, Petr Mladek wrote:
> > Notice in this case that klp_target_state is KLP_PATCHED. Which means
> > that klp_complete_transition() would not call synchronize_rcu() at the
> > right time, nor would it call module_put(). It can be fixed with:
> >
> > @@ -387,7 +389,7 @@ static int __klp_enable_patch(struct klp_patch *patch)
> > pr_warn("failed to enable patch '%s'\n",
> > patch->mod->name);
> >
> > - klp_unpatch_objects(patch);
> > + klp_target_state = KLP_UNPATCHED;
> > klp_complete_transition();
> >
> > return ret;
>
> Great catch! Looks good to me.

As it turns out, klp_target_state isn't accessible from this file, so
I'll make a klp_cancel_transition() helper function which reverses
klp_target_state and calls klp_complete_transition().

> > This assumes that the 'if (klp_target_state == KLP_UNPATCHED)' clause in
> > klp_try_complete_transition() gets moved to klp_complete_transition() as
> > you suggested.
> >
> > > > > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> > > > > > index 5efa262..1a77f05 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);
> > > > > >
> > > > > > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> > > > > > {
> > > > > > struct klp_ops *ops;
> > > > > > struct klp_func *func;
> > > > > > + int patch_state;
> > > > > >
> > > > > > 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);
> > > > > > +
> > > > > > + /*
> > > > > > + * func should never be NULL because preemption should be disabled here
> > > > > > + * and unregister_ftrace_function() does the equivalent of a
> > > > > > + * synchronize_sched() before the func_stack removal.
> > > > > > + */
> > > > > > + if (WARN_ON_ONCE(!func))
> > > > > > + goto unlock;
> > > > > > +
> > > > > > + /*
> > > > > > + * Enforce the order of the ops->func_stack and func->transition reads.
> > > > > > + * The corresponding write barrier is in __klp_enable_patch().
> > > > > > + */
> > > > > > + smp_rmb();
> > > > >
> > > > > I was curious why the comment did not mention __klp_disable_patch().
> > > > > It was related to the hours of thinking. I would like to avoid this
> > > > > in the future and add a comment like.
> > > > >
> > > > > * This barrier probably is not needed when the patch is being
> > > > > * disabled. The patch is removed from the stack in
> > > > > * klp_try_complete_transition() and there we need to call
> > > > > * rcu_synchronize() to prevent seeing the patch on the stack
> > > > > * at all.
> > > > > *
> > > > > * Well, it still might be needed to see func->transition
> > > > > * when the patch is removed and the task is migrated. See
> > > > > * the write barrier in __klp_disable_patch().
> > > >
> > > > Agreed, though as you mentioned earlier, there's also the implicit
> > > > barrier in klp_update_patch_state(), which would execute first in such a
> > > > scenario. So I think I'll update the barrier comments in
> > > > klp_update_patch_state().
> > >
> > > You inspired me to a scenario with 3 CPUs:
> > >
> > > CPU0 CPU1 CPU2
> > >
> > > __klp_disable_patch()
> > >
> > > klp_init_transition()
> > >
> > > func->transition = true
> > >
> > > smp_wmb()
> > >
> > > klp_start_transition()
> > >
> > > set TIF_PATCH_PATCHPENDING
> > >
> > > klp_update_patch_state()
> > >
> > > task->patch_state
> > > = KLP_UNPATCHED
> > >
> > > smp_mb()
> > >
> > > klp_ftrace_handler()
> > > func = list_...
> > >
> > > smp_rmb() /*needed?*/
> > >
> > > if (func->transition)
> > >
> >
> > I think this isn't possible. Remember the comment I added to
> > klp_update_patch_state():
> >
> > * NOTE: If task is not 'current', the caller must ensure the task is inactive.
> > * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
> >
> > Right now klp_update_patch_state() is only called for current.
> > klp_ftrace_handler() on CPU2 would be running in the context of a
> > different task.
>
> I agree that it is impossible with the current code. In fact, I cannot
> imagine a way to migrate the task where the barrier would be needed.
> The question if we could/should somehow document it. Something like
>
> * The barrier is not really needed when the patch is being
> * disabled. The value of func->transition would change
> * the result of this handled only after the task is migrated.
> * But the conditions for the migration are very limited
> * and practically include a full barrier, see
> * klp_update_patch_state().

Well, I'd like to avoid over-commenting this issue. For v5 I've added
the following comment to klp_update_patch_state() -- see #2:

/*
* This test_and_clear_tsk_thread_flag() call also serves as a read
* barrier (smp_rmb) for two cases:
*
* 1) Enforce the order of the TIF_PATCH_PENDING read and the
* klp_target_state read. The corresponding write barrier is in
* klp_init_transition().
*
* 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
* of func->transition, if klp_ftrace_handler() is called later on
* the same CPU. See __klp_disable_patch().
*/

Is that sufficient? If not, I could maybe add another related comment
in klp_ftrace_handler() which refers to this comment.

--
Josh