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SubjectRe: [PATCH 4/4] livepatch: Remove the redundant enabled flag in struct klp_patch
On Wed, 16 Jan 2019, Petr Mladek wrote:

> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index 684766d306ad..8e644837e668 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -59,6 +59,17 @@ static bool klp_is_module(struct klp_object *obj)
> return obj->name;
> }
>
> +static bool klp_patch_enabled(struct klp_patch *patch)
> +{
> + if (patch == klp_transition_patch) {
> + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);

I think we'd have a race in the code then. enabled_show() does not take
klp_mutex() when calling klp_patch_enabled().

A patch sysfs attributes are added quite early during its enablement.
klp_init_transition() first sets klp_transition_patch, then
klp_target_state. It means one can call enabled_show() with patch ==
klp_transition_patch and klp_target_state == KLP_UNDEFINED. No?

The similar applies the disablement. klp_complete_transition() first
clears klp_target_state (sets it to KLP_UNDEFINED), then it clears
klp_transition_patch.

We could add locking to enabled_show() or swap the assignments with some
barriers on top.

Or we could remove WARN_ON_ONCE() and live with false results in
enabled_show(). It does not matter much, I think. All the other call sites
of klp_patch_enabled() should be fine.

> + return klp_target_state == KLP_PATCHED;
> + }
> +
> + return !list_empty(&patch->list);
> +}

Shouldn't we also change list_del(&patch->list) in klp_free_patch_start()
to list_del_init(&patch->list)?

[...]

> @@ -955,7 +964,7 @@ static int __klp_enable_patch(struct klp_patch *patch)
> if (klp_transition_patch)
> return -EBUSY;
>
> - if (WARN_ON(patch->enabled))
> + if (list_empty(&patch->list))
> return -EINVAL;

I wanted to ask why there is list_empty() and not klp_patch_enabled(), so
just to be sure... the patch was added to klp_patches list, so patch->list
is not empty (should not be). We could achieve the same by calling
!klp_patch_enabled() given its implementation, but it would look
counter-intuitive here.

The rest looks fine.

However, I am not sure if the outcome is better than what we have. Yes,
patch->enabled is not technically necessary and we can live with that (as
the patch proves). On the other hand, it gives the reader clear guidance
about the patch's state. klp_patch_enabled() is not a complete
replacement. We have to call list_empty() in __klp_enable_patch() or check
the original klp_target_state in klp_try_complete_transition().

I am not against the change, I am glad to see it is achievable, but I am
not sure if the code is better with it. Joe acked it. What do the others
think?

Thanks,
Miroslav

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