linux-kernel - Re: [PATCH v15 05/11] livepatch: Simplify API by removing registration step

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Date:   Thu, 10 Jan 2019 13:32:35 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Petr Mladek <pmladek@...e.com>
cc:     Jiri Kosina <jikos@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v15 05/11] livepatch: Simplify API by removing registration
 step

On Wed, 9 Jan 2019, Petr Mladek wrote:

> The possibility to re-enable a registered patch was useful for immediate
> patches where the livepatch module had to stay until the system reboot.
> The improved consistency model allows to achieve the same result by
> unloading and loading the livepatch module again.
> 
> Also we are going to add a feature called atomic replace. It will allow
> to create a patch that would replace all already registered patches.
> The aim is to handle dependent patches more securely. It will obsolete
> the stack of patches that helped to handle the dependencies so far.
> Then it might be unclear when a cumulative patch re-enabling is safe.
> 
> It would be complicated to support the many modes. Instead we could
> actually make the API and code easier to understand.
> 
> Therefore, remove the two step public API. All the checks and init calls
> are moved from klp_register_patch() to klp_enabled_patch(). Also the patch
> is automatically freed, including the sysfs interface when the transition
> to the disabled state is completed.
> 
> As a result, there is never a disabled patch on the top of the stack.
> Therefore we do not need to check the stack in __klp_enable_patch().
> And we could simplify the check in __klp_disable_patch().
> 
> Also the API and logic is much easier. It is enough to call
> klp_enable_patch() in module_init() call. The patch can be disabled
> by writing '0' into /sys/kernel/livepatch/<patch>/enabled. Then the module
> can be removed once the transition finishes and sysfs interface is freed.
> 
> The only problem is how to free the structures and kobjects safely.
> The operation is triggered from the sysfs interface. We could not put
> the related kobject from there because it would cause lock inversion
> between klp_mutex and kernfs locks, see kn->count lockdep map.
> 
> Therefore, offload the free task to a workqueue. It is perfectly fine:
> 
>   + The patch can no longer be used in the livepatch operations.
> 
>   + The module could not be removed until the free operation finishes
>     and module_put() is called.
> 
>   + The operation is asynchronous already when the first
>     klp_try_complete_transition() fails and another call
>     is queued with a delay.
> 
> Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
> Signed-off-by: Petr Mladek <pmladek@...e.com>

Acked-by: Miroslav Benes <mbenes@...e.cz>

Miroslav