linux-kernel - Re: [External] Re: [PATCH] livepatch: Only block the removal of KLP

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Date:   Thu, 3 Mar 2022 10:43:18 -0500
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Chengming Zhou <zhouchengming@...edance.com>,
        Miroslav Benes <mbenes@...e.cz>
Cc:     jpoimboe@...hat.com, jikos@...nel.org, pmladek@...e.com,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        songmuchun@...edance.com, qirui.001@...edance.com
Subject: Re: [External] Re: [PATCH] livepatch: Only block the removal of
 KLP_UNPATCHED forced transition patch

On 3/3/22 5:33 AM, Chengming Zhou wrote:
> On 2022/3/3 3:51 下午, Miroslav Benes wrote:
>> On Thu, 3 Mar 2022, Chengming Zhou wrote:
>>
>>> Hi,
>>>
>>> On 2022/3/2 5:55 下午, Miroslav Benes wrote:
>>>> Hi,
>>>>
>>>> On Tue, 1 Mar 2022, Chengming Zhou wrote:
>>>>
>>>>> module_put() is currently never called for a patch with forced flag, to block
>>>>> the removal of that patch module that might still be in use after a forced
>>>>> transition.
>>>>>
>>>>> But klp_force_transition() will flag all patches on the list to be forced, since
>>>>> commit d67a53720966 ("livepatch: Remove ordering (stacking) of the livepatches")
>>>>> has removed stack ordering of the livepatches, it will cause all other patches can't
>>>>> be unloaded after disabled even if they have completed the KLP_UNPATCHED transition.
>>>>>
>>>>> In fact, we don't need to flag a patch to forced if it's a KLP_PATCHED forced
>>>>> transition. It can still be unloaded only if it has passed through the consistency
>>>>> model in KLP_UNPATCHED transition.
>>>>>
>>>>> So this patch only set forced flag and block the removal of a KLP_UNPATCHED forced
>>>>> transition livepatch.
>>>>>
>>>>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>>>>> ---
>>>>>  kernel/livepatch/transition.c | 4 ++--
>>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
>>>>> index 5683ac0d2566..8b296ad9e407 100644
>>>>> --- a/kernel/livepatch/transition.c
>>>>> +++ b/kernel/livepatch/transition.c
>>>>> @@ -641,6 +641,6 @@ void klp_force_transition(void)
>>>>>  	for_each_possible_cpu(cpu)
>>>>>  		klp_update_patch_state(idle_task(cpu));
>>>>>  
>>>>> -	klp_for_each_patch(patch)
>>>>> -		patch->forced = true;
>>>>> +	if (klp_target_state == KLP_UNPATCHED)
>>>>> +		klp_transition_patch->forced = true;
>>>>
>>>> I do not think this would interact nicely with the atomic replace feature. 
>>>> If you force the transition of a patch with ->replace set to true, no 
>>>> existing patch would get ->forced set with this change, which means all 
>>>> patches will be removed at the end of klp_try_complete_transition(). And 
>>>> that is something we want to prevent.
>>>
>>> Good point, I should check if it's an atomic replace livepatch in the else
>>> branch, in which case we have to set all existing patches to forced.
>>
>> Yes, but that leads to a question if it then brings any value. Forcing a 
>> transition should be exceptional. If it is needed, there may be other 
>> issues involved which should probably be fixed. Have you come across a 
>> practical situation where the patch helped?
> 
> Yes, you're right, the correct way is to find and fix the issues that
> make us to use this "force" transition interface, until we don't need
> to use it.
> 
> Apart from this reason, another reason we may use "force" transition
> is that we want to speed up the transition process of some patches
> when load them, and we can make sure these patches are safe to do so.
> (just like a consistency model check disable option when load a patch)
> 

Interesting use case.  Can you share any example livepatches where the
transition time was exceptionally long and that lead to requiring this
patch?

>From a kpatch developer's perspective, it would be interesting to read
how you go about ensuring forced livepatch safety.  We don't generally
build forced livepatches, so I'm curious how the dev/review process goes.

Thanks,
-- 
Joe